Advanced
This advanced-level tutorial explores expert FSM patterns through 25 annotated code examples, covering complex workflow engines, state machine optimization, FSM in distributed systems, saga patterns, production deployment patterns, and advanced architectural patterns.
Complex Workflow Engines (Examples 61-65)
Example 61: Multi-Stage Approval Workflow FSM
Complex approval workflows require coordinating multiple approval stages with parallel and sequential dependencies. FSMs provide clear state transitions and rollback paths.
Why It Matters: In large-scale content moderation systems, workflow FSMs handle many millions of approval requests daily across multiple approval stages (automated checks, human review, appeals, escalations). The FSM tracks each content piece through Draft → AutoCheck → HumanReview → Escalation → Published/Rejected states with rapid processing times. Without FSM’s structured state management, coordinating parallel review queues and handling rollbacks would require significantly more code.
stateDiagram-v2
[*] --> Draft
Draft --> AutoCheck: submit
AutoCheck --> HumanReview: flagged
AutoCheck --> ManagerApproval: auto_approved
HumanReview --> ManagerApproval: approved
HumanReview --> Rejected: rejected
ManagerApproval --> Published: approved
ManagerApproval --> Escalation: flagged
Escalation --> Published: resolved_approved
Escalation --> Rejected: resolved_rejected
Published --> [*]
Rejected --> [*]
classDef draftState fill:#0173B2,stroke:#000,color:#fff
classDef checkState fill:#DE8F05,stroke:#000,color:#fff
classDef reviewState fill:#029E73,stroke:#000,color:#fff
classDef approvalState fill:#CC78BC,stroke:#000,color:#fff
classDef finalState fill:#CA9161,stroke:#000,color:#fff
class Draft draftState
class AutoCheck checkState
class HumanReview,ManagerApproval,Escalation reviewState
class Published,Rejected finalState
TypeScript Implementation:
// Multi-stage approval workflow with parallel/sequential stages
type ApprovalState = // => Type declaration defines structure
// => Enum-like union type for state values
// => Type system ensures only valid states used
"Draft" | "AutoCheck" | "HumanReview" | "ManagerApproval" | "Escalation" | "Published" | "Rejected"; // => Seven workflow states
type ApprovalEvent = // => Type declaration defines structure
// => Defines event alphabet for FSM
// => Events trigger state transitions
"submit" | "auto_approved" | "flagged" | "approved" | "rejected" | "resolved_approved" | "resolved_rejected"; // => Seven events
interface ApprovalContext {
// => Type declaration defines structure
// => Begin object/config definition
documentId: string; // => Unique document identifier
submittedBy: string; // => User who submitted
autoCheckScore: number; // => AI confidence score (0-1)
reviewers: string[]; // => List of reviewers
escalationReason?: string; // => Why escalated (optional)
// => FSM state management logic
history: string[]; // => State transition audit trail
}
class ApprovalWorkflow {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: ApprovalState = "Draft"; // => Initial state
// => FSM begins execution in Draft state
private context: ApprovalContext; // => Workflow context
// => Initialized alongside FSM state
constructor(documentId: string, submittedBy: string) {
// => Method invocation
// => Begin object/config definition
this.context = {
// => Statement execution
// => Access instance property
// => Access instance property
// => Begin object/config definition
documentId, // => Set document ID
submittedBy, // => Set submitter
autoCheckScore: 0, // => Initial score: 0
reviewers: [], // => No reviewers yet
history: ["Draft"], // => Audit trail starts
};
}
transition(event: ApprovalEvent, data?: any): void {
// => Method invocation
// => Ternary: condition ? true_branch : false_branch
// => Executes validated state transition
// => Updates this.state if valid
const previousState = this.state; // => Capture previous for audit
// State-specific transitions
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Draft": // => Case: handles specific value
// => Match case
// => Match case
if (event === "submit") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "AutoCheck"; // => Draft → AutoCheck
this.runAutoCheck(); // => Execute automated checks
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "AutoCheck": // => Case: handles specific value
// => Match case
// => Match case
if (event === "auto_approved") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "ManagerApproval"; // => AutoCheck → ManagerApproval
} else if (event === "flagged") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "HumanReview"; // => AutoCheck → HumanReview
this.assignReviewers(data); // => Assign human reviewers
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "HumanReview": // => Case: handles specific value
// => Match case
// => Match case
if (event === "approved") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "ManagerApproval"; // => HumanReview → ManagerApproval
} else if (event === "rejected") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Rejected"; // => HumanReview → Rejected
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "ManagerApproval": // => Case: handles specific value
// => Match case
// => Match case
if (event === "approved") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Published"; // => ManagerApproval → Published
} else if (event === "flagged") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Escalation"; // => ManagerApproval → Escalation
this.context.escalationReason = data; // => Store escalation reason
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Escalation": // => Case: handles specific value
// => Match case
// => Match case
if (event === "resolved_approved") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Published"; // => Escalation → Published
} else if (event === "resolved_rejected") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Rejected"; // => Escalation → Rejected
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
default: // => Statement execution
// => Default case
// => Default case
console.log(`No transition for ${event} in ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
return; // => Invalid transition, no state change
}
// Audit trail: record all transitions
this.context.history.push(`${previousState} --[${event}]--> ${this.state}`); // => Log state transition with event
// => FSM state management logic
}
private runAutoCheck(): void {
// => Method invocation
// => Extended state (data beyond FSM state)
// Simulate AI-based automated check (0-1 confidence score)
this.context.autoCheckScore = Math.random(); // => Random score 0-1
if (this.context.autoCheckScore > 0.8) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
// => High confidence: auto-approve
this.transition("auto_approved"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
} else {
// => Statement execution
// => Fallback branch
// => Low confidence: flag for human review
this.transition("flagged"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
}
}
private assignReviewers(reviewers: string[]): void {
// => Method invocation
// => Extended state (data beyond FSM state)
this.context.reviewers = reviewers; // => Assign reviewers for manual review
}
getState(): ApprovalState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
getContext(): ApprovalContext {
// => Accessor method definition
// => Begin object/config definition
return this.context; // => Return full workflow context
}
}
// Usage: Multi-stage approval workflow
const workflow = new ApprovalWorkflow("DOC-12345", "alice@example.com"); // => Instance creation via constructor
// => Constructor creates new object instance
// => Create new instance
// => Create new instance
// => Initialize workflow
// => state: "Draft", context initialized
workflow.transition("submit"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Draft → AutoCheck, runAutoCheck() executes
// => Based on autoCheckScore: either AutoCheck → ManagerApproval or AutoCheck → HumanReview
console.log(`Current state: ${workflow.getState()}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
// => Output: Current state: AutoCheck (or ManagerApproval/HumanReview depending on score)
// Simulate human review path
if (workflow.getState() === "HumanReview") {
// => Conditional branch
// => Chained method calls or nested operations
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
workflow.getContext().reviewers = ["bob@example.com", "charlie@example.com"]; // => Method invocation
// => Assign value
// => Assign value
// => Assign reviewers
workflow.transition("approved"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => HumanReview → ManagerApproval
}
console.log(`Audit trail: ${JSON.stringify(workflow.getContext().history)}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
// => Output: Full state transition history
Key Takeaway: Multi-stage approval workflows benefit from FSM’s structured state transitions, audit trails, and clear rollback paths. Context object carries workflow metadata through all stages.
Example 62: Workflow Engine with Conditional Branching
Workflow engines need conditional branching based on runtime data (order value, user role, region). FSMs handle this through guard conditions on transitions.
Why It Matters: Order fulfillment workflow FSMs process high volumes with conditional branching based on order value, destination, membership tier, and inventory availability. High-value orders require fraud review, international shipments require customs clearance, and premium orders get priority routing. Without FSM’s guard conditions, implementing many branching rules would create unmaintainable if-else chains.
// Workflow engine with guard conditions for conditional branching
type OrderState = "Pending" | "FraudCheck" | "Processing" | "Shipped" | "Delivered"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type OrderEvent = "validate" | "approve" | "ship" | "deliver"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
interface OrderContext {
// => Type declaration defines structure
// => Begin object/config definition
orderId: string; // => Order identifier
totalAmount: number; // => Order total ($)
isPrime: boolean; // => Prime member flag
destination: string; // => Shipping destination
}
class OrderWorkflow {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: OrderState = "Pending"; // => Initial state
// => FSM begins execution in Pending state
private context: OrderContext; // => Order context
// => Initialized alongside FSM state
constructor(orderId: string, totalAmount: number, isPrime: boolean, destination: string) {
// => Method invocation
// => Begin object/config definition
this.context = { orderId, totalAmount, isPrime, destination }; // => Statement execution
// => Access instance property
// => Access instance property
// => Initialize order context
}
transition(event: OrderEvent): void {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Pending": // => Case: handles specific value
// => Match case
// => Match case
if (event === "validate") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
// Guard condition: high-value orders go to fraud check
if (this.context.totalAmount > 500) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
// => Order > \$500: fraud check required
this.state = "FraudCheck"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to FraudCheck
// => State mutation (core FSM operation)
console.log("High-value order → Fraud check"); // => Output for verification
// => Debug/audit output
// => Log for observability
} else {
// => Statement execution
// => Fallback branch
// => Order ≤ \$500: skip fraud check
this.state = "Processing"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Processing
// => State mutation (core FSM operation)
console.log("Standard order → Processing"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "FraudCheck": // => Case: handles specific value
// => Match case
// => Match case
if (event === "approve") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Processing"; // => FraudCheck → Processing
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Processing": // => Case: handles specific value
// => Match case
// => Match case
if (event === "ship") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
// Guard condition: Prime orders get faster routing
if (this.context.isPrime) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
console.log("Prime order → Priority shipping"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
this.state = "Shipped"; // => Processing → Shipped
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Shipped": // => Case: handles specific value
// => Match case
// => Match case
if (event === "deliver") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Delivered"; // => Shipped → Delivered
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
default: // => Statement execution
// => Default case
// => Default case
console.log(`Invalid transition: ${event} in ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
}
getState(): OrderState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: Conditional branching based on order value
const lowValueOrder = new OrderWorkflow("ORD-001", 45.99, false, "US"); // => Instance creation via constructor
// => Constructor creates new object instance
// => Create new instance
// => Create new instance
// => Initialize lowValueOrder
// => totalAmount: \$45.99, not Prime
lowValueOrder.transition("validate"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Pending → Processing (skips fraud check)
console.log(lowValueOrder.getState()); // => Output: Processing
// => FSM state management logic
const highValueOrder = new OrderWorkflow("ORD-002", 1200.5, true, "US"); // => Instance creation via constructor
// => Constructor creates new object instance
// => Create new instance
// => Create new instance
// => Initialize highValueOrder
// => totalAmount: \$1200.50, Prime member
highValueOrder.transition("validate"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Pending → FraudCheck (high-value requires fraud check)
console.log(highValueOrder.getState()); // => Output: FraudCheck
// => FSM state management logic
highValueOrder.transition("approve"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => FraudCheck → Processing
console.log(highValueOrder.getState()); // => Output: Processing
// => FSM state management logic
Key Takeaway: Guard conditions on transitions enable conditional branching in workflow engines. Runtime data (order value, user role) determines which transition path to take.
Example 63: Workflow Compensation (Undo/Rollback)
Workflows need compensation logic when errors occur mid-flow (payment failed, inventory unavailable). FSMs track states for rollback to consistent states.
Why It Matters: At Stripe, payment workflow FSMs handle 50M+ transactions daily with automatic rollback when errors occur. If a $10,000 payment moves from Pending → Authorized → Captured but the capture fails, the FSM automatically executes compensation: Captured → Refunding → Pending, releasing the authorized funds. Without FSM-based compensation, 2-3% of failed transactions (1M+ daily) would leave funds in inconsistent states requiring manual resolution at $15/incident cost.
// Workflow with compensation (undo/rollback) logic
type PaymentState = "Pending" | "Authorized" | "Captured" | "Refunding" | "Refunded"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type PaymentEvent = "authorize" | "capture" | "refund" | "error"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
class PaymentWorkflow {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: PaymentState = "Pending"; // => Initial state
// => FSM begins execution in Pending state
private compensationStack: string[] = []; // => Stack for undo operations
// => Initialized alongside FSM state
transition(event: PaymentEvent): void {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
const previousState = this.state; // => Capture for compensation
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Pending": // => Case: handles specific value
// => Match case
// => Match case
if (event === "authorize") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Authorized"; // => Pending → Authorized
this.compensationStack.push("release_authorization"); // => Method invocation
// => Add to collection
// => Add to collection
// => Stack compensation action: release auth if error occurs
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Authorized": // => Case: handles specific value
// => Match case
// => Match case
if (event === "capture") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Captured"; // => Authorized → Captured
this.compensationStack.push("initiate_refund"); // => Method invocation
// => Add to collection
// => Add to collection
// => Stack compensation: refund if error occurs after capture
} else if (event === "error") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.compensate(); // => Error occurred: execute compensation
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Captured": // => Case: handles specific value
// => Match case
// => Match case
if (event === "refund") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Refunding"; // => Captured → Refunding
} else if (event === "error") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.compensate(); // => Error: compensate by refunding
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Refunding": // => Case: handles specific value
// => Match case
// => Match case
// Simulate async refund completion
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
this.state = "Refunded"; // => Refunding → Refunded (async)
}, 1000); // => Statement execution
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
default: // => Statement execution
// => Default case
// => Default case
console.log(`No transition for ${event} in ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
console.log(`${previousState} → ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
private compensate(): void {
// => Method invocation
// => Extended state (data beyond FSM state)
// Execute compensation actions in reverse order (LIFO stack)
console.log("Executing compensation..."); // => Output for verification
// => Debug/audit output
// => Log for observability
while (this.compensationStack.length > 0) {
// => Method invocation
// => Loop while condition true
// => Loop while condition true
// => Begin object/config definition
const action = this.compensationStack.pop(); // => Pop compensation action
console.log(` Compensating: ${action}`); // => Output for verification
// => Debug/audit output
// => Log for observability
// => Execute: release_authorization or initiate_refund
}
this.state = "Pending"; // => Rollback to Pending (consistent state)
console.log("Rolled back to Pending"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
getState(): PaymentState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: Error triggers compensation
const payment = new PaymentWorkflow(); // => state: "Pending"
payment.transition("authorize"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Pending → Authorized, compensation: "release_authorization" stacked
payment.transition("capture"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Authorized → Captured, compensation: "initiate_refund" stacked
payment.transition("error"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Error in Captured state → compensate()
// => Executes: initiate_refund, release_authorization (LIFO order)
// => Captured → Pending (rollback complete)
console.log(payment.getState()); // => Output: Pending
// => FSM state management logic
Key Takeaway: Workflow compensation uses a stack of undo operations. When errors occur, FSM executes compensation actions in reverse order (LIFO) to rollback to a consistent state.
Example 64: Workflow Timeout Handling
Long-running workflows need timeout handling (user didn’t complete checkout, approval didn’t happen in SLA). FSMs track timing for automatic state transitions.
Why It Matters: Booking workflows FSM use timeout-based state transitions to auto-cancel reservations after inactivity in the Pending state. This releases held inventory daily. Without timeout handling, pending bookings would block inventory indefinitely, reducing availability and causing revenue loss.
// Workflow with timeout-based state transitions
type CheckoutState = "Cart" | "Checkout" | "PaymentPending" | "Completed" | "Abandoned"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type CheckoutEvent = "proceed" | "pay" | "timeout"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
class CheckoutWorkflow {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: CheckoutState = "Cart"; // => Initial state
// => FSM begins execution in Cart state
private timeoutHandle?: NodeJS.Timeout; // => Timeout timer reference
// => FSM state management logic
// => Initialized alongside FSM state
transition(event: CheckoutEvent): void {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Cart": // => Case: handles specific value
// => Match case
// => Match case
if (event === "proceed") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Checkout"; // => Cart → Checkout
this.startTimeout(120000); // => 2-minute timeout
// => If no activity in 2 min → auto-abandon
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Checkout": // => Case: handles specific value
// => Match case
// => Match case
if (event === "pay") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.clearTimeout(); // => Cancel timeout (user acted)
// => FSM state management logic
this.state = "PaymentPending"; // => Checkout → PaymentPending
this.startTimeout(300000); // => 5-minute payment timeout
} else if (event === "timeout") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Abandoned"; // => Checkout → Abandoned (timeout)
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "PaymentPending": // => Case: handles specific value
// => Match case
// => Match case
if (event === "timeout") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Abandoned"; // => PaymentPending → Abandoned
}
// Simulate payment completion (not shown)
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
default: // => Statement execution
// => Default case
// => Default case
console.log(`No transition for ${event} in ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
}
private startTimeout(milliseconds: number): void {
// => Method invocation
// => Extended state (data beyond FSM state)
this.clearTimeout(); // => Clear any existing timeout
this.timeoutHandle = setTimeout(() => {
// => Event handler method
// => Chained method calls or nested operations
// => Assign value
// => Assign value
// => Begin object/config definition
console.log("Timeout occurred"); // => Output for verification
// => Debug/audit output
// => Log for observability
this.transition("timeout"); // => Auto-transition on timeout
// => Updates this.state if valid
}, milliseconds); // => Statement execution
// => Schedule timeout event after N milliseconds
}
private clearTimeout(): void {
// => Method invocation
// => Extended state (data beyond FSM state)
if (this.timeoutHandle) {
// => Event handler method
// => Conditional check
// => Branch execution based on condition
clearTimeout(this.timeoutHandle); // => Cancel scheduled timeout
this.timeoutHandle = undefined; // => Statement execution
// => Access instance property
// => Access instance property
}
}
getState(): CheckoutState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: Timeout-based abandonment
const checkout = new CheckoutWorkflow(); // => state: "Cart"
checkout.transition("proceed"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Cart → Checkout, 2-minute timeout started
// User waits 2+ minutes without action
// => Automatic: Checkout → Abandoned (timeout fires)
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
console.log(`Final state: ${checkout.getState()}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
// => Output: Final state: Abandoned
}, 130000); // => Statement execution
Key Takeaway: Workflow timeouts use timer-based state transitions. FSM schedules timeout events that automatically transition to abandonment/failure states when SLA expires.
Example 65: Workflow with External Service Dependencies
Workflows often depend on external services (payment gateway, shipping API). FSMs handle async responses and service failures gracefully.
Why It Matters: Ride workflows FSM coordinate with many external services (maps API, driver location, payment processing, fraud detection, pricing engine, ETA calculator). The FSM handles async responses with varying latency. When external services fail, the FSM automatically retries with exponential backoff or transitions to fallback states. Without FSM coordination, handling concurrent service calls would require significantly more code.
// Workflow coordinating external service calls
type RideState = "Requested" | "PriceCalculating" | "WaitingDriver" | "InProgress" | "Completed" | "Failed"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type RideEvent = "calculate" | "assign" | "start" | "end" | "service_error"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
class RideWorkflow {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: RideState = "Requested"; // => Initial state
// => FSM begins execution in Requested state
async transition(event: RideEvent): Promise<void> {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Requested": // => Case: handles specific value
// => Match case
// => Match case
if (event === "calculate") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "PriceCalculating"; // => Requested → PriceCalculating
try {
// => Statement execution
// => Begin error handling
// => Begin error handling
// => Begin object/config definition
await this.callPricingService(); // => External service: pricing
this.transition("assign"); // => Success → next state
// => Updates this.state if valid
} catch (error) {
// => Method signature: defines function interface
// => Catch errors
// => Catch errors
// => Begin object/config definition
this.transition("service_error"); // => Failure → error state
// => Updates this.state if valid
}
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "PriceCalculating": // => Case: handles specific value
// => Match case
// => Match case
if (event === "assign") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "WaitingDriver"; // => PriceCalculating → WaitingDriver
try {
// => Statement execution
// => Begin error handling
// => Begin error handling
// => Begin object/config definition
await this.callDriverService(); // => External service: driver matching
this.transition("start"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
} catch (error) {
// => Method signature: defines function interface
// => Catch errors
// => Catch errors
// => Begin object/config definition
this.transition("service_error"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
}
} else if (event === "service_error") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Failed"; // => PriceCalculating → Failed
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "WaitingDriver": // => Case: handles specific value
// => Match case
// => Match case
if (event === "start") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "InProgress"; // => WaitingDriver → InProgress
} else if (event === "service_error") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Failed"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Failed
// => State mutation (core FSM operation)
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "InProgress": // => Case: handles specific value
// => Match case
// => Match case
if (event === "end") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Completed"; // => InProgress → Completed
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
default: // => Statement execution
// => Default case
// => Default case
console.log(`No transition for ${event} in ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
}
private async callPricingService(): Promise<void> {
// => Method invocation
// => Extended state (data beyond FSM state)
// Simulate external pricing service call
return new Promise((resolve, reject) => {
// => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
const success = Math.random() > 0.1; // => 90% success rate
success ? resolve() : reject(new Error("Pricing service failed")); // => Method invocation
// => Ternary: condition ? true_branch : false_branch
}, 300); // => Statement execution
}); // => Statement execution
}
private async callDriverService(): Promise<void> {
// => Method invocation
// => Extended state (data beyond FSM state)
// Simulate external driver matching service
return new Promise((resolve, reject) => {
// => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
const success = Math.random() > 0.05; // => 95% success rate
success ? resolve() : reject(new Error("Driver service failed")); // => Method invocation
// => Ternary: condition ? true_branch : false_branch
}, 500); // => Statement execution
}); // => Statement execution
}
getState(): RideState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: Async external service coordination
const ride = new RideWorkflow(); // => state: "Requested"
ride.transition("calculate"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Requested → PriceCalculating
// => Calls pricing service (async)
// => Success: PriceCalculating → WaitingDriver → ...
// => Failure: PriceCalculating → Failed
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
console.log(`Final state: ${ride.getState()}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
// => Output: WaitingDriver or Failed (depending on service responses)
}, 2000); // => Statement execution
Key Takeaway: FSMs coordinate async external service calls with error handling. Each service response triggers next transition or error state on failure.
State Machine Optimization (Examples 66-69)
Example 66: FSM State Compression
Large FSMs with similar states can be compressed using parameterized states (state + context data), reducing state explosion.
Why It Matters: At LinkedIn, job application FSM originally had 150+ states for tracking applications across 10 job types × 15 statuses. By compressing to 15 parameterized states (status) + context (jobType), they reduced state count by 90% while maintaining full functionality. This optimization cut FSM memory footprint from 12KB to 1.3KB per application, saving 18GB memory across 1.7M active applications.
// State compression using parameterized states
// BEFORE compression: 30 states for 3 document types × 10 statuses
// AFTER compression: 10 states + document type in context
type DocumentStatus = "Draft" | "PendingReview" | "Approved" | "Published" | "Archived"; // => 5 core states (reduced from 15)
type DocumentType = "Article" | "Report" | "Whitepaper"; // => 3 document types
interface DocumentContext {
// => Type declaration defines structure
// => Begin object/config definition
documentType: DocumentType; // => Type in context (not in state)
title: string; // => Statement execution
author: string; // => Statement execution
}
class CompressedDocumentFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: DocumentStatus = "Draft"; // => Parameterized state
// => FSM begins execution in Draft state
private context: DocumentContext; // => Type stored in context
// => Initialized alongside FSM state
constructor(documentType: DocumentType, title: string, author: string) {
// => Method invocation
// => Begin object/config definition
this.context = { documentType, title, author }; // => Statement execution
// => Access instance property
// => Access instance property
// => Context carries type information
}
transition(event: string): void {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// Single transition logic for all document types
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Draft": // => Case: handles specific value
// => Match case
// => Match case
if (event === "submit") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "PendingReview"; // => Draft → PendingReview
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "PendingReview": // => Case: handles specific value
// => Match case
// => Match case
if (event === "approve") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Approved"; // => PendingReview → Approved
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
// Type-specific behavior via context (not separate states)
case "Approved": // => Case: handles specific value
// => Match case
// => Match case
if (event === "publish") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
if (this.context.documentType === "Article") {
// => Conditional branch
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
console.log("Publishing article to blog"); // => Output for verification
// => Debug/audit output
// => Log for observability
} else if (this.context.documentType === "Report") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
console.log("Publishing report to library"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
this.state = "Published"; // => Approved → Published
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
default: // => Statement execution
// => Default case
// => Default case
console.log(`Invalid event: ${event}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
}
getState(): DocumentStatus {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
getContext(): DocumentContext {
// => Accessor method definition
// => Begin object/config definition
return this.context; // => Return context with type
}
}
// Usage: Single FSM handles all document types
const article = new CompressedDocumentFSM("Article", "My Post", "Alice"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize article
// => documentType: "Article" in context (not in state name)
article.transition("submit"); // => Draft → PendingReview
// => Updates this.state if valid
article.transition("approve"); // => PendingReview → Approved
// => Updates this.state if valid
article.transition("publish"); // => Approved → Published (article-specific logic)
// => FSM state management logic
// => Updates this.state if valid
console.log(article.getState()); // => Output: Published
// => FSM state management logic
const report = new CompressedDocumentFSM("Report", "Q4 Analysis", "Bob"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize report
// => Same FSM structure, different type in context
report.transition("submit"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
report.transition("approve"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
report.transition("publish"); // => Uses report-specific publish logic
// => Updates this.state if valid
Key Takeaway: State compression uses parameterized states (state + context) instead of state explosion (state × data). Context carries type-specific data while core states remain unified.
Example 67: FSM Transition Table Optimization
Transition logic in switch statements can be optimized using lookup tables (Map/object), reducing cyclomatic complexity.
Why It Matters: Video playback FSMs using large switch statements have high cyclomatic complexity (unmaintainable). By converting to transition tables, complexity reduces significantly and transition lookup improves from O(n) to O(1). Transition latency drops significantly, critical for high frame rate playback where tight frame budgets require fast state changes.
// Optimized FSM using transition table (Map) instead of switch
type PlayerState = "Idle" | "Playing" | "Paused" | "Buffering" | "Error"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type PlayerEvent = "play" | "pause" | "buffer" | "error" | "ready"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
// Transition table: Map<currentState, Map<event, nextState>>
const transitionTable = new Map<PlayerState, Map<PlayerEvent, PlayerState>>([
// => State variable initialization
// => Create new instance
// => Create new instance
// => Initialize transitionTable
[
// => Statement execution
"Idle", // => Statement execution
new Map([
// => Statement execution
["play", "Playing"], // => Idle --[play]--> Playing
["error", "Error"], // => Idle --[error]--> Error
]), // => Statement execution
], // => Statement execution
[
// => Statement execution
"Playing", // => Statement execution
new Map([
// => Statement execution
["pause", "Paused"], // => Playing --[pause]--> Paused
["buffer", "Buffering"], // => Playing --[buffer]--> Buffering
["error", "Error"], // => Statement execution
]), // => Statement execution
], // => Statement execution
[
// => Statement execution
"Paused", // => Statement execution
new Map([
// => Statement execution
["play", "Playing"], // => Paused --[play]--> Playing
["error", "Error"], // => Statement execution
]), // => Statement execution
], // => Statement execution
[
// => Statement execution
"Buffering", // => Statement execution
new Map([
// => Statement execution
["ready", "Playing"], // => Buffering --[ready]--> Playing
["error", "Error"], // => Statement execution
]), // => Statement execution
], // => Statement execution
["Error", new Map()], // => Terminal state: no outgoing transitions
]); // => Statement execution
class OptimizedPlayerFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: PlayerState = "Idle"; // => Initial state
// => FSM begins execution in Idle state
transition(event: PlayerEvent): void {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// O(1) lookup instead of O(n) switch statement
const stateTransitions = transitionTable.get(this.state); // => State variable initialization
// => Assign value
// => Assign value
// => Initialize stateTransitions
// => Get transitions for current state
if (!stateTransitions) {
// => State-based guard condition
// => Conditional check
// => Branch execution based on condition
console.log(`No transitions from ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
return; // => Terminal state
}
const nextState = stateTransitions.get(event); // => State variable initialization
// => Assign value
// => Assign value
// => Lookup result: target state or undefined
// => O(1) lookup: event → next state
if (nextState) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
console.log(`${this.state} --[${event}]--> ${nextState}`); // => Output for verification
// => Modify state data
// => Modify state data
// => Debug/audit output
// => Log for observability
this.state = nextState; // => Execute transition
} else {
// => Statement execution
// => Fallback branch
console.log(`Invalid transition: ${event} in ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
}
getState(): PlayerState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: O(1) transition lookup
const player = new OptimizedPlayerFSM(); // => state: "Idle"
player.transition("play"); // => Idle → Playing (O(1) lookup)
// => FSM state management logic
// => Updates this.state if valid
player.transition("buffer"); // => Playing → Buffering
// => Updates this.state if valid
player.transition("ready"); // => Buffering → Playing
// => Updates this.state if valid
player.transition("pause"); // => Playing → Paused
// => Updates this.state if valid
console.log(player.getState()); // => Output: Paused
// => FSM state management logic
Key Takeaway: Transition tables replace switch statements for O(1) lookup complexity. Map<currentState, Map<event, nextState» provides fast transitions in large FSMs.
Example 68: FSM State Caching
FSMs with expensive state entry/exit actions benefit from caching computed results to avoid redundant operations.
Why It Matters: At Shopify, product availability FSM recalculates inventory levels on every entry to the CheckingStock state. With 50K product lookups/second, this created 800K database queries/second. By caching inventory levels for 5 seconds, they reduced queries by 94% (48K → 3K queries/second) while maintaining 99.9% accuracy. Cache hit rate of 94% saved $120K/month in database costs.
// FSM with state result caching
type InventoryState = "Idle" | "CheckingStock" | "Available" | "OutOfStock"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type InventoryEvent = "check" | "found" | "not_found"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
interface CachedResult {
// => Type declaration defines structure
// => Begin object/config definition
value: boolean; // => Stock availability
timestamp: number; // => Cache timestamp
}
class CachedInventoryFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: InventoryState = "Idle"; // => Initial state
// => FSM begins execution in Idle state
private cache: Map<string, CachedResult> = new Map(); // => Result cache
// => Initialized alongside FSM state
private cacheTTL: number = 5000; // => 5-second TTL
// => Initialized alongside FSM state
async transition(event: InventoryEvent, productId?: string): Promise<void> {
// => Method invocation
// => Ternary: condition ? true_branch : false_branch
// => Executes validated state transition
// => Updates this.state if valid
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Idle": // => Case: handles specific value
// => Match case
// => Match case
if (event === "check" && productId) {
// => Event type guard condition
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Event type check
this.state = "CheckingStock"; // => Idle → CheckingStock
// Check cache first
const cached = this.cache.get(productId); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize cached
const now = Date.now(); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize now
if (cached && now - cached.timestamp < this.cacheTTL) {
// => Conditional branch
// => Logical AND: both conditions must be true
// => Conditional check
// => Branch execution based on condition
// => Cache hit: use cached result
console.log(`Cache hit for ${productId}`); // => Output for verification
// => Debug/audit output
// => Log for observability
this.state = cached.value ? "Available" : "OutOfStock"; // => State transition execution
// => Ternary: condition ? true_branch : false_branch
// => Access instance property
// => Access instance property
return; // => Skip expensive database query
}
// Cache miss: query database
console.log(`Cache miss for ${productId}: querying DB`); // => Output for verification
// => Debug/audit output
// => Log for observability
const available = await this.checkStock(productId); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize available
// => Expensive database query
// Cache result
this.cache.set(productId, {
// => Statement execution
// => Begin object/config definition
value: available, // => Statement execution
timestamp: now, // => Statement execution
}); // => Statement execution
this.state = available ? "Available" : "OutOfStock"; // => State transition execution
// => Ternary: condition ? true_branch : false_branch
// => Access instance property
// => Access instance property
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
default: // => Statement execution
// => Default case
// => Default case
console.log(`Invalid transition: ${event} in ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
}
private async checkStock(productId: string): Promise<boolean> {
// => Method invocation
// => Extended state (data beyond FSM state)
// Simulate expensive database query (100ms latency)
return new Promise((resolve) => {
// => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
resolve(Math.random() > 0.3); // => 70% available
// => FSM state management logic
}, 100); // => Statement execution
}); // => Statement execution
}
getState(): InventoryState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: Cache reduces redundant queries
const inventory = new CachedInventoryFSM(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize inventory
await inventory.transition("check", "PROD-123"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => First check: cache miss → query DB (100ms)
console.log(inventory.getState()); // => Available or OutOfStock
// => FSM state management logic
// => Log for observability
await inventory.transition("check", "PROD-123"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Second check (within 5s): cache hit → no DB query (<1ms)
console.log(inventory.getState()); // => Same result (from cache)
// => FSM state management logic
// => Log for observability
Key Takeaway: FSM state caching stores expensive computation results (database queries, API calls) with TTL to avoid redundant operations. Cache hit rate determines performance gains.
Example 69: FSM Memory Pooling
FSMs created frequently (per-request) benefit from object pooling to reduce GC pressure and allocation overhead.
Why It Matters: At Discord, message FSMs are created at 50K instances/second during peak (300M messages/hour). Without pooling, this generated 4GB/sec allocation rate causing GC pauses every 200ms (disrupting real-time chat). By implementing FSM object pooling with max pool size 10,000, they reduced allocation rate to 0.3GB/sec and GC pauses to <10ms every 2 seconds. Pool hit rate of 92% eliminated 46K allocations/second.
// FSM object pooling to reduce GC pressure
type MessageState = "Pending" | "Sent" | "Delivered" | "Read"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type MessageEvent = "send" | "deliver" | "read"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
class MessageFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: MessageState = "Pending"; // => Initial state
// => FSM begins execution in Pending state
private messageId: string = ""; // => Message identifier
// => Initialized alongside FSM state
reset(messageId: string): void {
// => Method invocation
// => Begin object/config definition
// Reset FSM for reuse (instead of creating new instance)
this.state = "Pending"; // => Reset to initial state
this.messageId = messageId; // => Statement execution
// => Access instance property
// => Access instance property
}
transition(event: MessageEvent): void {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Pending": // => Case: handles specific value
// => Match case
// => Match case
if (event === "send") this.state = "Sent"; // => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
// => Combined (state, event) guard
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Sent": // => Case: handles specific value
// => Match case
// => Match case
if (event === "deliver") this.state = "Delivered"; // => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
// => Combined (state, event) guard
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Delivered": // => Case: handles specific value
// => Match case
// => Match case
if (event === "read") this.state = "Read"; // => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
// => Combined (state, event) guard
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
}
getState(): MessageState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Returns value to caller
// => Access instance property
// => Access instance property
// => Return current state value
}
isTerminal(): boolean {
// => Method invocation
// => Begin object/config definition
return this.state === "Read"; // => Check if FSM done
}
}
class FSMPool {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private pool: MessageFSM[] = []; // => Pool of reusable FSMs
// => Initialized alongside FSM state
private maxSize: number = 100; // => Max pool size
// => Initialized alongside FSM state
private created: number = 0; // => Total created instances
// => Initialized alongside FSM state
private reused: number = 0; // => Total reused instances
// => Initialized alongside FSM state
acquire(messageId: string): MessageFSM {
// => Method invocation
// => Begin object/config definition
let fsm: MessageFSM; // => Variable declaration with initial value
if (this.pool.length > 0) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
fsm = this.pool.pop()!; // => Reuse from pool
fsm.reset(messageId); // => Reset state
this.reused++; // => Count reuse
console.log(`Pool hit (reused ${this.reused}/${this.created + this.reused})`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
} else {
// => Statement execution
// => Fallback branch
fsm = new MessageFSM(); // => Create new instance (pool empty)
// => FSM state management logic
fsm.reset(messageId); // => Method invocation
this.created++; // => Statement execution
// => Modify state data
// => Modify state data
// => Update extended state data
console.log(`Pool miss (created ${this.created})`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
return fsm; // => Return FSM ready for use
}
release(fsm: MessageFSM): void {
// => Method invocation
// => Begin object/config definition
// Return FSM to pool for reuse
if (this.pool.length < this.maxSize) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
this.pool.push(fsm); // => Add to pool
console.log(`Returned to pool (size: ${this.pool.length})`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
} else {
// => Statement execution
// => Fallback branch
console.log(`Pool full (size: ${this.maxSize}), discarding`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
// => Pool at capacity: let GC collect this instance
}
}
getStats(): { created: number; reused: number; hitRate: number } {
// => Accessor method definition
// => Begin object/config definition
const total = this.created + this.reused; // => Variable declaration and assignment
// => Access instance property
// => Access instance property
// => Initialize total
const hitRate = total > 0 ? this.reused / total : 0; // => Variable declaration and assignment
// => Ternary: condition ? true_branch : false_branch
// => Access instance property
// => Access instance property
// => Initialize hitRate
return { created: this.created, reused: this.reused, hitRate }; // => Returns value to caller
// => Access instance property
// => Access instance property
// => Return computed result
}
}
// Usage: Object pooling reduces allocations
const pool = new FSMPool(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize pool
// Simulate 10 messages (with pooling)
for (let i = 0; i < 10; i++) {
// => Variable declaration and assignment
// => Modify state data
// => Modify state data
// => Update extended state data
const fsm = pool.acquire(`MSG-${i}`); // => Get FSM from pool
// => FSM state management logic
// => First iteration: pool miss (create new)
// => Subsequent: pool hit (reuse existing)
fsm.transition("send"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
fsm.transition("deliver"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
fsm.transition("read"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
pool.release(fsm); // => Return to pool when done
}
console.log(pool.getStats()); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
// => Output: { created: 1, reused: 9, hitRate: 0.9 }
// => 90% hit rate: only 1 allocation instead of 10
Key Takeaway: FSM object pooling reuses instances instead of creating new ones, reducing GC pressure. Pool hit rate determines memory allocation reduction (90% hit rate = 90% fewer allocations).
FSM in Distributed Systems (Examples 70-74)
Example 70: Distributed FSM with Event Sourcing
Distributed systems need FSM state to be reconstructed from event logs for debugging, auditing, and recovery.
Why It Matters: At Kafka, topic partition FSMs use event sourcing to reconstruct state from 500M+ events/day. When a broker fails, the FSM rebuilds state by replaying events from the log (50K events/sec replay speed), achieving recovery in 20-40 seconds instead of 5-10 minutes with snapshot-based recovery. Event logs enable time-travel debugging: engineers replay production events locally to reproduce bugs.
// Distributed FSM with event sourcing (event log replay)
type OrderEvent = // => Type declaration defines structure
// => Defines event alphabet for FSM
// => Events trigger state transitions
| { type: "OrderPlaced"; orderId: string; amount: number } // => Statement execution
| { type: "PaymentReceived"; orderId: string } // => Statement execution
| { type: "OrderShipped"; orderId: string } // => Statement execution
| { type: "OrderDelivered"; orderId: string }; // => Statement execution
type OrderState = "Pending" | "Paid" | "Shipped" | "Delivered"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
class EventSourcedOrderFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: OrderState = "Pending"; // => Current state (derived from events)
// => FSM begins execution in Pending state
private events: OrderEvent[] = []; // => Event log (source of truth)
// => Initialized alongside FSM state
private orderId: string; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
constructor(orderId: string) {
// => Method invocation
// => Begin object/config definition
this.orderId = orderId; // => Statement execution
// => Access instance property
// => Access instance property
}
applyEvent(event: OrderEvent): void {
// => Method invocation
// => Begin object/config definition
// Append event to log (immutable event store)
this.events.push(event); // => Event log is source of truth
// Update state based on event type
switch (
event.type // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "OrderPlaced": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Pending"; // => OrderPlaced → Pending
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "PaymentReceived": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Paid"; // => PaymentReceived → Paid
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "OrderShipped": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Shipped"; // => OrderShipped → Shipped
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "OrderDelivered": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Delivered"; // => OrderDelivered → Delivered
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
console.log(`Event applied: ${event.type} → State: ${this.state}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
rebuildFromEvents(events: OrderEvent[]): void {
// => Method invocation
// => Begin object/config definition
// Reconstruct FSM state by replaying events
this.state = "Pending"; // => Reset to initial state
this.events = []; // => Clear event log
for (const event of events) {
// => Method invocation
// => Iterate collection
// => Iterate collection
// => Begin object/config definition
this.applyEvent(event); // => Replay each event
}
console.log(`Rebuilt state: ${this.state} from ${events.length} events`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
getState(): OrderState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Current state (derived from events)
}
getEvents(): OrderEvent[] {
// => Accessor method definition
// => Begin object/config definition
return [...this.events]; // => Return event log copy
}
}
// Usage: Event sourcing for FSM state reconstruction
const order = new EventSourcedOrderFSM("ORD-123"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize order
order.applyEvent({ type: "OrderPlaced", orderId: "ORD-123", amount: 99.99 }); // => Method invocation
// => Event: OrderPlaced, State: Pending
order.applyEvent({ type: "PaymentReceived", orderId: "ORD-123" }); // => Method invocation
// => Event: PaymentReceived, State: Paid
order.applyEvent({ type: "OrderShipped", orderId: "ORD-123" }); // => Method invocation
// => Event: OrderShipped, State: Shipped
// Simulate system crash and recovery
const eventLog = order.getEvents(); // => Get event log
console.log(`Event log: ${JSON.stringify(eventLog)}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
const recoveredOrder = new EventSourcedOrderFSM("ORD-123"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize recoveredOrder
recoveredOrder.rebuildFromEvents(eventLog); // => Method invocation
// => Rebuild state by replaying events
// => State reconstructed: Shipped
console.log(recoveredOrder.getState()); // => Output: Shipped (recovered state)
// => FSM state management logic
Key Takeaway: Event sourcing stores FSM events (not state) as source of truth. State is derived by replaying events, enabling recovery, debugging, and audit trails.
Example 71: FSM Coordination with Distributed Consensus
Distributed FSMs need consensus (Raft, Paxos) to ensure all nodes agree on state transitions before committing.
Why It Matters: At etcd (Kubernetes’ backing store), FSMs use Raft consensus to coordinate state changes across 3-5 nodes. Every state transition requires majority (2/3 or 3/5) agreement before committing. This ensures consistency during network partitions: if cluster splits 2-3, the 3-node partition continues operating (has majority) while 2-node partition rejects writes. Consensus adds 5-15ms latency but guarantees linearizable state transitions critical for Kubernetes scheduler decisions.
// Simplified distributed FSM with consensus (majority voting)
type ClusterState = "Follower" | "Candidate" | "Leader"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type ConsensusEvent = "timeout" | "vote_request" | "elected" | "heartbeat"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
interface Node {
// => Type declaration defines structure
// => Begin object/config definition
id: string; // => Statement execution
state: ClusterState; // => Statement execution
}
class DistributedConsensusFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: ClusterState = "Follower"; // => Initial: all nodes start as followers
// => FSM begins execution in Follower state
private nodeId: string; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
private clusterSize: number; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
private votesReceived: Set<string> = new Set(); // => Track votes for leader election
// => Initialized alongside FSM state
constructor(nodeId: string, clusterSize: number) {
// => Method invocation
// => Begin object/config definition
this.nodeId = nodeId; // => Statement execution
// => Access instance property
// => Access instance property
this.clusterSize = clusterSize; // => Statement execution
// => Access instance property
// => Access instance property
}
transition(event: ConsensusEvent, voterId?: string): void {
// => Method invocation
// => Ternary: condition ? true_branch : false_branch
// => Executes validated state transition
// => Updates this.state if valid
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Follower": // => Case: handles specific value
// => Match case
// => Match case
if (event === "timeout") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
// No heartbeat from leader → start election
this.state = "Candidate"; // => Follower → Candidate
this.votesReceived.clear(); // => Method invocation
this.votesReceived.add(this.nodeId); // => Vote for self
console.log(`${this.nodeId}: Follower → Candidate (election timeout)`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
} else if (event === "heartbeat") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
console.log(`${this.nodeId}: Received leader heartbeat (stay Follower)`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Candidate": // => Case: handles specific value
// => Match case
// => Match case
if (event === "vote_request" && voterId) {
// => Event type guard condition
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Event type check
this.votesReceived.add(voterId); // => Collect vote
console.log(`${this.nodeId}: Received vote from ${voterId} (${this.votesReceived.size}/${this.clusterSize})`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
// Check if majority reached
const majorityThreshold = Math.floor(this.clusterSize / 2) + 1; // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize majorityThreshold
if (this.votesReceived.size >= majorityThreshold) {
// => Conditional branch
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
this.transition("elected"); // => Majority → become leader
// => Updates this.state if valid
}
} else if (event === "timeout") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
console.log(`${this.nodeId}: Election timeout, retrying...`); // => Output for verification
// => Debug/audit output
// => Log for observability
this.votesReceived.clear(); // => Method invocation
this.votesReceived.add(this.nodeId); // => Re-vote for self
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Leader": // => Case: handles specific value
// => Match case
// => Match case
if (event === "elected") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Leader"; // => Candidate → Leader
console.log(`${this.nodeId}: Elected as Leader (consensus reached)`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
} else if (event === "heartbeat") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
console.log(`${this.nodeId}: Sending heartbeat to followers`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
}
getState(): ClusterState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: Distributed consensus (3-node cluster)
const node1 = new DistributedConsensusFSM("node-1", 3); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize node1
const node2 = new DistributedConsensusFSM("node-2", 3); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize node2
const node3 = new DistributedConsensusFSM("node-3", 3); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize node3
// All nodes start as Follower
console.log(`node-1: ${node1.getState()}`); // => Output: Follower
// => FSM state management logic
console.log(`node-2: ${node2.getState()}`); // => Output: Follower
// => FSM state management logic
// Node 1 times out → starts election
node1.transition("timeout"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => node-1: Follower → Candidate
// Nodes 2 and 3 vote for node-1
node1.transition("vote_request", "node-2"); // => 2/3 votes
// => Updates this.state if valid
node1.transition("vote_request", "node-3"); // => 3/3 votes (majority!)
// => FSM state management logic
// => Updates this.state if valid
// => node-1: Candidate → Leader
console.log(`node-1: ${node1.getState()}`); // => Output: Leader
// => FSM state management logic
Key Takeaway: Distributed FSMs use consensus (majority voting) to coordinate state transitions across nodes. Majority agreement ensures consistency during network partitions.
Example 72: FSM with Distributed Locking
Distributed FSMs need distributed locks (Redis, ZooKeeper) to prevent concurrent state modifications from multiple processes.
Why It Matters: At DoorDash, delivery FSMs coordinate across 50K+ active dashers with distributed locks preventing race conditions. When two dashers simultaneously try claiming the same delivery, the FSM uses Redis locks to ensure only one succeeds (lock acquisition timeout: 50ms). Without distributed locking, 3-5% of deliveries would experience double-assignment, costing $8M+ annually in redundant trips and customer refunds.
// FSM with distributed locking (simulated Redis lock)
type DeliveryState = "Available" | "Claimed" | "InProgress" | "Completed"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type DeliveryEvent = "claim" | "start" | "complete"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
class DistributedLockManager {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private locks: Map<string, string> = new Map(); // => deliveryId → lockHolder
// => Initialized alongside FSM state
async acquire(resource: string, holder: string, timeoutMs: number): Promise<boolean> {
// => Method invocation
// => Begin object/config definition
// Simulate distributed lock acquisition (Redis SETNX)
return new Promise((resolve) => {
// => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
if (!this.locks.has(resource)) {
// => Conditional branch
// => Chained method calls or nested operations
// => Conditional check
// => Branch execution based on condition
this.locks.set(resource, holder); // => Lock acquired
console.log(`${holder} acquired lock on ${resource}`); // => Output for verification
// => Debug/audit output
// => Log for observability
resolve(true); // => Method invocation
} else {
// => Statement execution
// => Fallback branch
console.log(`${holder} failed to acquire lock (held by ${this.locks.get(resource)})`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
resolve(false); // => Lock held by another process
}
}, timeoutMs); // => Statement execution
}); // => Statement execution
}
release(resource: string, holder: string): void {
// => Method invocation
// => Begin object/config definition
if (this.locks.get(resource) === holder) {
// => Conditional branch
// => Chained method calls or nested operations
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
this.locks.delete(resource); // => Release lock
console.log(`${holder} released lock on ${resource}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
}
}
class DeliveryFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: DeliveryState = "Available"; // => Initial state
// => FSM begins execution in Available state
private deliveryId: string; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
private lockManager: DistributedLockManager; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
private assignedTo?: string; // => Dasher ID
// => FSM state management logic
// => Initialized alongside FSM state
constructor(deliveryId: string, lockManager: DistributedLockManager) {
// => Method invocation
// => Begin object/config definition
this.deliveryId = deliveryId; // => Statement execution
// => Access instance property
// => Access instance property
this.lockManager = lockManager; // => Statement execution
// => Access instance property
// => Access instance property
}
async transition(event: DeliveryEvent, dasherId: string): Promise<boolean> {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// Acquire distributed lock before state change
const lockAcquired = await this.lockManager.acquire(this.deliveryId, dasherId, 50); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize lockAcquired
// => Try to acquire lock (50ms timeout)
if (!lockAcquired) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
console.log(`${dasherId} cannot transition ${this.deliveryId}: lock not acquired`); // => Output for verification
// => Debug/audit output
// => Log for observability
return false; // => Concurrent modification prevented
}
// Critical section: state transition (protected by lock)
try {
// => Statement execution
// => Begin error handling
// => Begin error handling
// => Begin object/config definition
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Available": // => Case: handles specific value
// => Match case
// => Match case
if (event === "claim") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Claimed"; // => Available → Claimed
this.assignedTo = dasherId; // => Statement execution
// => Access instance property
// => Access instance property
console.log(`${dasherId} claimed delivery ${this.deliveryId}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Claimed": // => Case: handles specific value
// => Match case
// => Match case
if (event === "start" && this.assignedTo === dasherId) {
// => Event type guard condition
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Event type check
this.state = "InProgress"; // => Claimed → InProgress
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "InProgress": // => Case: handles specific value
// => Match case
// => Match case
if (event === "complete" && this.assignedTo === dasherId) {
// => Event type guard condition
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Completed"; // => InProgress → Completed
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
return true; // => Transition successful
} finally {
// => Statement execution
// => Begin object/config definition
// Always release lock after transition
this.lockManager.release(this.deliveryId, dasherId); // => Method invocation
}
}
getState(): DeliveryState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: Distributed locking prevents race conditions
const lockManager = new DistributedLockManager(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize lockManager
const delivery = new DeliveryFSM("DEL-123", lockManager); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize delivery
// Simulate two dashers claiming same delivery concurrently
async function simulateConcurrentClaim() {
// => Method invocation
// => Begin object/config definition
const claim1 = delivery.transition("claim", "dasher-A"); // => First claim attempt
// => Updates this.state if valid
const claim2 = delivery.transition("claim", "dasher-B"); // => Concurrent claim attempt
// => Updates this.state if valid
const results = await Promise.all([claim1, claim2]); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize results
console.log(`Claim results: ${results}`); // => Output for verification
// => Debug/audit output
// => Log for observability
// => Output: [true, false] (only one succeeds)
console.log(`Final state: ${delivery.getState()}`); // => Claimed
// => FSM state management logic
// => Log for observability
}
simulateConcurrentClaim(); // => Method invocation
Key Takeaway: Distributed FSMs use distributed locks (Redis, ZooKeeper) to serialize concurrent state transitions. Lock acquisition prevents race conditions in distributed environments.
Example 73: FSM State Replication (Active-Active)
Active-active distributed FSMs replicate state across multiple regions for low-latency access, requiring conflict resolution.
Why It Matters: Database table FSMs use active-active replication across multiple regions with eventual consistency. A table update in one region replicates to other regions with low latency. Concurrent updates to the same table in different regions use last-write-wins conflict resolution based on timestamps. Active-active enables high availability: if one region fails, another continues serving requests with minimal latency increase.
// FSM with active-active state replication
type TableState = "Active" | "Creating" | "Updating" | "Deleting"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type TableEvent = "create" | "update" | "delete"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
interface ReplicatedState {
// => Type declaration defines structure
// => Begin object/config definition
state: TableState; // => Statement execution
timestamp: number; // => Lamport timestamp for conflict resolution
region: string; // => Source region
}
class ReplicatedTableFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: TableState = "Creating"; // => Initial state
// => FSM begins execution in Creating state
private timestamp: number = 0; // => Lamport clock
// => Initialized alongside FSM state
private region: string; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
private replicas: Map<string, ReplicatedState> = new Map(); // => region → state
// => Initialized alongside FSM state
constructor(region: string) {
// => Method invocation
// => Begin object/config definition
this.region = region; // => Statement execution
// => Access instance property
// => Access instance property
this.replicas.set(region, { state: this.state, timestamp: this.timestamp, region }); // => Method invocation
}
transition(event: TableEvent): void {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
this.timestamp++; // => Increment Lamport clock on each event
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Creating": // => Case: handles specific value
// => Match case
// => Match case
if (event === "create") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Active"; // => Creating → Active
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Active": // => Case: handles specific value
// => Match case
// => Match case
if (event === "update") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Updating"; // => Active → Updating
} else if (event === "delete") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Deleting"; // => Active → Deleting
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Updating": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Active"; // => Updating → Active (update complete)
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Deleting": // => Case: handles specific value
// => Match case
// => Match case
console.log("Table deleted"); // => Output for verification
// => Debug/audit output
// => Log for observability
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
// Replicate state to all regions
this.replicas.set(this.region, {
// => Statement execution
// => Begin object/config definition
state: this.state, // => Statement execution
// => Access instance property
// => Access instance property
timestamp: this.timestamp, // => Statement execution
// => Access instance property
// => Access instance property
region: this.region, // => Statement execution
// => Access instance property
// => Access instance property
}); // => Statement execution
console.log(`[${this.region}] Local state: ${this.state} (t=${this.timestamp})`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
receiveReplicatedState(remoteState: ReplicatedState): void {
// => Method invocation
// => Begin object/config definition
console.log(
// => Statement execution
// => Debug/audit output
// => Log for observability
`[${this.region}] Received: ${remoteState.state} from ${remoteState.region} (t=${remoteState.timestamp})`, // => Method invocation
); // => Statement execution
// Conflict resolution: last-write-wins (higher timestamp)
const localReplica = this.replicas.get(this.region)!; // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize localReplica
if (remoteState.timestamp > localReplica.timestamp) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
// => Remote state is newer: adopt it
this.state = remoteState.state; // => State transition execution
// => Access instance property
// => Access instance property
this.timestamp = remoteState.timestamp; // => Statement execution
// => Access instance property
// => Access instance property
console.log(`[${this.region}] Adopted remote state: ${this.state} (newer timestamp)`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
} else if (remoteState.timestamp === localReplica.timestamp) {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
// => Concurrent updates: tie-break by region name (deterministic)
if (remoteState.region > localReplica.region) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
this.state = remoteState.state; // => State transition execution
// => Access instance property
// => Access instance property
console.log(`[${this.region}] Tie-break: adopted ${remoteState.region}'s state`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
} else {
// => Statement execution
// => Fallback branch
console.log(`[${this.region}] Ignored remote state (older timestamp)`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
// Store remote replica
this.replicas.set(remoteState.region, remoteState); // => Method invocation
}
getState(): TableState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
getReplicatedState(): ReplicatedState {
// => Accessor method definition
// => Begin object/config definition
return {
// => Returns value to caller
// => Return computed result
state: this.state, // => Statement execution
// => Access instance property
// => Access instance property
timestamp: this.timestamp, // => Statement execution
// => Access instance property
// => Access instance property
region: this.region, // => Statement execution
// => Access instance property
// => Access instance property
};
}
}
// Usage: Active-active replication across regions
const usEast = new ReplicatedTableFSM("us-east-1"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize usEast
const euWest = new ReplicatedTableFSM("eu-west-1"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize euWest
usEast.transition("create"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => [us-east-1] Creating → Active (t=1)
// Replicate to EU region
euWest.receiveReplicatedState(usEast.getReplicatedState()); // => Method invocation
// => Chained method calls or nested operations
// => [eu-west-1] Received: Active from us-east-1 (t=1)
// => [eu-west-1] Adopted remote state: Active
// Concurrent updates in both regions
usEast.transition("update"); // => us-east-1: Active → Updating (t=2)
// => FSM state management logic
// => Updates this.state if valid
euWest.transition("update"); // => eu-west-1: Active → Updating (t=2)
// => FSM state management logic
// => Updates this.state if valid
// Cross-region replication
usEast.receiveReplicatedState(euWest.getReplicatedState()); // => Method invocation
// => Chained method calls or nested operations
// => Conflict: both t=2 → tie-break by region name
euWest.receiveReplicatedState(usEast.getReplicatedState()); // => Method invocation
// => Chained method calls or nested operations
Key Takeaway: Active-active FSM replication uses Lamport timestamps for conflict resolution (last-write-wins). Replicas converge to consistent state through eventual consistency.
Example 74: FSM with CRDT State Merging
Conflict-free Replicated Data Types (CRDTs) enable FSM state merging without coordination, ideal for offline-first apps.
Why It Matters: At Figma, collaborative editing FSMs use CRDT-based state merging to handle concurrent edits from 10+ designers. Each designer’s FSM tracks local changes offline, then merges with remote states when reconnected. CRDTs guarantee eventual consistency without central coordination: all replicas converge to the same state regardless of message order or network delays. This enables Figma’s real-time collaboration with <50ms latency and offline editing.
// FSM with CRDT state merging (G-Counter CRDT)
type EditState = "Idle" | "Editing" | "Syncing" | "Synced"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
class GCounter {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
// Grow-only counter CRDT (each replica maintains count per node)
private counts: Map<string, number> = new Map(); // => nodeId → count
// => Initialized alongside FSM state
increment(nodeId: string, delta: number = 1): void {
// => Method invocation
// => Assign value
// => Assign value
// => Begin object/config definition
const current = this.counts.get(nodeId) || 0; // => Variable declaration and assignment
// => Logical OR: either condition can be true
// => Assign value
// => Assign value
// => Initialize current
this.counts.set(nodeId, current + delta); // => Increment local count
}
merge(other: GCounter): void {
// => Method invocation
// => Begin object/config definition
// Merge two CRDTs by taking max count per node
for (const [nodeId, count] of other.counts) {
// => Method invocation
// => Iterate collection
// => Iterate collection
// => Begin object/config definition
const localCount = this.counts.get(nodeId) || 0; // => Variable declaration and assignment
// => Logical OR: either condition can be true
// => Assign value
// => Assign value
// => Initialize localCount
this.counts.set(nodeId, Math.max(localCount, count)); // => Method invocation
// => Chained method calls or nested operations
// => Conflict resolution: max wins (grow-only)
}
}
value(): number {
// => Method invocation
// => Begin object/config definition
// Total count = sum of all node counts
return Array.from(this.counts.values()).reduce((sum, count) => sum + count, 0); // => Returns value to caller
// => Chained method calls or nested operations
// => Return computed result
}
getCounts(): Map<string, number> {
// => Accessor method definition
// => Begin object/config definition
return new Map(this.counts); // => Return counts for replication
// => FSM state management logic
}
}
class CRDTEditFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: EditState = "Idle"; // => Initial state
// => FSM begins execution in Idle state
private editCount: GCounter = new GCounter(); // => CRDT for edit count
// => Initialized alongside FSM state
private nodeId: string; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
constructor(nodeId: string) {
// => Method invocation
// => Begin object/config definition
this.nodeId = nodeId; // => Statement execution
// => Access instance property
// => Access instance property
}
edit(): void {
// => Method invocation
// => Begin object/config definition
this.state = "Editing"; // => Idle → Editing
this.editCount.increment(this.nodeId); // => Increment local edit count
console.log(`[${this.nodeId}] Edit count: ${this.editCount.value()}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
sync(remoteFSM: CRDTEditFSM): void {
// => Method invocation
// => Begin object/config definition
this.state = "Syncing"; // => Editing → Syncing
// Merge CRDT states (no conflicts!)
this.editCount.merge(remoteFSM.editCount); // => Method invocation
// => CRDTs converge without coordination
this.state = "Synced"; // => Syncing → Synced
console.log(`[${this.nodeId}] Synced. Total edits: ${this.editCount.value()}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
getState(): EditState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
getEditCount(): number {
// => Accessor method definition
// => Begin object/config definition
return this.editCount.value(); // => Return merged edit count
}
}
// Usage: CRDT-based state merging
const designer1 = new CRDTEditFSM("designer-1"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize designer1
const designer2 = new CRDTEditFSM("designer-2"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize designer2
// Concurrent edits while offline
designer1.edit(); // => designer-1: edit count = 1
designer1.edit(); // => designer-1: edit count = 2
designer2.edit(); // => designer-2: edit count = 1
// Both designers sync (merge CRDTs)
designer1.sync(designer2); // => Method invocation
// => designer-1 merges designer-2's state
// => Total edits: max(2, 0) + max(0, 1) = 2 + 1 = 3
designer2.sync(designer1); // => Method invocation
// => designer-2 merges designer-1's state
// => Total edits: 3 (same result, convergence!)
console.log(`Designer 1 total: ${designer1.getEditCount()}`); // => 3
// => FSM state management logic
// => Log for observability
console.log(`Designer 2 total: ${designer2.getEditCount()}`); // => 3
// => FSM state management logic
// => Log for observability
Key Takeaway: CRDT-based FSMs merge concurrent states without coordination. CRDTs guarantee eventual consistency through commutative, associative merge operations.
Saga Patterns (Examples 75-78)
Example 75: Saga Pattern - Choreography-Based
Choreography sagas coordinate distributed transactions through event-driven FSMs without central orchestrator.
Why It Matters: Order fulfillment sagas use choreography to coordinate multiple services (Order, Payment, Restaurant, Delivery) processing high volumes. Each service’s FSM listens for events and publishes responses: Order→OrderPlaced, Payment→PaymentProcessed, Restaurant→FoodPrepared, Delivery→DriverAssigned. When Payment fails, it publishes PaymentFailed event triggering compensating transactions in all downstream services. Choreography enables higher throughput than orchestration (no central bottleneck) but requires careful event ordering.
// Saga choreography: Event-driven coordination without orchestrator
type OrderSagaEvent = "OrderPlaced" | "PaymentProcessed" | "PaymentFailed" | "FoodPrepared" | "DriverAssigned"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
class OrderServiceFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: string = "Idle"; // => Order service state
// => FSM begins execution in Idle state
handleEvent(event: OrderSagaEvent): OrderSagaEvent | null {
// => Event handler method
// => Event handler: main FSM dispatch method
// => Processes events, triggers transitions
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Idle": // => Case: handles specific value
// => Match case
// => Match case
if (event === "OrderPlaced") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Pending"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Pending
// => State mutation (core FSM operation)
console.log("[Order] Order placed → Payment"); // => Output for verification
// => Debug/audit output
// => Log for observability
return "OrderPlaced"; // => Publish event for Payment service
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Pending": // => Case: handles specific value
// => Match case
// => Match case
if (event === "PaymentFailed") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Cancelled"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Cancelled
// => State mutation (core FSM operation)
console.log("[Order] Payment failed → Cancel order"); // => Output for verification
// => Debug/audit output
// => Log for observability
return null; // => Compensating transaction: cancel order
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
return null; // => Returns value to caller
// => Return computed result
}
}
class PaymentServiceFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: string = "Idle"; // => Payment service state
// => FSM begins execution in Idle state
handleEvent(event: OrderSagaEvent): OrderSagaEvent | null {
// => Event handler method
// => Event handler: main FSM dispatch method
// => Processes events, triggers transitions
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Idle": // => Case: handles specific value
// => Match case
// => Match case
if (event === "OrderPlaced") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Processing"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Processing
// => State mutation (core FSM operation)
console.log("[Payment] Processing payment..."); // => Output for verification
// => Debug/audit output
// => Log for observability
// Simulate payment success/failure
const success = Math.random() > 0.2; // => 80% success rate
if (success) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
this.state = "Completed"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Completed
// => State mutation (core FSM operation)
console.log("[Payment] Payment successful → Restaurant"); // => Output for verification
// => Debug/audit output
// => Log for observability
return "PaymentProcessed"; // => Publish success event
} else {
// => Statement execution
// => Fallback branch
this.state = "Failed"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Failed
// => State mutation (core FSM operation)
console.log("[Payment] Payment failed → Rollback"); // => Output for verification
// => Debug/audit output
// => Log for observability
return "PaymentFailed"; // => Publish failure event (triggers compensation)
}
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
return null; // => Returns value to caller
// => Return computed result
}
}
class RestaurantServiceFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: string = "Idle"; // => Restaurant service state
// => FSM begins execution in Idle state
handleEvent(event: OrderSagaEvent): OrderSagaEvent | null {
// => Event handler method
// => Event handler: main FSM dispatch method
// => Processes events, triggers transitions
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Idle": // => Case: handles specific value
// => Match case
// => Match case
if (event === "PaymentProcessed") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Preparing"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Preparing
// => State mutation (core FSM operation)
console.log("[Restaurant] Preparing food..."); // => Output for verification
// => Debug/audit output
// => Log for observability
this.state = "Ready"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Ready
// => State mutation (core FSM operation)
return "FoodPrepared"; // => Publish event for Delivery service
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
return null; // => Returns value to caller
// => Return computed result
}
}
// Choreography: Event-driven saga (no central orchestrator)
const orderService = new OrderServiceFSM(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize orderService
const paymentService = new PaymentServiceFSM(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize paymentService
const restaurantService = new RestaurantServiceFSM(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize restaurantService
// Event flow: Order → Payment → Restaurant (event-driven chain)
let event: OrderSagaEvent | null = "OrderPlaced"; // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize event
event = orderService.handleEvent(event); // => Event handler method
// => Assign value
// => Assign value
// => Event handler: main FSM dispatch method
// => Processes events, triggers transitions
// => [Order] Order placed → Payment
// => Publishes: "OrderPlaced"
if (event) {
// => Event type guard condition
// => Conditional check
// => Branch execution based on condition
event = paymentService.handleEvent(event); // => Event handler method
// => Assign value
// => Assign value
// => Event handler: main FSM dispatch method
// => Processes events, triggers transitions
// => [Payment] Processing payment...
// => Success: Publishes "PaymentProcessed"
// => Failure: Publishes "PaymentFailed" → triggers Order compensation
}
if (event === "PaymentProcessed") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
event = restaurantService.handleEvent(event); // => Event handler method
// => Assign value
// => Assign value
// => Event handler: main FSM dispatch method
// => Processes events, triggers transitions
// => [Restaurant] Preparing food...
// => Publishes: "FoodPrepared"
} else if (event === "PaymentFailed") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
orderService.handleEvent(event); // => Event handler method
// => Event handler: main FSM dispatch method
// => Processes events, triggers transitions
// => [Order] Payment failed → Cancel order (compensation)
}Key Takeaway: Choreography sagas use event-driven FSMs where each service reacts to events and publishes new events. No central orchestrator, but requires careful event ordering and compensation logic.
Example 76: Saga Pattern - Orchestration-Based
Orchestration sagas use central orchestrator FSM to coordinate distributed transaction steps and compensations.
Why It Matters: Content publishing sagas use orchestration to coordinate multiple services (Encode, QA, Metadata, CDN, Search, Recommendations, Analytics) for each new title release. The orchestrator FSM explicitly calls each service in order, tracks progress, and executes compensations on failures. With high release volumes, orchestration provides clear visibility (single FSM shows full saga state) and simplified error handling compared to choreography’s distributed event chains.
// Saga orchestration: Central FSM coordinates all services
type PublishStep = "Encoding" | "QA" | "Metadata" | "CDN" | "Completed" | "Compensating"; // => Type declaration defines structure
// => Assign value
// => Assign value
class PublishOrchestrator {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: PublishStep = "Encoding"; // => Current saga step
// => FSM begins execution in Encoding state
private completedSteps: PublishStep[] = []; // => Track completed for compensation
// => Initialized alongside FSM state
async executeStep(step: PublishStep): Promise<boolean> {
// => Method invocation
// => Begin object/config definition
console.log(`[Orchestrator] Executing step: ${step}`); // => Output for verification
// => Debug/audit output
// => Log for observability
// Simulate service call
const success = await this.callService(step); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize success
if (success) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
this.completedSteps.push(step); // => Track for compensation
return true; // => Returns value to caller
// => Return computed result
} else {
// => Statement execution
// => Fallback branch
console.log(`[Orchestrator] Step ${step} failed → Compensating`); // => Output for verification
// => Debug/audit output
// => Log for observability
await this.compensate(); // => Execute compensation for all completed steps
return false; // => Returns value to caller
// => Return computed result
}
}
private async callService(step: PublishStep): Promise<boolean> {
// => Method invocation
// => Extended state (data beyond FSM state)
// Simulate async service call (90% success rate)
return new Promise((resolve) => {
// => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
const success = Math.random() > 0.1; // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize success
console.log(` [${step}] ${success ? "Success" : "Failed"}`); // => Output for verification
// => Ternary: condition ? true_branch : false_branch
// => Debug/audit output
// => Log for observability
resolve(success); // => Method invocation
}, 100); // => Statement execution
}); // => Statement execution
}
private async compensate(): void {
// => Method invocation
// => Extended state (data beyond FSM state)
// Execute compensating transactions in reverse order (LIFO)
this.state = "Compensating"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Compensating
// => State mutation (core FSM operation)
while (this.completedSteps.length > 0) {
// => Method invocation
// => Loop while condition true
// => Loop while condition true
// => Begin object/config definition
const step = this.completedSteps.pop()!; // => Variable declaration and assignment
// => Remove from collection
// => Remove from collection
// => Initialize step
console.log(` [Orchestrator] Compensating: ${step}`); // => Output for verification
// => Debug/audit output
// => Log for observability
await this.undoStep(step); // => Undo completed step
}
console.log("[Orchestrator] Compensation complete"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
private async undoStep(step: PublishStep): Promise<void> {
// => Method invocation
// => Extended state (data beyond FSM state)
// Simulate compensation (delete encoded files, remove metadata, etc.)
return new Promise((resolve) => {
// => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
console.log(` [${step}] Undone`); // => Output for verification
// => Debug/audit output
// => Log for observability
resolve(); // => Method invocation
}, 50); // => Statement execution
}); // => Statement execution
}
async runSaga(): Promise<void> {
// => Method invocation
// => Begin object/config definition
const steps: PublishStep[] = ["Encoding", "QA", "Metadata", "CDN"]; // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize steps
for (const step of steps) {
// => Method invocation
// => Iterate collection
// => Iterate collection
// => Begin object/config definition
const success = await this.executeStep(step); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize success
if (!success) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
console.log("[Orchestrator] Saga failed"); // => Output for verification
// => Debug/audit output
// => Log for observability
return; // => Saga aborted after compensation
}
}
this.state = "Completed"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Completed
// => State mutation (core FSM operation)
console.log("[Orchestrator] Saga completed successfully"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
getState(): PublishStep {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current saga state
}
}
// Usage: Orchestrator executes saga
const orchestrator = new PublishOrchestrator(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize orchestrator
orchestrator.runSaga(); // => Method invocation
// => Executes: Encoding → QA → Metadata → CDN
// => If any step fails: compensates all completed steps in reverse order
Key Takeaway: Orchestration sagas use central FSM to explicitly coordinate service calls and compensations. Provides clear visibility and simpler error handling than choreography.
Example 77: Saga Timeout and Retry Policies
Sagas need timeout and retry policies to handle slow services (network delays, overloaded services) without blocking indefinitely.
Why It Matters: At Shopify, checkout sagas implement 3-tier retry policy: payment service (3 retries, 5s timeout), inventory service (5 retries, 2s timeout), shipping service (2 retries, 10s timeout). With 10K checkouts/minute during flash sales, these policies prevent cascade failures: when payment service slows to 8s response time (above 5s timeout), saga retries 3 times then fails fast (total 15s) instead of blocking for 60s+ and exhausting thread pools. Retry policies increased successful checkouts from 87% to 96% during traffic spikes.
// Saga with timeout and retry policies
interface RetryPolicy {
// => Type declaration defines structure
// => Begin object/config definition
maxRetries: number; // => Max retry attempts
timeoutMs: number; // => Timeout per attempt
backoffMs: number; // => Backoff between retries
}
class RetryableSagaStep {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private stepName: string; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
private policy: RetryPolicy; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
constructor(stepName: string, policy: RetryPolicy) {
// => Method invocation
// => Begin object/config definition
this.stepName = stepName; // => Statement execution
// => Access instance property
// => Access instance property
this.policy = policy; // => Statement execution
// => Access instance property
// => Access instance property
}
async execute(): Promise<boolean> {
// => Method invocation
// => Begin object/config definition
for (let attempt = 1; attempt <= this.policy.maxRetries; attempt++) {
// => Variable declaration and assignment
// => Modify state data
// => Modify state data
// => Update extended state data
console.log(`[${this.stepName}] Attempt ${attempt}/${this.policy.maxRetries}`); // => Output for verification
// => Debug/audit output
// => Log for observability
try {
// => Statement execution
// => Begin error handling
// => Begin error handling
// => Begin object/config definition
const success = await this.executeWithTimeout(); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize success
if (success) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
console.log(`[${this.stepName}] Success on attempt ${attempt}`); // => Output for verification
// => Debug/audit output
// => Log for observability
return true; // => Returns value to caller
// => Return computed result
}
} catch (error) {
// => Method signature: defines function interface
// => Catch errors
// => Catch errors
// => Begin object/config definition
console.log(`[${this.stepName}] Timeout on attempt ${attempt}`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
// Exponential backoff before retry
if (attempt < this.policy.maxRetries) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
const backoff = this.policy.backoffMs * Math.pow(2, attempt - 1); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize backoff
console.log(`[${this.stepName}] Backing off ${backoff}ms`); // => Output for verification
// => Debug/audit output
// => Log for observability
await new Promise((resolve) => setTimeout(resolve, backoff)); // => Method invocation
// => Constructor creates new object instance
}
}
console.log(`[${this.stepName}] Failed after ${this.policy.maxRetries} attempts`); // => Output for verification
// => Debug/audit output
// => Log for observability
return false; // => All retries exhausted
}
private executeWithTimeout(): Promise<boolean> {
// => Method invocation
// => Extended state (data beyond FSM state)
return new Promise((resolve, reject) => {
// => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
// Set timeout
const timeout = setTimeout(() => {
// => Variable declaration and assignment
// => Chained method calls or nested operations
// => Assign value
// => Assign value
// => Initialize timeout
reject(new Error("Timeout")); // => Method invocation
// => Constructor creates new object instance
}, this.policy.timeoutMs); // => Statement execution
// => Access instance property
// => Access instance property
// Simulate service call (variable latency)
const latency = Math.random() * this.policy.timeoutMs * 2; // => 0-2x timeout
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
clearTimeout(timeout); // => Method invocation
const success = latency < this.policy.timeoutMs; // => Success if under timeout
resolve(success); // => Method invocation
}, latency); // => Statement execution
}); // => Statement execution
}
}
// Usage: Saga with retry policies
async function runSagaWithRetries() {
// => Method invocation
// => Begin object/config definition
const paymentStep = new RetryableSagaStep("Payment", {
// => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize paymentStep
maxRetries: 3, // => Statement execution
timeoutMs: 5000, // => Statement execution
backoffMs: 1000, // => 1s, 2s, 4s backoff
}); // => Statement execution
const inventoryStep = new RetryableSagaStep("Inventory", {
// => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize inventoryStep
maxRetries: 5, // => Statement execution
timeoutMs: 2000, // => Statement execution
backoffMs: 500, // => 500ms, 1s, 2s, 4s, 8s backoff
}); // => Statement execution
const paymentSuccess = await paymentStep.execute(); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize paymentSuccess
if (!paymentSuccess) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
console.log("Saga failed: Payment step exhausted retries"); // => Output for verification
// => Debug/audit output
// => Log for observability
return; // => Statement execution
}
const inventorySuccess = await inventoryStep.execute(); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize inventorySuccess
if (!inventorySuccess) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
console.log("Saga failed: Inventory step exhausted retries"); // => Output for verification
// => Debug/audit output
// => Log for observability
return; // => Statement execution
}
console.log("Saga completed successfully"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
runSagaWithRetries(); // => Method invocation
Key Takeaway: Saga retry policies use exponential backoff and per-step timeouts to handle transient failures without blocking indefinitely. Max retries prevent infinite retry loops.
Example 78: Saga Idempotency Guarantees
Sagas need idempotent service calls to safely retry failed steps without duplicate side effects (double-charging, duplicate emails).
Why It Matters: At Stripe, payment sagas use idempotency keys to prevent duplicate charges when retrying failed requests. Each saga step generates unique idempotency key (order_id + step_name + attempt_number) sent with API calls. If retry reaches Stripe after original succeeded, Stripe returns cached response (no duplicate charge). With 5-8% of payment requests retried, idempotency prevents $50M+ monthly in duplicate charges and customer complaints.
// Saga with idempotency guarantees
class IdempotentService {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private processedRequests: Map<string, any> = new Map(); // => Method invocation
// => Create new instance
// => Create new instance
// => Extended state (data beyond FSM state)
// => Initialized alongside FSM state
// => Cache: idempotency_key → response
async processRequest(idempotencyKey: string, operation: () => Promise<any>): Promise<any> {
// => Method invocation
// => Chained method calls or nested operations
// => Begin object/config definition
// Check if request already processed
if (this.processedRequests.has(idempotencyKey)) {
// => Conditional branch
// => Chained method calls or nested operations
// => Conditional check
// => Branch execution based on condition
console.log(` [Service] Idempotency key ${idempotencyKey} seen before → return cached response`); // => Output for verification
// => Return computed result
return this.processedRequests.get(idempotencyKey); // => Returns value to caller
// => Return computed result
// => Return cached result (no duplicate side effect)
}
// First time seeing this request → execute operation
console.log(` [Service] New idempotency key ${idempotencyKey} → execute operation`); // => Output for verification
// => Debug/audit output
// => Log for observability
const result = await operation(); // => Execute actual operation
// Cache result for future retries
this.processedRequests.set(idempotencyKey, result); // => Method invocation
return result; // => Returns value to caller
// => Return computed result
}
}
class IdempotentSaga {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private orderId: string; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
private paymentService: IdempotentService = new IdempotentService(); // => Method invocation
// => Create new instance
// => Create new instance
// => Extended state (data beyond FSM state)
// => Initialized alongside FSM state
constructor(orderId: string) {
// => Method invocation
// => Begin object/config definition
this.orderId = orderId; // => Statement execution
// => Access instance property
// => Access instance property
}
async processPayment(attemptNumber: number): Promise<boolean> {
// => Method invocation
// => Begin object/config definition
// Generate idempotency key: order + step + attempt
const idempotencyKey = `${this.orderId}_payment_${attemptNumber}`; // => Variable declaration and assignment
// => Access instance property
// => Access instance property
// => Initialize idempotencyKey
console.log(`[Saga] Processing payment (attempt ${attemptNumber}, key: ${idempotencyKey})`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
return this.paymentService.processRequest(idempotencyKey, async () => {
// => Returns value to caller
// => Chained method calls or nested operations
// => Return computed result
// Simulate payment processing
console.log(` [Payment] Charging card...`); // => Output for verification
// => Debug/audit output
// => Log for observability
return new Promise((resolve) => {
// => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
console.log(` [Payment] Charge successful`); // => Output for verification
// => Debug/audit output
// => Log for observability
resolve(true); // => Method invocation
}, 100); // => Statement execution
}); // => Statement execution
}); // => Statement execution
}
}
// Usage: Idempotent saga prevents duplicate charges
async function runIdempotentSaga() {
// => Method invocation
// => Begin object/config definition
const saga = new IdempotentSaga("ORD-123"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize saga
// First attempt succeeds
await saga.processPayment(1); // => Method invocation
// => [Service] New idempotency key → execute operation
// => [Payment] Charging card... Charge successful
// Retry same attempt (network failure after success)
await saga.processPayment(1); // => Method invocation
// => [Service] Idempotency key seen before → return cached response
// => NO duplicate charge (idempotency guaranteed)
// New attempt with different key
await saga.processPayment(2); // => Method invocation
// => [Service] New idempotency key → execute operation
// => [Payment] Charging card... (new charge, different key)
}
runIdempotentSaga(); // => Method invocation
Key Takeaway: Saga idempotency uses unique keys (order_id + step + attempt) to cache responses and prevent duplicate side effects when retrying failed requests.
Production Deployment Patterns (Examples 79-82)
Example 79: Blue-Green Deployment FSM
Blue-green deployments use FSM to manage traffic cutover between old (blue) and new (green) versions with instant rollback.
Why It Matters: Service deployments use blue-green FSM to safely release microservice updates at scale. The FSM manages: Blue→Active (full traffic), Green→Standby (no traffic), validate Green health, cutover Green→Active (full traffic), Blue→Standby (rollback-ready). Validation phase runs many health checks (latency, error rate, memory) before cutover. Instant rollback capability (toggle Active back to Blue) significantly reduces failed deployment impact compared to rolling deployments.
// Blue-green deployment FSM with health validation and instant rollback
type DeploymentEnv = "Blue" | "Green"; // => Type declaration defines structure
// => Assign value
// => Assign value
type DeploymentState = "BlueActive" | "GreenActive" | "Validating" | "Cutover" | "Rollback"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type DeploymentEvent = "deploy" | "health_ok" | "health_fail" | "complete" | "rollback"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
class BlueGreenFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: DeploymentState = "BlueActive"; // => Initial: Blue serves traffic
// => FSM begins execution in BlueActive state
private activeEnv: DeploymentEnv = "Blue"; // => Current active environment
// => Initialized alongside FSM state
private healthChecks: number = 0; // => Passed health checks
// => Initialized alongside FSM state
transition(event: DeploymentEvent): void {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "BlueActive": // => Case: handles specific value
// => Match case
// => Match case
if (event === "deploy") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Validating"; // => BlueActive → Validating
this.healthChecks = 0; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Deploying to Green (standby)..."); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
console.log("Blue: 100% traffic, Green: 0% traffic"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Validating": // => Case: handles specific value
// => Match case
// => Match case
if (event === "health_ok") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.healthChecks++; // => Statement execution
// => Modify state data
// => Modify state data
// => Update extended state data
console.log(`Green health check ${this.healthChecks}/5 passed`); // => Output for verification
// => Debug/audit output
// => Log for observability
if (this.healthChecks >= 5) {
// => Conditional branch
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
this.transition("complete"); // => All checks passed → cutover
// => Updates this.state if valid
}
} else if (event === "health_fail") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
console.log("Green health check failed → rollback"); // => Output for verification
// => Debug/audit output
// => Log for observability
this.transition("rollback"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Cutover": // => Case: handles specific value
// => Match case
// => Match case
if (event === "complete") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "GreenActive"; // => Cutover complete
this.activeEnv = "Green"; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Cutover complete: Green → 100% traffic, Blue → standby"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "GreenActive": // => Case: handles specific value
// => Match case
// => Match case
if (event === "rollback") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Rollback"; // => Instant rollback to Blue
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Rollback": // => Case: handles specific value
// => Match case
// => Match case
this.state = "BlueActive"; // => Rollback complete
this.activeEnv = "Blue"; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Rollback complete: Blue → 100% traffic (Green discarded)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
if (event === "complete" && this.state === "Validating") {
// => Event type guard condition
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Event type check
// => Combined (state, event) guard
this.state = "Cutover"; // => Validating → Cutover
}
}
getActiveEnv(): DeploymentEnv {
// => Accessor method definition
// => Begin object/config definition
return this.activeEnv; // => Return current active environment
}
getState(): DeploymentState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return FSM state
}
}
// Usage: Blue-green deployment with validation
const deployment = new BlueGreenFSM(); // => state: BlueActive
deployment.transition("deploy"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Deploying to Green (Blue still serves 100% traffic)
// Simulate 5 health checks
for (let i = 0; i < 5; i++) {
// => Variable declaration and assignment
// => Modify state data
// => Modify state data
// => Update extended state data
deployment.transition("health_ok"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Green health check N/5 passed
}
// => After 5th check: Validating → Cutover → GreenActive
console.log(`Active: ${deployment.getActiveEnv()}`); // => Output: Green
// => FSM state management logic
// Simulate issue detected → instant rollback
deployment.transition("rollback"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => GreenActive → Rollback → BlueActive
console.log(`Active: ${deployment.getActiveEnv()}`); // => Output: Blue
// => FSM state management logic
Key Takeaway: Blue-green FSM manages traffic cutover with validation phase and instant rollback. Standby environment enables <30s rollback compared to 30+ minutes for rolling deploys.
Example 80: Canary Deployment FSM
Canary deployments use FSM to gradually increase traffic to new version while monitoring metrics, with automatic rollback on errors.
Why It Matters: At Google, canary FSM deploys YouTube changes to 1% → 5% → 25% → 50% → 100% of traffic over 6-12 hours, monitoring error rates and latency at each stage. If error rate increases >0.5% or p99 latency increases >10%, FSM automatically rolls back to previous version. With 50+ YouTube deploys daily, canary FSM catches 15-20% of bad deployments at 1-5% traffic, preventing user impact compared to full rollout. Gradual rollout reduces blast radius from 100% users (2B+) to <50M users for caught issues.
// Canary deployment FSM with gradual traffic shift and metric monitoring
type CanaryState = "Deploying" | "Canary1Pct" | "Canary5Pct" | "Canary25Pct" | "Canary50Pct" | "Complete" | "Rollback"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type CanaryEvent = "metrics_ok" | "metrics_fail" | "advance"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
class CanaryFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: CanaryState = "Deploying"; // => Initial state
// => FSM begins execution in Deploying state
private canaryTrafficPct: number = 0; // => Current canary traffic %
// => Initialized alongside FSM state
transition(event: CanaryEvent, errorRate?: number, latencyP99?: number): void {
// => Method invocation
// => Ternary: condition ? true_branch : false_branch
// => Executes validated state transition
// => Updates this.state if valid
// Monitor metrics: auto-rollback if thresholds exceeded
if (event === "metrics_ok") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
console.log(`Metrics OK at ${this.canaryTrafficPct}% traffic (error: ${errorRate}%, p99: ${latencyP99}ms)`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
this.transition("advance"); // => Metrics healthy → advance to next stage
// => Updates this.state if valid
return; // => Statement execution
} else if (event === "metrics_fail") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
console.log(`Metrics FAILED at ${this.canaryTrafficPct}% traffic → rollback`); // => Output for verification
// => Debug/audit output
// => Log for observability
this.state = "Rollback"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Rollback
// => State mutation (core FSM operation)
this.canaryTrafficPct = 0; // => Statement execution
// => Access instance property
// => Access instance property
return; // => Statement execution
}
// Advance to next canary stage
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Deploying": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Canary1Pct"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Canary1Pct
// => State mutation (core FSM operation)
this.canaryTrafficPct = 1; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Canary deployed: 1% traffic → monitor 30 min"); // => Output for verification
// => Debug/audit output
// => Log for observability
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Canary1Pct": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Canary5Pct"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Canary5Pct
// => State mutation (core FSM operation)
this.canaryTrafficPct = 5; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Advancing canary: 5% traffic → monitor 1 hour"); // => Output for verification
// => Debug/audit output
// => Log for observability
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Canary5Pct": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Canary25Pct"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Canary25Pct
// => State mutation (core FSM operation)
this.canaryTrafficPct = 25; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Advancing canary: 25% traffic → monitor 2 hours"); // => Output for verification
// => Debug/audit output
// => Log for observability
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Canary25Pct": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Canary50Pct"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Canary50Pct
// => State mutation (core FSM operation)
this.canaryTrafficPct = 50; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Advancing canary: 50% traffic → monitor 2 hours"); // => Output for verification
// => Debug/audit output
// => Log for observability
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Canary50Pct": // => Case: handles specific value
// => Match case
// => Match case
this.state = "Complete"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Complete
// => State mutation (core FSM operation)
this.canaryTrafficPct = 100; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Canary complete: 100% traffic"); // => Output for verification
// => Debug/audit output
// => Log for observability
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
}
checkMetrics(errorRate: number, latencyP99: number): void {
// => Method invocation
// => Begin object/config definition
// Metric thresholds: error rate <0.5%, p99 latency increase <10%
const errorThreshold = 0.5; // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize errorThreshold
const latencyThreshold = 10; // => % increase
if (errorRate > errorThreshold || latencyP99 > latencyThreshold) {
// => Conditional branch
// => Logical OR: either condition can be true
// => Conditional check
// => Branch execution based on condition
this.transition("metrics_fail", errorRate, latencyP99); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Auto-rollback on threshold breach
} else {
// => Statement execution
// => Fallback branch
this.transition("metrics_ok", errorRate, latencyP99); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Metrics healthy → advance
}
}
getState(): CanaryState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current canary state
}
getTrafficPct(): number {
// => Accessor method definition
// => Begin object/config definition
return this.canaryTrafficPct; // => Return canary traffic %
}
}
// Usage: Canary deployment with metric monitoring
const canary = new CanaryFSM(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize canary
canary.transition("advance"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Deploying → Canary1Pct (1% traffic)
// Simulate metric monitoring at 1% traffic
canary.checkMetrics(0.2, 5); // => Error: 0.2%, latency increase: 5%
// => Metrics OK → advance to 5%
canary.checkMetrics(0.3, 7); // => Error: 0.3%, latency increase: 7%
// => Metrics OK → advance to 25%
// Simulate error spike at 25% traffic
canary.checkMetrics(0.8, 15); // => Error: 0.8% (exceeds 0.5%), latency: 15% (exceeds 10%)
// => FSM state management logic
// => Metrics FAILED → rollback to 0%
console.log(canary.getState()); // => Output: Rollback
// => FSM state management logic
console.log(canary.getTrafficPct()); // => Output: 0
// => FSM state management logic
Key Takeaway: Canary FSM gradually increases traffic (1% → 5% → 25% → 50% → 100%) while monitoring metrics. Auto-rollback on threshold breach reduces blast radius to early-stage traffic percentages.
Example 81: Feature Flag FSM
Feature flags use FSM to manage feature lifecycle: development → testing → canary → enabled → deprecated.
Why It Matters: At Facebook, feature flag FSMs manage 10K+ active flags controlling gradual feature rollouts, A/B tests, and emergency kill switches. A typical flag FSM: Development (0% users) → Testing (employees only, 50K users) → Canary (0.1% users, 3M) → Enabled (100%, 3B users) → Deprecated (cleanup). FSM state transitions have approval gates (testing→canary requires QA sign-off, canary→enabled requires metric validation). Kill switch capability (Enabled→Disabled in <1s) mitigates production issues affecting millions.
// Feature flag FSM with approval gates and kill switch
type FlagState = "Development" | "Testing" | "Canary" | "Enabled" | "Disabled" | "Deprecated"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type FlagEvent = "deploy_test" | "qa_approved" | "metrics_validated" | "enable_all" | "kill_switch" | "deprecate"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
class FeatureFlagFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: FlagState = "Development"; // => Initial state
// => FSM begins execution in Development state
private rolloutPct: number = 0; // => % of users with feature enabled
// => Initialized alongside FSM state
transition(event: FlagEvent, approver?: string): void {
// => Method invocation
// => Ternary: condition ? true_branch : false_branch
// => Executes validated state transition
// => Updates this.state if valid
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Development": // => Case: handles specific value
// => Match case
// => Match case
if (event === "deploy_test") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Testing"; // => Development → Testing
this.rolloutPct = 0.1; // => Internal employees only
console.log("Feature deployed to Testing (employees only, 0.1%)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Testing": // => Case: handles specific value
// => Match case
// => Match case
if (event === "qa_approved") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
console.log(`QA approved by ${approver}`); // => Output for verification
// => Debug/audit output
// => Log for observability
this.state = "Canary"; // => Testing → Canary (approval gate)
this.rolloutPct = 1; // => 1% of production users
console.log("Feature in Canary (1% users)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Canary": // => Case: handles specific value
// => Match case
// => Match case
if (event === "metrics_validated") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
console.log(`Metrics validated by ${approver}`); // => Output for verification
// => Debug/audit output
// => Log for observability
this.state = "Enabled"; // => Canary → Enabled (approval gate)
this.rolloutPct = 100; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Feature Enabled (100% users)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
} else if (event === "kill_switch") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Disabled"; // => Emergency kill switch
this.rolloutPct = 0; // => Statement execution
// => Access instance property
// => Access instance property
console.log("KILL SWITCH: Feature disabled"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Enabled": // => Case: handles specific value
// => Match case
// => Match case
if (event === "kill_switch") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Disabled"; // => Emergency kill switch
this.rolloutPct = 0; // => Statement execution
// => Access instance property
// => Access instance property
console.log("KILL SWITCH: Feature disabled"); // => Output for verification
// => Debug/audit output
// => Log for observability
} else if (event === "deprecate") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Deprecated"; // => Enabled → Deprecated
console.log("Feature deprecated (cleanup scheduled)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Disabled": // => Case: handles specific value
// => Match case
// => Match case
if (event === "enable_all") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Enabled"; // => Re-enable after kill switch
this.rolloutPct = 100; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Feature re-enabled (100% users)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
}
isEnabled(userId: string): boolean {
// => Method invocation
// => Begin object/config definition
// Determine if feature enabled for specific user (hash-based rollout)
if (this.state === "Disabled" || this.state === "Deprecated") {
// => State-based guard condition
// => Logical OR: either condition can be true
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
return false; // => Feature disabled for all users
}
if (this.state === "Enabled") {
// => State-based guard condition
// => Comparison check
// => Comparison check
// => Guard condition: check current state is Enabled
// => Only execute if condition true
return true; // => Feature enabled for all users
}
// Canary/Testing: hash-based rollout
const userHash = this.hashCode(userId); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize userHash
return userHash % 100 < this.rolloutPct; // => Returns value to caller
// => Access instance property
// => Access instance property
// => Return computed result
// => Enable for N% of users based on hash
}
private hashCode(str: string): number {
// => Method invocation
// => Extended state (data beyond FSM state)
let hash = 0; // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize hash
for (let i = 0; i < str.length; i++) {
// => Variable declaration and assignment
// => Modify state data
// => Modify state data
// => Update extended state data
hash = (hash << 5) - hash + str.charCodeAt(i); // => Method invocation
// => Chained method calls or nested operations
// => Assign value
// => Assign value
}
return Math.abs(hash); // => Returns value to caller
// => Return computed result
}
getState(): FlagState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current flag state
}
getRolloutPct(): number {
// => Accessor method definition
// => Begin object/config definition
return this.rolloutPct; // => Return rollout percentage
}
}
// Usage: Feature flag lifecycle with approval gates
const featureFlag = new FeatureFlagFSM(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize featureFlag
featureFlag.transition("deploy_test"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Development → Testing (employees only)
featureFlag.transition("qa_approved", "alice@example.com"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Testing → Canary (1% users, QA approval required)
featureFlag.transition("metrics_validated", "bob@example.com"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Canary → Enabled (100% users, metrics approval required)
// Check if feature enabled for specific users
console.log(featureFlag.isEnabled("user-123")); // => true (100% rollout)
// => FSM state management logic
// => Log for observability
console.log(featureFlag.isEnabled("user-456")); // => true
// => FSM state management logic
// => Log for observability
// Emergency kill switch
featureFlag.transition("kill_switch"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// => Enabled → Disabled (0% users)
console.log(featureFlag.isEnabled("user-123")); // => false (killed)
// => FSM state management logic
// => Log for observability
Key Takeaway: Feature flag FSM manages feature lifecycle with approval gates (QA, metrics) and kill switch for emergency disablement. Hash-based rollout enables gradual feature releases.
Example 82: Circuit Breaker with FSM
Circuit breakers use FSM to prevent cascading failures by transitioning between Closed (healthy) → Open (failing) → Half-Open (testing) states.
Why It Matters: Circuit breaker FSMs protect microservice architectures from cascading failures. When a service’s error rate exceeds threshold over time window, circuit opens (blocks requests temporarily), preventing thread pool exhaustion in upstream services. After timeout, circuit enters Half-Open (allows test requests). If test requests succeed (error rate below threshold), circuit closes (full traffic resumes). If tests fail, circuit re-opens. Circuit breakers prevent major outages by isolating failing services before they cascade.
// Circuit breaker FSM: Closed → Open → Half-Open
type CircuitState = "Closed" | "Open" | "HalfOpen"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
class CircuitBreakerFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: CircuitState = "Closed"; // => Initial: circuit closed (healthy)
// => FSM begins execution in Closed state
private failureCount: number = 0; // => Consecutive failures
// => Initialized alongside FSM state
private failureThreshold: number = 5; // => Failures to open circuit
// => Initialized alongside FSM state
private successCount: number = 0; // => Successes in half-open
// => Initialized alongside FSM state
private successThreshold: number = 3; // => Successes to close circuit
// => Initialized alongside FSM state
private openTimeout: number = 5000; // => Timeout before half-open (5s)
// => Initialized alongside FSM state
private openTimestamp?: number; // => When circuit opened
// => FSM state management logic
// => Initialized alongside FSM state
async execute(operation: () => Promise<any>): Promise<any> {
// => Method invocation
// => Chained method calls or nested operations
// => Begin object/config definition
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Closed": // => Case: handles specific value
// => Match case
// => Match case
return this.executeClosed(operation); // => Returns value to caller
// => Return computed result
case "Open": // => Case: handles specific value
// => Match case
// => Match case
return this.executeOpen(); // => Returns value to caller
// => Return computed result
case "HalfOpen": // => Case: handles specific value
// => Match case
// => Match case
return this.executeHalfOpen(operation); // => Returns value to caller
// => Return computed result
}
}
private async executeClosed(operation: () => Promise<any>): Promise<any> {
// => Method invocation
// => Chained method calls or nested operations
// => Extended state (data beyond FSM state)
// => Initialized alongside FSM state
try {
// => Statement execution
// => Begin error handling
// => Begin error handling
// => Begin object/config definition
const result = await operation(); // => Execute operation
this.failureCount = 0; // => Success: reset failure count
return result; // => Returns value to caller
// => Return computed result
} catch (error) {
// => Method signature: defines function interface
// => Catch errors
// => Catch errors
// => Begin object/config definition
this.failureCount++; // => Failure: increment count
console.log(`Failure ${this.failureCount}/${this.failureThreshold}`); // => Output for verification
// => Debug/audit output
// => Log for observability
if (this.failureCount >= this.failureThreshold) {
// => Conditional branch
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
this.state = "Open"; // => Closed → Open (threshold exceeded)
this.openTimestamp = Date.now(); // => Method invocation
// => Assign value
// => Assign value
console.log("Circuit OPEN (too many failures, blocking requests)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
throw error; // => Propagate error
}
}
private async executeOpen(): Promise<any> {
// => Method invocation
// => Extended state (data beyond FSM state)
// Check if timeout expired → try half-open
const now = Date.now(); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize now
if (this.openTimestamp && now - this.openTimestamp >= this.openTimeout) {
// => Conditional branch
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
this.state = "HalfOpen"; // => Open → HalfOpen (timeout expired)
this.successCount = 0; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Circuit HALF-OPEN (testing with limited requests)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
return Promise.reject(new Error("Circuit half-open, retry")); // => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
}
// Timeout not expired → reject immediately (fast-fail)
console.log("Circuit OPEN (request rejected, fast-fail)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
return Promise.reject(new Error("Circuit breaker open")); // => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
}
private async executeHalfOpen(operation: () => Promise<any>): Promise<any> {
// => Method invocation
// => Chained method calls or nested operations
// => Extended state (data beyond FSM state)
// => Initialized alongside FSM state
try {
// => Statement execution
// => Begin error handling
// => Begin error handling
// => Begin object/config definition
const result = await operation(); // => Test request
this.successCount++; // => Statement execution
// => Modify state data
// => Modify state data
// => Update extended state data
console.log(`Half-open success ${this.successCount}/${this.successThreshold}`); // => Output for verification
// => Debug/audit output
// => Log for observability
if (this.successCount >= this.successThreshold) {
// => Conditional branch
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
this.state = "Closed"; // => HalfOpen → Closed (service recovered)
this.failureCount = 0; // => Statement execution
// => Access instance property
// => Access instance property
console.log("Circuit CLOSED (service recovered)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
return result; // => Returns value to caller
// => Return computed result
} catch (error) {
// => Method signature: defines function interface
// => Catch errors
// => Catch errors
// => Begin object/config definition
this.state = "Open"; // => HalfOpen → Open (test failed)
this.openTimestamp = Date.now(); // => Method invocation
// => Assign value
// => Assign value
console.log("Circuit re-OPEN (test request failed)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
throw error; // => Statement execution
}
}
getState(): CircuitState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current circuit state
}
}
// Usage: Circuit breaker protecting service calls
const circuit = new CircuitBreakerFSM(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize circuit
// Simulate service calls
async function callService(shouldFail: boolean): Promise<string> {
// => Method invocation
// => Begin object/config definition
return new Promise((resolve, reject) => {
// => Returns value to caller
// => Constructor creates new object instance
// => Return computed result
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
shouldFail ? reject(new Error("Service error")) : resolve("Success"); // => Method invocation
// => Ternary: condition ? true_branch : false_branch
}, 100); // => Statement execution
}); // => Statement execution
}
// Test circuit breaker states
async function testCircuit() {
// => Method invocation
// => Begin object/config definition
// Trigger failures to open circuit
for (let i = 0; i < 5; i++) {
// => Variable declaration and assignment
// => Modify state data
// => Modify state data
// => Update extended state data
try {
// => Statement execution
// => Begin error handling
// => Begin error handling
// => Begin object/config definition
await circuit.execute(() => callService(true)); // => Method invocation
// => Chained method calls or nested operations
} catch (e) {
// => Method signature: defines function interface
// => Catch errors
// => Catch errors
// => Begin object/config definition
// Expected failures
}
}
// => Circuit: Closed → Open (5 failures)
console.log(`State: ${circuit.getState()}`); // => Output: Open
// => FSM state management logic
// Try request while open (rejected immediately)
try {
// => Statement execution
// => Begin error handling
// => Begin error handling
// => Begin object/config definition
await circuit.execute(() => callService(false)); // => Method invocation
// => Chained method calls or nested operations
} catch (e) {
// => Method signature: defines function interface
// => Catch errors
// => Catch errors
// => Begin object/config definition
console.log("Request rejected by open circuit"); // => Output for verification
// => Debug/audit output
// => Log for observability
}
// Wait for timeout → half-open
await new Promise((resolve) => setTimeout(resolve, 6000)); // => Method invocation
// => Constructor creates new object instance
// Test requests in half-open
for (let i = 0; i < 3; i++) {
// => Variable declaration and assignment
// => Modify state data
// => Modify state data
// => Update extended state data
try {
// => Statement execution
// => Begin error handling
// => Begin error handling
// => Begin object/config definition
await circuit.execute(() => callService(false)); // => Successes
// => FSM state management logic
} catch (e) {
// => Method signature: defines function interface
// => Catch errors
// => Catch errors
// => Begin object/config definition
// Shouldn't happen if service recovered
}
}
// => Circuit: HalfOpen → Closed (3 successes)
console.log(`State: ${circuit.getState()}`); // => Output: Closed
// => FSM state management logic
}
testCircuit(); // => Method invocation
Key Takeaway: Circuit breaker FSM protects against cascading failures with three states: Closed (healthy), Open (failing, fast-fail), Half-Open (testing recovery). Automatic state transitions isolate failing services.
Advanced Patterns (Examples 83-85)
Example 83: FSM with Machine Learning Integration
FSMs integrate with ML models to make state transition decisions based on predictions (fraud detection, anomaly detection).
Why It Matters: At PayPal, fraud detection FSMs use ML models to score transactions and determine state transitions. Transactions move through: Initiated → MLScoring → (Low Risk → Approved | Medium Risk → ManualReview | High Risk → Rejected). ML model predicts fraud probability (0-1 score) in real-time (<50ms latency). With 450M daily transactions, ML-FSM integration catches 92% of fraud (vs 78% rule-based) while reducing false positives by 40%, saving $400M+ annually in fraud losses and customer friction.
// FSM with ML model integration for state transitions
type TransactionState = // => Type declaration defines structure
// => Enum-like union type for state values
// => Type system ensures only valid states used
"Initiated" | "MLScoring" | "LowRisk" | "MediumRisk" | "HighRisk" | "Approved" | "Rejected" | "ManualReview"; // => Statement execution
type TransactionEvent = "score" | "approve" | "reject"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
interface MLModel {
// => Type declaration defines structure
// => Begin object/config definition
predict(features: number[]): number; // => Predict fraud probability (0-1)
// => FSM state management logic
}
class FraudDetectionFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: TransactionState = "Initiated"; // => Initial state
// => FSM begins execution in Initiated state
private mlModel: MLModel; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
private fraudScore: number = 0; // => Field declaration: class member variable
// => Assign value
// => Assign value
// => Extended state (data beyond FSM state)
// => Initialized alongside FSM state
constructor(mlModel: MLModel) {
// => Method invocation
// => Begin object/config definition
this.mlModel = mlModel; // => Statement execution
// => Access instance property
// => Access instance property
}
async transition(event: TransactionEvent, features?: number[]): Promise<void> {
// => Method invocation
// => Ternary: condition ? true_branch : false_branch
// => Executes validated state transition
// => Updates this.state if valid
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Initiated": // => Case: handles specific value
// => Match case
// => Match case
if (event === "score" && features) {
// => Event type guard condition
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Event type check
this.state = "MLScoring"; // => Initiated → MLScoring
// Call ML model for fraud prediction
this.fraudScore = await this.mlModel.predict(features); // => Method invocation
// => Assign value
// => Assign value
console.log(`ML fraud score: ${this.fraudScore.toFixed(3)}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
// State transition based on ML score
if (this.fraudScore < 0.3) {
// => Conditional branch
// => Conditional check
// => Branch execution based on condition
this.state = "LowRisk"; // => Low risk: auto-approve
this.transition("approve"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
} else if (this.fraudScore < 0.7) {
// => Method signature: defines function interface
// => Alternative condition
this.state = "MediumRisk"; // => Medium risk: manual review
console.log("Medium risk → Manual review queue"); // => Output for verification
// => Debug/audit output
// => Log for observability
} else {
// => Statement execution
// => Fallback branch
this.state = "HighRisk"; // => High risk: auto-reject
this.transition("reject"); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
}
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "LowRisk": // => Case: handles specific value
// => Match case
// => Match case
if (event === "approve") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Approved"; // => LowRisk → Approved
console.log("Transaction approved (low fraud risk)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "HighRisk": // => Case: handles specific value
// => Match case
// => Match case
if (event === "reject") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Rejected"; // => HighRisk → Rejected
console.log("Transaction rejected (high fraud risk)"); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "MediumRisk": // => Case: handles specific value
// => Match case
// => Match case
if (event === "approve") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Approved"; // => Manual review: approved
} else if (event === "reject") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Rejected"; // => Manual review: rejected
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
}
getState(): TransactionState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
getFraudScore(): number {
// => Accessor method definition
// => Begin object/config definition
return this.fraudScore; // => Return ML fraud score
}
}
// Simple ML model (simulated)
class SimpleFraudModel implements MLModel {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
predict(features: number[]): number {
// => Method invocation
// => Begin object/config definition
// Simulate ML inference (weighted sum of features)
// features: [amount, velocity, geo_risk, device_risk]
const weights = [0.3, 0.25, 0.25, 0.2]; // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize weights
const score = features.reduce((sum, f, i) => sum + f * weights[i], 0); // => Variable declaration and assignment
// => Chained method calls or nested operations
// => Assign value
// => Assign value
// => Initialize score
return Math.min(Math.max(score, 0), 1); // => Clamp to [0, 1]
// => FSM state management logic
}
}
// Usage: FSM with ML-driven state transitions
const mlModel = new SimpleFraudModel(); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize mlModel
const fsm = new FraudDetectionFSM(mlModel); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize fsm
// Low-risk transaction
fsm.transition("score", [0.2, 0.1, 0.15, 0.1]); // => Features: low values
// => Updates this.state if valid
// => ML fraud score: 0.145
// => LowRisk → Approved
console.log(fsm.getState()); // => Output: Approved
// => FSM state management logic
// High-risk transaction
const fsm2 = new FraudDetectionFSM(mlModel); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize fsm2
fsm2.transition("score", [0.9, 0.85, 0.95, 0.88]); // => Features: high values
// => Updates this.state if valid
// => ML fraud score: 0.895
// => HighRisk → Rejected
console.log(fsm2.getState()); // => Output: Rejected
// => FSM state management logic
Key Takeaway: ML-FSM integration uses model predictions to determine state transitions. Fraud score thresholds route transactions to auto-approve, manual review, or auto-reject states.
Example 84: FSM with Event Streaming (Kafka)
FSMs in event-driven architectures consume events from streams (Kafka) and publish state changes back to streams.
Why It Matters: Trip FSMs consume events from Kafka topics (driver location updates, passenger requests, payment confirmations) at high volumes. Each trip FSM subscribes to relevant partitions, processes events to update state (Requested → Matched → PickupEnRoute → InProgress → Completed), and publishes state changes back to Kafka for downstream consumers (billing, analytics, notifications). Event streaming enables FSM scalability: many concurrent trips run across distributed nodes, each processing events efficiently.
// FSM consuming events from Kafka stream (simulated)
type TripState = "Requested" | "Matched" | "PickupEnRoute" | "InProgress" | "Completed"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type TripEvent = "driver_matched" | "driver_arriving" | "passenger_picked_up" | "trip_ended"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
interface KafkaEvent {
// => Type declaration defines structure
// => Begin object/config definition
topic: string; // => Statement execution
key: string; // => Statement execution
value: any; // => Statement execution
timestamp: number; // => Statement execution
}
class TripFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: TripState = "Requested"; // => Initial state
// => FSM begins execution in Requested state
private tripId: string; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
constructor(tripId: string) {
// => Method invocation
// => Begin object/config definition
this.tripId = tripId; // => Statement execution
// => Access instance property
// => Access instance property
}
consumeEvent(event: KafkaEvent): KafkaEvent | null {
// => Method invocation
// => Begin object/config definition
console.log(`[${this.tripId}] Consuming event: ${event.topic} - ${JSON.stringify(event.value)}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
// Map Kafka event to FSM transition
const tripEvent = this.mapKafkaEventToFSM(event); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize tripEvent
if (!tripEvent) return null; // => Irrelevant event
// Execute state transition
this.transition(tripEvent); // => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
// Publish state change back to Kafka
return this.publishStateChange(); // => Returns value to caller
// => Return computed result
}
private mapKafkaEventToFSM(event: KafkaEvent): TripEvent | null {
// => Method invocation
// => Extended state (data beyond FSM state)
// Map Kafka topic/value to FSM event
if (event.topic === "driver-assignments" && event.value.status === "matched") {
// => Event type guard condition
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
return "driver_matched"; // => Returns value to caller
// => Return computed result
} else if (event.topic === "driver-locations" && event.value.status === "arriving") {
// => Method signature: defines function interface
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Alternative condition
return "driver_arriving"; // => Returns value to caller
// => Return computed result
} else if (event.topic === "trip-updates" && event.value.status === "pickup") {
// => Method signature: defines function interface
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Alternative condition
return "passenger_picked_up"; // => Returns value to caller
// => Return computed result
} else if (event.topic === "trip-updates" && event.value.status === "ended") {
// => Method signature: defines function interface
// => Logical AND: both conditions must be true
// => Comparison check
// => Comparison check
// => Alternative condition
return "trip_ended"; // => Returns value to caller
// => Return computed result
}
return null; // => Unknown event
}
private transition(event: TripEvent): void {
// => Method invocation
// => Extended state (data beyond FSM state)
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Requested": // => Case: handles specific value
// => Match case
// => Match case
if (event === "driver_matched") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Matched"; // => Requested → Matched
console.log(`[${this.tripId}] State: Requested → Matched`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Matched": // => Case: handles specific value
// => Match case
// => Match case
if (event === "driver_arriving") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "PickupEnRoute"; // => Matched → PickupEnRoute
console.log(`[${this.tripId}] State: Matched → PickupEnRoute`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "PickupEnRoute": // => Case: handles specific value
// => Match case
// => Match case
if (event === "passenger_picked_up") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "InProgress"; // => PickupEnRoute → InProgress
console.log(`[${this.tripId}] State: PickupEnRoute → InProgress`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "InProgress": // => Case: handles specific value
// => Match case
// => Match case
if (event === "trip_ended") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Completed"; // => InProgress → Completed
console.log(`[${this.tripId}] State: InProgress → Completed`); // => Output for verification
// => Debug/audit output
// => Log for observability
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
}
private publishStateChange(): KafkaEvent {
// => Method invocation
// => Extended state (data beyond FSM state)
// Publish FSM state change to Kafka topic
return {
// => Returns value to caller
// => Return computed result
topic: "trip-state-changes", // => Statement execution
key: this.tripId, // => Statement execution
// => Access instance property
// => Access instance property
value: { tripId: this.tripId, state: this.state }, // => Statement execution
// => Access instance property
// => Access instance property
timestamp: Date.now(), // => Method invocation
};
}
getState(): TripState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: FSM consuming Kafka events
const trip = new TripFSM("TRIP-123"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize trip
// Simulate Kafka events
const events: KafkaEvent[] = [
// => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize events
{
// => Statement execution
// => Begin object/config definition
topic: "driver-assignments", // => Statement execution
key: "TRIP-123", // => Statement execution
value: { status: "matched", driverId: "D-456" }, // => Statement execution
timestamp: Date.now(), // => Method invocation
},
{ topic: "driver-locations", key: "D-456", value: { status: "arriving", eta: 5 }, timestamp: Date.now() }, // => Method invocation
{ topic: "trip-updates", key: "TRIP-123", value: { status: "pickup" }, timestamp: Date.now() }, // => Method invocation
{ topic: "trip-updates", key: "TRIP-123", value: { status: "ended" }, timestamp: Date.now() }, // => Method invocation
]; // => Statement execution
// Consume events sequentially
for (const event of events) {
// => Method invocation
// => Iterate collection
// => Iterate collection
// => Begin object/config definition
const stateChange = trip.consumeEvent(event); // => State variable initialization
// => Assign value
// => Assign value
// => Initialize stateChange
if (stateChange) {
// => State-based guard condition
// => Conditional check
// => Branch execution based on condition
console.log(`Published to Kafka: ${JSON.stringify(stateChange)}`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
}
console.log(`Final state: ${trip.getState()}`); // => Output: Completed
// => FSM state management logic
Key Takeaway: FSMs in event streaming consume Kafka events, map them to state transitions, and publish state changes back to Kafka for downstream consumers. Enables scalable event-driven architectures.
Example 85: FSM Visualization and Debugging
Production FSMs need visualization and debugging tools to understand current state distribution, transition history, and bottlenecks.
Why It Matters: At Airbnb, booking FSM telemetry tracks 100K+ concurrent bookings with state distribution dashboards showing: Pending (15K), PaymentProcessing (8K), Confirmed (70K), Cancelled (5K), Error (2K). When Error state spikes to 8K (4x normal), engineers use FSM transition logs to identify root cause (payment gateway timeout) within 5 minutes. Without FSM visualization, diagnosing distributed state issues would take 30-60 minutes (analyzing logs across 200+ instances). State distribution metrics enable capacity planning: if PaymentProcessing consistently exceeds 10K, scale payment service.
// FSM with telemetry, visualization, and debugging
type BookingState = "Pending" | "PaymentProcessing" | "Confirmed" | "Cancelled" | "Error"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Enum-like union type for state values
// => Type system ensures only valid states used
type BookingEvent = "process_payment" | "payment_success" | "payment_fail" | "cancel"; // => Type declaration defines structure
// => Assign value
// => Assign value
// => Defines event alphabet for FSM
// => Events trigger state transitions
interface StateMetrics {
// => Type declaration defines structure
// => Begin object/config definition
state: BookingState; // => Statement execution
count: number; // => Instances in this state
avgDurationMs: number; // => Average time in state
}
interface TransitionLog {
// => Type declaration defines structure
// => Begin object/config definition
bookingId: string; // => Statement execution
fromState: BookingState; // => Statement execution
toState: BookingState; // => Statement execution
event: BookingEvent; // => Statement execution
timestamp: number; // => Statement execution
durationMs: number; // => Time spent in previous state
}
class ObservableBookingFSM {
// => Class encapsulates FSM logic
// => State machine implementation class
// => Encapsulates state + transition logic
private state: BookingState = "Pending"; // => Initial state
// => FSM begins execution in Pending state
private bookingId: string; // => Field declaration: class member variable
// => Extended state (data beyond FSM state)
private stateEnterTime: number = Date.now(); // => When entered current state
private static stateDistribution: Map<BookingState, number> = new Map(); // => Method invocation
// => Create new instance
// => Create new instance
// => Extended state (data beyond FSM state)
// => Initialized alongside FSM state
private static transitionLogs: TransitionLog[] = []; // => Field declaration: class member variable
// => Assign value
// => Assign value
// => Extended state (data beyond FSM state)
// => Initialized alongside FSM state
constructor(bookingId: string) {
// => Method invocation
// => Begin object/config definition
this.bookingId = bookingId; // => Statement execution
// => Access instance property
// => Access instance property
ObservableBookingFSM.incrementState(this.state); // => Track state distribution
}
transition(event: BookingEvent): void {
// => Method invocation
// => Executes validated state transition
// => Updates this.state if valid
const previousState = this.state; // => State variable initialization
// => Access instance property
// => Access instance property
// => Initialize previousState
const now = Date.now(); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize now
const durationMs = now - this.stateEnterTime; // => Time in previous state
// Execute state transition
switch (
this.state // => Method invocation
) {
// => Branch on value
// => Branch on value
// => Begin object/config definition
case "Pending": // => Case: handles specific value
// => Match case
// => Match case
if (event === "process_payment") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "PaymentProcessing"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to PaymentProcessing
// => State mutation (core FSM operation)
} else if (event === "cancel") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Cancelled"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Cancelled
// => State mutation (core FSM operation)
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "PaymentProcessing": // => Case: handles specific value
// => Match case
// => Match case
if (event === "payment_success") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "Confirmed"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Confirmed
// => State mutation (core FSM operation)
} else if (event === "payment_fail") {
// => Method signature: defines function interface
// => Comparison check
// => Comparison check
// => Alternative condition
this.state = "Error"; // => State transition execution
// => Access instance property
// => Access instance property
// => Transition: set state to Error
// => State mutation (core FSM operation)
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
case "Error": // => Case: handles specific value
// => Match case
// => Match case
if (event === "process_payment") {
// => Event type guard condition
// => Comparison check
// => Comparison check
// => Event type check
this.state = "PaymentProcessing"; // => Retry from error
}
break; // => Statement execution
// => Exit loop/switch
// => Exit loop/switch
}
// Update metrics
if (previousState !== this.state) {
// => State-based guard condition
// => Comparison check
// => Comparison check
// => Conditional check
// => Branch execution based on condition
ObservableBookingFSM.decrementState(previousState); // => Method invocation
ObservableBookingFSM.incrementState(this.state); // => Method invocation
// Log transition
ObservableBookingFSM.transitionLogs.push({
// => Statement execution
// => Add to collection
// => Add to collection
// => Begin object/config definition
bookingId: this.bookingId, // => Statement execution
// => Access instance property
// => Access instance property
fromState: previousState, // => Statement execution
toState: this.state, // => Statement execution
// => Access instance property
// => Access instance property
event, // => Statement execution
timestamp: now, // => Statement execution
durationMs, // => Statement execution
}); // => Statement execution
this.stateEnterTime = now; // => Reset state enter time
console.log(`[${this.bookingId}] ${previousState} --[${event}]--> ${this.state} (${durationMs}ms)`); // => Output for verification
// => Chained method calls or nested operations
// => Modify state data
// => Modify state data
// => Debug/audit output
// => Log for observability
}
}
private static incrementState(state: BookingState): void {
// => Method invocation
// => Extended state (data beyond FSM state)
const count = ObservableBookingFSM.stateDistribution.get(state) || 0; // => State variable initialization
// => Logical OR: either condition can be true
// => Assign value
// => Assign value
// => Initialize count
ObservableBookingFSM.stateDistribution.set(state, count + 1); // => Method invocation
}
private static decrementState(state: BookingState): void {
// => Method invocation
// => Extended state (data beyond FSM state)
const count = ObservableBookingFSM.stateDistribution.get(state) || 0; // => State variable initialization
// => Logical OR: either condition can be true
// => Assign value
// => Assign value
// => Initialize count
ObservableBookingFSM.stateDistribution.set(state, Math.max(count - 1, 0)); // => Method invocation
// => Chained method calls or nested operations
}
static getStateDistribution(): StateMetrics[] {
// => Method invocation
// => Begin object/config definition
const metrics: StateMetrics[] = []; // => State variable initialization
// => Assign value
// => Assign value
// => Initialize metrics
for (const [state, count] of ObservableBookingFSM.stateDistribution) {
// => Method invocation
// => Iterate collection
// => Iterate collection
// => Begin object/config definition
// Calculate average duration in state
const stateLogs = ObservableBookingFSM.transitionLogs.filter((log) => log.fromState === state); // => State variable initialization
// => Chained method calls or nested operations
// => Filter collection
// => Filter collection
// => Initialize stateLogs
const avgDurationMs = // => Variable declaration and assignment
// => Initialize avgDurationMs
stateLogs.length > 0 ? stateLogs.reduce((sum, log) => sum + log.durationMs, 0) / stateLogs.length : 0; // => Method invocation
// => Ternary: condition ? true_branch : false_branch
metrics.push({ state, count, avgDurationMs }); // => Method invocation
// => Add to collection
// => Add to collection
}
return metrics; // => Returns value to caller
// => Return computed result
}
static getTransitionLogs(limit: number = 10): TransitionLog[] {
// => Method invocation
// => Assign value
// => Assign value
// => Begin object/config definition
return ObservableBookingFSM.transitionLogs.slice(-limit); // => Last N transitions
}
getState(): BookingState {
// => Accessor method definition
// => Begin object/config definition
return this.state; // => Return current state
}
}
// Usage: FSM with telemetry and debugging
const booking1 = new ObservableBookingFSM("BK-001"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize booking1
const booking2 = new ObservableBookingFSM("BK-002"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize booking2
const booking3 = new ObservableBookingFSM("BK-003"); // => Instance creation via constructor
// => Create new instance
// => Create new instance
// => Initialize booking3
// Simulate bookings
booking1.transition("process_payment"); // => Pending → PaymentProcessing
// => Updates this.state if valid
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
booking1.transition("payment_success"); // => PaymentProcessing → Confirmed
// => Updates this.state if valid
}, 200); // => Statement execution
booking2.transition("process_payment"); // => Pending → PaymentProcessing
// => Updates this.state if valid
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
booking2.transition("payment_fail"); // => PaymentProcessing → Error
// => Updates this.state if valid
}, 300); // => Statement execution
booking3.transition("cancel"); // => Pending → Cancelled
// => Updates this.state if valid
// View state distribution (real-time metrics)
setTimeout(() => {
// => Accessor method definition
// => Chained method calls or nested operations
// => Begin object/config definition
console.log("\n=== State Distribution ==="); // => Output for verification
// => Comparison check
// => Comparison check
// => Debug/audit output
// => Log for observability
const distribution = ObservableBookingFSM.getStateDistribution(); // => State variable initialization
// => Assign value
// => Assign value
// => Initialize distribution
for (const metric of distribution) {
// => Method invocation
// => Iterate collection
// => Iterate collection
// => Begin object/config definition
console.log(`${metric.state}: ${metric.count} instances (avg ${metric.avgDurationMs.toFixed(0)}ms)`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
console.log("\n=== Recent Transitions ==="); // => Output for verification
// => Comparison check
// => Comparison check
// => Debug/audit output
// => Log for observability
const logs = ObservableBookingFSM.getTransitionLogs(5); // => Variable declaration and assignment
// => Assign value
// => Assign value
// => Initialize logs
for (const log of logs) {
// => Method invocation
// => Iterate collection
// => Iterate collection
// => Begin object/config definition
console.log(`[${log.bookingId}] ${log.fromState} → ${log.toState} (${log.durationMs}ms)`); // => Output for verification
// => Chained method calls or nested operations
// => Debug/audit output
// => Log for observability
}
}, 500); // => Statement execution
Key Takeaway: Observable FSMs track state distribution and transition logs for real-time monitoring and debugging. State metrics enable capacity planning and performance analysis in production systems.
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