saga-architecture

saga-architecture

When to Activate

When coordinating distributed operations across multiple services or data stores where ACID transactions are not available and compensating actions are needed to maintain eventual consistency.

Steps

1. Identify the saga participants - Determine which services or components participate in the distributed operation.
2. Define the saga choreography or orchestration - Choose whether sagas will be choreographed (event-driven) or orchestrated (central coordinator).
3. Define each step with corresponding compensation - For every forward action, define what compensating action undoes it.
4. Implement idempotent operations - Ensure each step can be safely retried and compensation can be safely reapplied.
5. Handle saga failures with compensation - On failure, execute compensations in reverse order (for orchestrating sagas) or react to failure events (for choreographing sagas).
6. Persist saga state - Store saga state to survive process crashes and enable recovery.
7. Add timeout and retry logic - Detect stuck sagas and advance or compensate accordingly.

Examples

// Saga State
interface SagaState<T> {
  id: string
  currentStep: number
  data: T
  status: 'pending' | 'in_progress' | 'completed' | 'compensating' | 'failed'
}

// Orchestrating Saga - Central coordinator manages steps
class OrderProcessingSaga {
  private readonly steps: SagaStep[]

  constructor(
    private readonly sagaOrchestrator: SagaOrchestrator,
    private readonly inventoryService: InventoryService,
    private readonly paymentService: PaymentService,
    private readonly shippingService: ShippingService
  ) {
    this.steps = [
      {
        name: 'reserve_inventory',
        execute: (state) => this.inventoryService.reserve(state.orderId, state.items),
        compensate: (state) => this.inventoryService.release(state.orderId, state.items)
      },
      {
        name: 'process_payment',
        execute: (state) => this.paymentService.charge(state.orderId, state.total),
        compensate: (state) => this.paymentService.refund(state.orderId, state.total)
      },
      {
        name: 'initiate_shipping',
        execute: (state) => this.shippingService.createShipment(state.orderId),
        compensate: (state) => this.shippingService.cancelShipment(state.shipmentId)
      }
    ]
  }

  async execute(orderId: string): Promise<void> {
    const state: SagaState<OrderSagaData> = {
      id: generateId(),
      currentStep: 0,
      data: { orderId, items: [], total: 0 },
      status: 'in_progress'
    }

    await this.sagaOrchestrator.start(state, this.steps)
  }
}

// Choreography-based Saga - Events trigger reactions
class OrderCreatedHandler {
  constructor(private readonly eventBus: EventBus) {}

  async handle(event: OrderCreatedEvent): Promise<void> {
    // Step 1: Reserve inventory
    try {
      await this.inventoryService.reserve(event.orderId, event.items)
      this.eventBus.publish(new InventoryReservedEvent(event.orderId))
    } catch (error) {
      this.eventBus.publish(new InventoryReservationFailedEvent(event.orderId, error.message))
    }
  }
}

class InventoryReservedHandler {
  async handle(event: InventoryReservedEvent): Promise<void> {
    // Step 2: Process payment
    try {
      await this.paymentService.charge(event.orderId, event.total)
      this.eventBus.publish(new PaymentProcessedEvent(event.orderId))
    } catch (error) {
      // Compensate by releasing inventory
      this.eventBus.publish(new InventoryReleaseRequestedEvent(event.orderId))
    }
  }
}

// Idempotent Step Implementation
class PaymentService {
  async charge(orderId: string, amount: Money): Promise<TransactionId> {
    const existingTx = await this.transactionRepo.findByOrderId(orderId)
    if (existingTx) {
      return existingTx.id // Idempotent: return existing instead of charging again
    }

    const transaction = await this.paymentGateway.charge(amount)
    await this.transactionRepo.save({ orderId, transaction })
    return transaction.id
  }
}