Saga Pattern: Distributed Transactions in Microservices#

In a monolith, transactions are simple — wrap everything in a database transaction, and it either all commits or all rolls back. In microservices, each service has its own database. There's no shared transaction.

The saga pattern solves this: break the transaction into local transactions with compensating actions for rollback.

The Problem#

E-commerce checkout in microservices:

1. Order Service: create order ✓
2. Payment Service: charge card ✓
3. Inventory Service: reserve items ✗ (out of stock!)
4. ??? — order exists, card charged, but items unavailable

Without a saga, you have inconsistent state. With a saga, step 4 triggers compensation:

4. Compensate Payment: refund card
5. Compensate Order: cancel order

Two Approaches#

Choreography (Event-Driven)#

Services react to each other's events. No central coordinator.

Order Service → publishes "OrderCreated"
  ↓
Payment Service → listens → charges card → publishes "PaymentCompleted"
  ↓
Inventory Service → listens → reserves items → publishes "ItemsReserved"
  ↓
Shipping Service → listens → creates shipment → publishes "ShipmentCreated"

On failure:
Inventory Service → publishes "ItemsReservationFailed"
  ↓
Payment Service → listens → refunds → publishes "PaymentRefunded"
  ↓
Order Service → listens → cancels order

Pros: Simple, decoupled, no single point of failure Cons: Hard to track flow, implicit logic, debugging is painful Best for: Simple flows (3-4 steps)

Orchestration (Coordinator)#

A saga orchestrator explicitly controls the flow:

Saga Orchestrator:
  1. Tell Order Service: "Create order" → success
  2. Tell Payment Service: "Charge card" → success
  3. Tell Inventory Service: "Reserve items" → FAILED
  4. Tell Payment Service: "Refund card" (compensate)
  5. Tell Order Service: "Cancel order" (compensate)
  6. Mark saga as failed

Pros: Explicit flow, easy to debug, centralized state Cons: Orchestrator is a single point of failure, coupling to coordinator Best for: Complex flows (5+ steps), flows with conditional logic

Comparison#

Compensating Actions#

Every step needs a compensating action for rollback:

Rules:

1. Compensations must be idempotent (safe to retry)
2. Compensations must always succeed (or have their own retry)
3. Some actions can't be compensated (sent emails) — use semantic compensation (send correction)

Saga State Machine#

Track saga progress:

States: STARTED → PAYMENT_PENDING → PAYMENT_COMPLETED
        → INVENTORY_PENDING → INVENTORY_RESERVED
        → SHIPPING_PENDING → COMPLETED

Failure states: PAYMENT_FAILED → COMPENSATING → COMPENSATED
                INVENTORY_FAILED → COMPENSATING_PAYMENT → COMPENSATED

interface SagaState {
  id: string;
  status: "started" | "completed" | "compensating" | "compensated" | "failed";
  steps: {
    name: string;
    status: "pending" | "completed" | "failed" | "compensated";
    result?: unknown;
  }[];
}

Implementation Example#

Orchestrator in TypeScript#

class OrderSaga {
  private steps: SagaStep[] = [
    {
      name: "createOrder",
      execute: (ctx) => orderService.create(ctx.order),
      compensate: (ctx) => orderService.cancel(ctx.orderId),
    },
    {
      name: "chargePayment",
      execute: (ctx) => paymentService.charge(ctx.amount, ctx.paymentMethod),
      compensate: (ctx) => paymentService.refund(ctx.chargeId),
    },
    {
      name: "reserveInventory",
      execute: (ctx) => inventoryService.reserve(ctx.items),
      compensate: (ctx) => inventoryService.release(ctx.reservationId),
    },
  ];

  async execute(input: OrderInput): Promise<SagaResult> {
    const ctx: Record<string, unknown> = { ...input };
    const completedSteps: SagaStep[] = [];

    for (const step of this.steps) {
      try {
        const result = await step.execute(ctx);
        Object.assign(ctx, result);
        completedSteps.push(step);
      } catch (error) {
        // Compensate in reverse order
        for (const completed of completedSteps.reverse()) {
          await completed.compensate(ctx);
        }
        return { status: "failed", error, compensated: true };
      }
    }
    return { status: "completed", ctx };
  }
}

Failure Scenarios#

What if compensation fails?#

Retry with exponential backoff. If it still fails, send to a dead letter queue for manual resolution.

Step fails → compensate → compensation fails → retry (3x)
  → still fails → dead letter queue → alert ops team

What if the orchestrator crashes?#

Store saga state in a durable store (PostgreSQL, Redis). On restart, resume from last known state.

Network partition during saga?#

Timeout + retry. Design all steps to be idempotent so retries are safe.

When NOT to Use Sagas#

1. Single database — use a regular transaction
2. All-or-nothing required — sagas are eventually consistent, not ACID
3. Simple workflows — if you have 2 steps, just handle errors directly
4. Performance critical — sagas add latency (multiple service calls)

Architecture Example#

E-Commerce Order Saga#

Client → API Gateway → Order Saga Orchestrator
                            → Order Service (create) ← compensate: cancel
                            → Payment Service (charge) ← compensate: refund
                            → Inventory Service (reserve) ← compensate: release
                            → Shipping Service (schedule) ← compensate: cancel
                            → Notification Service (email) ← compensate: correction email

Saga state stored in PostgreSQL
Failed sagas → Dead Letter Queue → Manual review

Generate saga architecture →

Summary#

1. Choreography for simple flows (3-4 steps), decoupled services
2. Orchestration for complex flows (5+ steps), better visibility
3. Every step needs a compensating action — design them upfront
4. Make everything idempotent — retries are inevitable
5. Store saga state durably — survive orchestrator crashes
6. Dead letter queue for unrecoverable failures
7. Don't use sagas when a database transaction works — simpler is better

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