Saga Pattern vs 2PC — Real Tradeoffs

Saga Pattern vs 2PC — Real Tradeoffs

Distributed transactions across microservices force a choice between strong consistency and availability. Two-phase commit (2PC) offers atomicity but is operationally fragile. Sagas trade atomicity for availability and scalability.

Two-Phase Commit (2PC)

2PC coordinates participants through a prepare and commit phase. If any participant fails during prepare, the coordinator aborts.

Strengths:

• Atomic commit across services.
• Simplifies reasoning about consistency.

Weaknesses:

• Blocking: participants hold locks while waiting for commit.
• Single coordinator becomes a critical dependency.
• Poor fit for cloud-native environments with transient failures.

Saga Pattern

A Saga is a sequence of local transactions, each with a compensating action.

Strengths:

• No distributed locks.
• Works across heterogeneous data stores.
• Scales better and tolerates partial failures.

Weaknesses:

• Eventual consistency is visible to clients.
• Complex compensation logic.
• Requires careful orchestration and observability.

Choosing Between Them

Dimension	2PC	Saga
Consistency	Strong	Eventual
Availability	Lower	Higher
Latency	Higher	Lower
Operational Complexity	Moderate	High
Failure Handling	Blocking	Compensating

2PC is suitable for tightly coupled systems within a single datastore or when the infrastructure guarantees low failure rates. Sagas are superior for microservices that value availability and autonomy.

Orchestrated Saga Example (Spring Boot)

@Service
public class OrderSaga {
    private final OrderService orderService;
    private final PaymentService paymentService;
    private final ShippingService shippingService;

    public OrderSaga(OrderService orderService,
                     PaymentService paymentService,
                     ShippingService shippingService) {
        this.orderService = orderService;
        this.paymentService = paymentService;
        this.shippingService = shippingService;
    }

    @Transactional
    public void placeOrder(PlaceOrderCommand command) {
        Order order = orderService.createOrder(command);
        try {
            paymentService.capturePayment(order);
            shippingService.reserveShipment(order);
        } catch (Exception ex) {
            shippingService.releaseShipment(order);
            paymentService.refundPayment(order);
            orderService.cancelOrder(order);
            throw ex;
        }
    }
}

Implementation Details That Matter

• Outbox pattern: ensure events are published atomically with database updates.
• Idempotent compensations: compensations must be safe to run multiple times.
• Saga state: store saga state explicitly to enable recovery.

Failure Modes to Consider

• Compensations failing and requiring manual intervention.
• Orchestrator crashes between steps.
• Duplicate events leading to double compensation.

Summary

2PC provides strong consistency but is brittle and latency-heavy. Sagas fit microservices better but require explicit state management, idempotent compensations, and robust observability to handle failures safely.