spring-boot-saga-pattern

# Spring Boot Saga Pattern

## Overview

Implements distributed transactions across microservices using the Saga Pattern. Replaces two-phase commit with a sequence of local transactions and compensating actions. Supports choreography (event-driven) and orchestration (centralized coordinator) approaches with Kafka, RabbitMQ, or Axon Framework.

## When to Use

- Building distributed transactions across multiple microservices
- Replacing two-phase commit (2PC) with a more scalable solution
- Handling transaction rollback when a service fails
- Ensuring eventual consistency in microservices architecture
- Implementing compensating transactions for failed operations
- Coordinating complex business processes spanning multiple services

**Trigger phrases**: distributed transactions, saga pattern, compensating transactions, microservices transaction, eventual consistency, rollback across services, orchestration pattern, choreography pattern

## Instructions

### 1. Design Transaction Flow

Map the sequence of operations and their compensating transactions:

```
Order → Payment → Inventory → Shipment
  ↓        ↓        ↓          ↓
Cancel  Refund   Release    Cancel
```

**Validation**: Verify every forward step has a corresponding compensation.

### 2. Choose Implementation Approach

| Approach | Use Case | Stack |
|----------|----------|-------|
| Choreography | Greenfield, few participants | Spring Cloud Stream + Kafka/RabbitMQ |
| Orchestration | Complex workflows, brownfield | Axon Framework, Eventuate Tram, Camunda |

**Validation**: Review team expertise and system complexity before choosing.

### 3. Implement Services with Local Transactions

Each service completes its local ACID transaction atomically:

```java
@Service
@RequiredArgsConstructor
public class OrderService {
    private final OrderRepository orderRepository;
    private final KafkaTemplate<String, Object> kafka;

    @Transactional
    public Order createOrder(CreateOrderCommand cmd) {
        Order order = orderRepository.save(new Order(cmd.orderId(), cmd.items()));
        kafka.send("order.created", new OrderCreatedEvent(order.getId(), order.getItems()));
        return order;
    }
}
```

**Validation**: Test that local transaction commits before event is published.

### 4. Implement Compensating Transactions

Every forward operation requires an idempotent compensation:

```java
@Service
@RequiredArgsConstructor
public class PaymentService {
    private final PaymentRepository paymentRepository;
    private final KafkaTemplate<String, Object> kafka;

    public void processPayment(PaymentRequest request) {
        Payment payment = paymentRepository.save(new Payment(request.orderId(), request.amount()));
        kafka.send("payment.processed", new PaymentProcessedEvent(payment.getId(), request.orderId()));
    }

    @Transactional
    public void refundPayment(String paymentId) {
        paymentRepository.findById(paymentId)
            .ifPresent(p -> {
                p.setStatus(REFUNDED);
                paymentRepository.save(p);
                kafka.send("payment.refunded", new PaymentRefundedEvent(paymentId));
            });
    }
}
```

**Validation**: Confirm compensation can execute safely multiple times (idempotency).

### 5. Set Up Message Broker

Configure Kafka with idempotent consumers:

```java
@Configuration
@EnableKafka
public class KafkaConfig {
    @Bean
    public ConcurrentKafkaListenerContainerFactory<String, Object> kafkaListenerContainerFactory(
            ConsumerFactory<String, Object> consumerFactory) {
        ConcurrentKafkaListenerContainerFactory<String, Object> factory =
            new ConcurrentKafkaListenerContainerFactory<>();
        factory.setConsumerFactory(consumerFactory);
        factory.setCommonErrorHandler(new DefaultErrorHandler());
        return factory;
    }
}
```

**Validation**: Enable transactional ID and verify exactly-once semantics.

### 6. Implement Saga Orchestrator (Orchestration Only)

```java
@Service
@RequiredArgsConstructor
public class OrderSagaOrchestrator {
    private final KafkaTemplate<String, Object> kafka;
    private final SagaStateRepository sagaStateRepo;

    public void startSaga(OrderRequest request) {
        String sagaId = UUID.randomUUID().toString();
        sagaStateRepo.save(new SagaState(sagaId, STARTED, LocalDateTime.now()));
        kafka.send("saga.order.start", new StartOrderSagaCommand(sagaId, request));
    }

    @KafkaListener(topics = "payment.failed")
    public void handlePaymentFailed(PaymentFailedEvent event) {
        kafka.send("order.compensate", new CompensateOrderCommand(event.getSagaId()));
        kafka.send("inventory.compensate", new ReleaseInventoryCommand(event.getSagaId()));
        sagaStateRepo.updateStatus(event.getSagaId(), FAILED);
    }
}
```

**Validation**: Verify saga state persists before sending commands. Check compensation triggers on each failure path.

### 7. Implement Event Handlers (Choreography Only)

```java
@Service
public class OrderEventHandler {
    private final OrderService orderService;
    private final KafkaTemplate<String, Object> kafka;

    @KafkaListener(topics = "payment.processed", groupId = "order-service")
    public void onPaymentProcessed(PaymentProcessedEvent event) {
        try {
            InventoryReservedEvent result = orderService.reserveInventory(event.toInventoryRequest());
            kafka.send("inventory.reserved", result);
        } catch (InsufficientInventoryException e) {
            kafka.send("inventory.insufficient", new InsufficientInventoryEvent(event.getOrderId(), event.getPaymentId()));
        }
    }
}
```

**Validation**: Test that each event handler correctly triggers the next step or compensation.

### 8. Add Monitoring and Observability

```java
@Configuration
public class SagaMetricsConfig {
    @Bean
    public MeterRegistry meterRegistry() {
        return new PrometheusMeterRegistry(PrometheusConfig.DEFAULT);
    }
}
```

Track: saga