The Saga pattern is a design pattern used in microservices architecture to manage long-running transactions that span multiple microservices. A saga is a sequence of local transactions that are coordinated using a Saga Orchestrator, which ensures that each step of the saga is executed in the correct order and compensates for any failed transactions.
In Java microservices, the Saga pattern can be implemented using frameworks such as Eventuate or Axon Framework. Here’s an example of how the Saga pattern can be implemented in Java using the Axon Framework:
public class OrderSaga {
@Inject
private transient CommandGateway commandGateway;
@StartSaga
@SagaEventHandler(associationProperty = “orderId”)
public void handle(OrderCreatedEvent event) {
commandGateway.send(new ReserveCreditCommand(event.getOrderId(), event.getOrderTotal()));
}
@SagaEventHandler(associationProperty = “orderId”)
public void handle(CreditReservedEvent event) {
commandGateway.send(new ShipOrderCommand(event.getOrderId()));
}
@SagaEventHandler(associationProperty = “orderId”)
public void handle(OrderShippedEvent event) {
commandGateway.send(new MarkOrderAsCompletedCommand(event.getOrderId()));
}
@EndSaga
@SagaEventHandler(associationProperty = “orderId”)
public void handle(OrderCompletedEvent event) {
// Saga completed
}
@SagaEventHandler(associationProperty = “orderId”)
public void handle(OrderCancelledEvent event) {
commandGateway.send(new ReleaseCreditCommand(event.getOrderId(), event.getOrderTotal()));
}
}
In this example, the OrderSaga handles the OrderCreatedEvent, which is triggered when a new order is created. The @StartSaga annotation indicates that this is the first step in the saga. The handle method sends a ReserveCreditCommand to reserve credit for the order.
The @SagaEventHandler annotation indicates that the method handles events that are part of the saga. The CreditReservedEvent is handled by sending a ShipOrderCommand to ship the order. The OrderShippedEvent is handled by sending a MarkOrderAsCompletedCommand to mark the order as completed. The @EndSaga annotation indicates that this is the final step in the saga. The OrderCompletedEvent marks the end of the saga.
If any of the commands fail, the Saga Orchestrator will compensate for the failed transaction by sending a ReleaseCreditCommand to release the reserved credit.
By using the Saga pattern, long-running transactions that span multiple microservices can be managed effectively. The Saga Orchestrator ensures that each step of the saga is executed in the correct order and compensates for any failed transactions, making the system more resilient and reliable.