spring-boot-event-driven-patterns

Event-driven architecture in Spring Boot uses domain events published through ApplicationEventPublisher and consumed via @EventListener or @TransactionalEventListener. This decouples services and enables asynchronous, eventual-consistency communication patterns for distributed systems.

@EventListener processes events immediately within the same transaction, while @TransactionalEventListener waits until the transaction commits before firing, ensuring transactional consistency and preventing event processing if the transaction rolls back.

Define domain events as POJOs, publish them via ApplicationEventPublisher, and bind them to Kafka topics using Spring Cloud Stream's messaging abstraction. This integrates in-process events with distributed Kafka brokers for cross-service communication.

Yes. Event-driven patterns combined with transactional listeners and the outbox pattern ensure eventual consistency. Services publish domain events transactionally, listeners process them idempotently, and Kafka guarantees delivery across service boundaries.

The transactional outbox pattern writes events to a database outbox table atomically with business data, then publishes them to Kafka. Use it to guarantee event delivery and prevent lost messages when publishing to external systems.

Implement idempotent event handlers by tracking processed event IDs, using unique constraints on side effects, or designing handlers as pure functions. This prevents duplicate processing when Kafka redelivers events or listeners retry failed operations.