Why Transaction Boundaries Make or Break Event Processing in Spring Boot

When building event-driven systems, ensuring correct behavior during partial failures is a critical challenge, particularly around the Inbox pattern used to make event consumers idempotent. The Inbox pattern tracks whether an event has already been processed, but problems arise when the Inbox update and the business operation do not share the same database transaction boundary. If the business logic commits successfully but the Inbox update fails separately, the broker may redeliver the event and trigger duplicate processing. Conversely, if the Inbox is marked as processed before the business transaction commits and that transaction then rolls back, the system enters a contradictory state where recovery becomes impossible. The key insight is that Inbox state and business state must be managed within the same transactional boundary to guarantee consistency and reliable idempotency.