Spring Boot 4 Kafka Event Starter — Outbox, Idempotent Consumers & DLT

Reliable messaging takes days to wire: outbox, idempotency, retries, DLTs, Kafka KRaft Compose… This starter ships that done and tested on Spring Boot 4.1 and Java 21.

Companion to the REST/JWT API Starter (https://payhip.com/b/wXQ7a) — no auth here on purpose; events only.

What's included

- Transactional outbox + scheduled publisher

- Idempotent consumers

- Exponential retries + DLT

- Sample Order → Inventory saga

- Minimal REST to place/query orders

- Flyway + JPA (H2 in tests, Postgres in Compose)

- 16 tests (9 with EmbeddedKafka, no Docker for CI)

- compose.yaml, Dockerfile, GitHub Actions

- Guides in English and Spanish + commercial license

Requirements: JDK 21+. Docker optional for local Compose.

License: personal license for 1 developer; use and modify in unlimited projects. Reselling or redistributing the source as a template is not allowed.

Notice: source-code foundation, not a hosted service. Spring, Spring Boot, Kafka and Java are trademarks of their respective owners; this independent product is not affiliated with or endorsed by Broadcom, VMware, Oracle or the Apache Software Foundation.

---

v1.0.1 (7 Oct 2026): Fixed Dead Letter Topic routing. Failed messages from any partition now reach the DLT (DLTs have 3 partitions; Kafka picks the partition). New multi-partition tests, 16 in total. No API, topic or DB changes.