Production-ready event-driven backend implementing the Transactional Outbox Pattern — the canonical solution to the dual-write problem in distributed architectures, used at scale by Netflix, Uber, and Amazon. Guarantees atomic, exactly-once event delivery across service boundaries without distributed transactions or two-phase commit. Routes 4 domain event types (USER_CREATED, ORDER_PLACED, PAYMENT_RECEIVED, INVENTORY_UPDATE) through a strict three-state lifecycle machine with sub-second latency, HikariCP connection pooling, type-based multi-path routing, and idempotency enforcement to make the entire pipeline self-healing.
In distributed systems, writing to a database and publishing to an event bus in a single atomic operation is inherently unreliable — if the DB write succeeds but the message publish fails (or vice versa), data silently diverges across service boundaries with no clean, consistent recovery path.
Applied the Transactional Outbox Pattern: events are written atomically into an outbox table inside the same DB transaction as the business operation, guaranteeing zero message loss. A separate n8n async processor polls the table, routes events by type to the correct handler, transitions each event through a strict state machine (PENDING → PROCESSING → COMPLETED), and enforces idempotency to eliminate duplicate side-effects.