The Outbox pattern guarantees reliable publishing. But what about the consumer side? The Inbox pattern ensures each incoming message is processed exactly once, even when the broker retries or delivers duplicates. Here's how to implement it in .NET with MassTransit and PostgreSQL.

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Milan Jovanović

The Inbox pattern is the consumer-side counterpart to the Outbox pattern, solving the duplicate message processing problem that arises from at-least-once broker delivery. The implementation uses a PostgreSQL inbox_messages table with ON CONFLICT DO NOTHING for idempotent inserts, a generic MassTransit IConsumer that writes incoming messages to the table instead of processing them directly, and a background processor that fetches unprocessed messages in batches using FOR UPDATE SKIP LOCKED for safe concurrent processing. Batch updates use UNNEST to minimize round-trips. The post also covers operational concerns like table growth, poison messages, ordering guarantees, and monitoring, plus a comparison between the Inbox pattern and the Idempotent Consumer pattern explaining when to use each.

Implementing the Inbox Pattern for Reliable Message Consumption