Implementing the Inbox Pattern for Reliable Message Consumption

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.