You Wrote To Two Databases. Did Both Succeed?
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When writing to two databases (e.g., MongoDB and ClickHouse), a silent consistency gap can occur if the app crashes between writes. Two patterns address this: RabbitMQ with a Dead Letter Queue offers sub-second delivery but has an unrecoverable gap if the app crashes before publishing the message. The Transactional Outbox Pattern eliminates this gap by atomically writing both the entity and an outbox record in the same MongoDB transaction, then using a relay worker to deliver to ClickHouse with at-least-once semantics and deduplication via ReplacingMergeTree. The author recommends the outbox pattern for data-integrity-critical workloads, with RabbitMQ reserved for cases requiring sub-second latency or multiple consumers.
