The dual-write problem occurs when an application must write to both a database and a message queue atomically — if either write fails, the system becomes silently inconsistent. The transactional outbox pattern solves this by writing an event row to an outbox table within the same database transaction as the business record, then using a separate relay worker to publish those rows to SQS asynchronously. A full Node.js implementation is walked through using PostgreSQL for the order service, SQS as the message queue, and DynamoDB for a downstream fulfillment service. Key implementation details include using FOR UPDATE SKIP LOCKED for safe concurrent relay instances, idempotent consumers via DynamoDB ConditionExpression, and local development using floci as an AWS emulator. Production considerations include dead-letter queues, retry counters, and change data capture as an alternative to polling.
