What is the Outbox Pattern?

Outbox Pattern is a microservice design technique where events are first stored in a local outbox table during a database transaction and then forwarded asynchronously to a message broker, ensuring reliable delivery without data loss.

When to Use

Use the Outbox Pattern when you need to update a database and publish an event together. It’s ideal in order-processing, payment, or user-signup workflows where losing events can cause inconsistencies.

Example

In an e-commerce app, placing an order writes to the database and also logs an “Order Placed” event into an outbox table. A dispatcher later sends that event to a broker so inventory and email services are updated reliably.

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Why Is It Important

It guarantees eventual consistency without distributed transactions. If the DB update succeeds, the event is guaranteed to be delivered—even after crashes.

Interview Tips

Explain how it solves the dual write problem. Mention the transactional insert + background dispatcher, stress idempotent consumers, and note that the outbox must be cleaned regularly.

Trade-offs

Pros: strong reliability, avoids two-phase commit. Cons: adds dispatcher complexity, introduces slight delivery delay, and requires duplicate handling.