Exactly-Once Effects with the Outbox Pattern

Interview Question

You need reliable event publication coupled with database writes. Describe how you’d implement the outbox pattern and ensure idempotency end to end.

Key Points to Cover

Evaluation Rubric

Couples DB write and event atomically35% weight

Ensures idempotent, deduplicated processing25% weight

Describes relay/monitoring/DLQ operations20% weight

Considers scaling of relays/consumers20% weight

Hints

Common Pitfalls to Avoid

⚠️Treating the outbox relay as a simple poll loop without proper concurrency control, leading to duplicate event publications or missed events.
⚠️Failing to make the outbox table write and the business data write atomic within the same transaction.
⚠️Implementing idempotency at the consumer without also ensuring the idempotency key is persisted with the event in the outbox.
⚠️Ignoring the possibility of the relay service itself failing and not implementing persistent storage for its progress or state.
⚠️Not having a clear strategy for handling events that consistently fail to be published or processed, leading to unbounded retry loops or orphaned messages.

Potential Follow-up Questions