AdvancedTechnical
5 min
Exactly-Once Effects with the Outbox Pattern
ReliabilityMessagingDatabases
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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
- Write DB change and event to the same transaction (outbox table)
- Background relay publishes outbox to the broker with retries
- Use idempotency keys and deduplication at consumers
- Monitor outbox backlog and poison records via DLQ
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
- 💡Make outbox records immutable; process in order when needed.
Potential Follow-up Questions
- ❓How do you handle schema changes in outbox?
- ❓What about exactly-once with Kafka transactions?
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