System designHardsd-g509

Subject Messaging queuesLevel Senior–Staff~50 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

Design dead-letter and replay tooling for a large event platform. When a consumer fails a message N times it goes to a DLQ. You need operators to: inspect DLQ messages, see the failure reason and original headers, edit/fix a payload, and replay one message, a filtered subset, or an entire DLQ back into the original topic — without re-triggering already-succeeded downstream side effects or causing an infinite poison loop. Scale: 10K DLQ messages/min during a bad deploy, replay batches of millions.

What a strong answer looks like

Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts — data model, bottlenecks, consistency, failure modes — and name the trade-offs you are making.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.