Code Room
On-callHardoc-g168
Subject Dead letter overflowLevel Senior–Staff~40 minCommon in Reliability & on-call interviewsIndustries Technology, Software development

Question

RabbitMQ setup: a `payments` work queue dead-letters (via DLX) to a `payments-retry` queue with a message TTL of 60s, and that retry queue dead-letters *back* to `payments` for another attempt (a delayed-retry pattern). At 08:00, broker CPU and message rates spike: the `payments-retry` queue depth oscillates around 80k, the `payments` queue shows a huge `redelivered` rate, and the broker's overall publish+deliver throughput is ~10x normal with no increase in real business traffic. Recent context: a downstream payment processor started returning errors at 07:45 for ~30% of charges. Triage and explain why the broker is melting, and how you mitigate.

What a strong answer looks like

Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.

Diagram & narrate the incident
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Run or narrate your approach, then ask the coach.