Lambda throttle backlog
Your payment-webhook fan-out runs SQS → Lambda. At 18:50 the `ApproximateNumberOfMessagesVisible` on the `webhook-dispatch` queue jumps from ~500 to 220k and keeps climbing; `ApproximateAgeOfOldestMessage` is now 9 minutes and rising. The Lambda's `ConcurrentExecutions` is pinned flat at 1,000 and `Throttles` is non-zero and growing. A marketing push went out at 18:45 that 3x'd inbound webhook volume. Downstream the merchant-notification API (called by the Lambda) shows p99 latency up from 80ms to 1,100ms. Walk through triage and mitigation.
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.
0:00 of about 30 min
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.