Message visibility timeout
An SQS-driven `pdf-render` worker fleet alerts at 04:00: rendered PDFs are appearing in S3 2–3 times for the same job, and downstream gets duplicate 'render complete' webhooks. Dashboards: `NumberOfMessagesReceived` is ~2.3x `NumberOfMessagesDeleted`; `ApproximateAgeOfOldestMessage` is rising; the queue's visibility timeout is 30s. Recent context: a deploy at 03:50 added an embedded-font fetch step that made the average render time climb from ~12s to ~45s. The handler deletes the message only AFTER the render+upload completes. How do you triage and 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.
0:00 of about 35 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.