The Message That Was Delivered Twice Because It Was Slow
Read what each party saw, commit to a cause, and only then find out which of them was right. The root cause, why the obvious reading was wrong, and the fixes are all held back until you have answered.
The symptom
What was reported, before anyone knew what was happening.
What each party saw, and what each concluded
The evidence, in the form it actually arrived in — several parties, several partial views, several confident conclusions.
| Who | What they could actually see | What they concluded | Verdict |
|---|---|---|---|
| Worker 1 | A message received at 10:00:00 with a 5-minute visibility timeout. Transcoding a 40-minute video took 7 minutes. Deleted the message at 10:07 successfully. | I processed this message and removed it. | ✕ wrong |
| Queue | No delete within the visibility timeout; message became visible again at 10:05. | The consumer failed. Redeliver. | ✕ wrong |
| Worker 2 | The message at 10:05. | An unprocessed job. | ✕ wrong |
| Engineer | No crashes, no errors, duplicates only on long videos. | The queue is redelivering messages it should not. | ✕ wrong |
4 of 4 parties reasoned correctly from what they could see and still reached the wrong conclusion. Nobody in this table is careless. Each one acted on complete-looking local information, and the information was local. That gap — between what a node can observe and what is true — is the whole domain, and one of these readings will usually be yours.
Commit before you read on
No worker crashed and every delete succeeded. Commit before reading on: state what a visibility timeout is actually a bet about, and explain why the duplicates correlate with video length.
Write it down even if you are unsure. An unwritten guess quietly becomes “that is what I thought” the moment you read the answer.