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Recorded on 26 October 2026, unchanged since

Entry #59 of UpFork’s ledger · A lesson's version: FOR UPDATE SKIP LOCKED rechecks only the conditions written in its own step: two workers can claim the same row Open it

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Recorded: 26 Oct 2026, 08:27 UTC
Signed with: k1 · Ed25519
Entry hash: 990183cb0316e5a078720983468a78f60a89416399ce85aac15d732fb59e672f
Before it: 356d62f1a990fa0704831c0db515a1a8c73016694319c6cfc1308bc0e72f546a
Record hash: ea1b42cf8faad8eebbd82741d99c2a1d87cd12de8d80cbbddadfebd3640adbd4
Signature: SqvRFmikohIi5B2HcpF243RQmzAFaqxH+hjh2f3oniC6ASHQezECUWo38Xa5eELR5zCYqpd7JbOJTKfJml/dAQ==

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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "article": "telegram-bots-on-postgresql-alone-sponsor-bot-s-queues-forced-join-gate-and",
  "articleRevision": 1,
  "assumed": "a claim query that picks candidates in one CTE and then locks them with `FOR UPDATE SKIP LOCKED` in the next could never hand the same row to two workers.",
  "author": {
    "account": "a_9d470b566acab5f65bdc",
    "handle": "ramin-valadan"
  },
  "evidence": "commit 7eecc9f (phase 12, \"a race that let two workers handle the same update\").",
  "fix": "the locking step repeats the status conditions (`status = 'Pending' AND next_attempt_at <= @now`, or an expired Processing lock). Exactly one answer per update since.",
  "happened": "under load, a worker's claim that finished between another worker's two steps had already set the row to Processing; the second worker's lock step, which only said `WHERE id IN (candidates)`, took it anyway. 24% more messages went out than people came in.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-9DDS",
  "packages": "",
  "product": "telegram-uploader-bot-forced-join",
  "productRevision": null,
  "publishedAt": "2026-10-26T08:26:45.701Z",
  "site": "https://upfork.dev",
  "title": "FOR UPDATE SKIP LOCKED rechecks only the conditions written in its own step: two workers can claim the same row",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-9DDS",
  "version": 1,
  "why": "after waiting for or skipping a lock, PostgreSQL rechecks only the conditions written in that same step, not the ones in an earlier CTE."
}