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live35 ptscloses 11/16/2026, 10:07:33 AM

Replica reads are a time machine

Users update their profile, hit save, and the page shows the old value. Refresh a few seconds later: correct. Sometimes 40s later.

The scene

Reported: "I saved my bio and it reverted!", then on a later refresh it's back to the new text. Mostly in the evenings.

  • Writes go to pg-primary. All GETs go to a pool of 2 streaming replicas "to offload the primary".
  • The save handler does UPDATE profiles ... then responds 302 → /profile, which does a SELECT (on a replica).
  • SELECT now() - pg_last_xact_replay_timestamp() on replica-2 at 20:10: 00:00:41.
  • The nightly VACUUM FULL analytics_events (a 60GB table) is scheduled for 20:00 on the primary.

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