Replaying and discarding messages - ep. 10

A new subscriber version can mistakenly acknowledge messages before processing them. Pub/Sub's seek operation offers a recovery path: change many messages back to unacknowledged so they can be delivered again. The episode compares restoring a saved snapshot with selecting a timestamp 00:00–01:03(opens in a new tab).

Key points

• Snapshots preserve a point to replay from: A snapshot retains messages unacknowledged at creation plus later publications. It can be used by subscriptions on the same topic. In the episode's example, a seven-day retention window with a one-day-old backlog leaves the snapshot six days before expiry 01:03–02:07(opens in a new tab).
• Time-based replay divides messages around a timestamp: Earlier messages become acknowledged; later ones become unacknowledged. This requires retaining acknowledged messages. The episode notes added cost and possible inaccuracies from differences between server clocks 01:03–03:10(opens in a new tab).
• Take a snapshot before changing subscriber code: The episode recommends including snapshots in deployment so a buggy release has a recovery point. The same snapshot can also supply the same input messages across repeated tests 02:07–03:10(opens in a new tab).
• Seeking into the future discards backlog by marking it acknowledged: That is a destructive purge operation, not a harmless reset 03:10(opens in a new tab).

Full video: https://www.youtube.com/watch?v=VyLmmamuOVo(opens in a new tab)