# Rolling deploy mixed versions: first 30 minutes checklist

Source: https://releasefailures.com/incidents/rolling-deploy-mixed-versions/
Content version: 1.0.0
Verified on: 2026-10-22
Licence: CC BY 4.0

Definition: Old and new versions serve traffic at once and disagree on a contract.
- [ ] FM-01 Check whether failed requests began right after a rolling deploy started, and split the errors by application version.
- [ ] FM-02 Check whether the web fleet and the background workers run different versions, such as a web canary ahead of the workers.
- [ ] FM-03 Drain the canary fleet to stop new pipelines or sessions reaching the mixed-version path.
- [ ] FM-04 Decide on the rollback knowing it opens a second mixed-version window, and run it with the widest deployment parallelism the fleet allows.
- [ ] FM-05 Mark the faulty version unsafe so a pipeline unlock or a queued deployment cannot redeploy it.
- [ ] FM-06 If the automated rollback is blocked, run a manual rollback to the previous code.
- [ ] FM-07 Post a status update that names the failing surface, such as web sessions or pipeline status, and the surfaces that still work.