TELEICU/Rollback strategy
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How changes are rolled back in both migration paths.
Path A: Ansible
Procedure:
- Identify the offending commit(s) in the ansible repo
git revert <commit-hash>- Push to the relevant branch
- Nodes apply on next ansible-pull run (default: 15-30 min timer)
- For urgent rollback: SSH into nodes and run
ansible-pull -U <repo>immediately
Limitation: If the change modified system state that is not idempotent, a simple revert may not fully restore. Ansible roles should be written idempotently.
Path B: NixOS
Procedure:
- CLI rollback:
nixos-rebuild switch --rollback— switches to the previous generation immediately. No rebuild needed — the generation is already in the Nix store. - Boot rollback: Reboot and hold space during systemd-boot. Select the previous generation from the menu. Useful if the system is unbootable on the current generation.
- comin rollback: Point the flake back to the previous revision in git. comin will pull and activate the older generation on next poll.
Key advantage: Every NixOS generation is atomic and bootable. Rollback is always available, even if the current generation is partially broken.
Comparison
| Dimension | Ansible | NixOS |
|---|---|---|
| Speed | Depends on pull interval | Instant (CLI) or at boot |
| Completeness | Idempotency-dependent | Full — complete generation restore |
| Offline rollback | Not possible (needs git access) | Yes — generation is in local store |
| Granularity | Per-playbook or per-role | Full OS generation |
| Safety | Can leave partial state | Atomic — all-or-nothing |