Perform a minor rollback

Example: MySQL 8.0.34 → MySQL 8.0.33
(including charm revision bump: e.g Revision 43 → Revision 42)

After a juju refresh, if there are any version incompatibilities in charm revisions, its dependencies, or any other unexpected failure in the upgrade process, the process will be halted and enter a failure state.

Even if the underlying MySQL cluster continue to work, it’s important to roll back the charm to a previous revision so that an update can be attempted after further inspection of the failure.

Summary of the rollback steps

1. Prepare the Charmed MySQL application for the in-place rollback.
2. Roll back. Once started, all units in a cluster will be executed sequentially. The rollback will be aborted (paused) if the unit rollback has failed.
3. Check. Make sure the charm and cluster are in healthy state again.

Step 1: Prepare

To execute a rollback, we use a similar procedure to the upgrade. The difference is the charm revision to upgrade to. In this guide’s example, we will refresh the charm back to revision 182.

It is necessary to re-run pre-upgrade-check action on the leader unit in order to enter the upgrade recovery state:

juju run mysql/leader pre-upgrade-check

Step 2: Rollback

When using charm from charmhub:

juju refresh mysql --revision=182

When deploying from a local charm file, one must have the previous revision charm file and run the following command:

juju refresh mysql --path=./mysql_ubuntu-22.04-amd64.charm

where mysql_ubuntu-22.04-amd64.charm is the previous revision charm file.

The first unit will be rolled out and should rejoin the cluster after settling down. After the refresh command, the juju controller revision for the application will be back in sync with the running Charmed MySQL revision.

Step 3: Check

Future improvements are planned to check the state on pods/clusters on a low level.

For now, check juju status to make sure the cluster state is OK.