Charmed PostgreSQL K8s How-to - Perform a minor rollback

Perform a minor rollback

Example: PostgreSQL 14.9 → PostgreSQL 14.8
(including simple charm revision bump: from revision 43 to revision 42)

Warning: Do NOT trigger rollback during the running upgrade action! It may cause an unpredictable PostgreSQL cluster state!

Note: All commands are written for juju >= v.3.0

If you are using an earlier version, be aware that:

  • juju run replaces juju run-action --wait in juju v.2.9
  • juju integrate replaces juju relate and juju add-relation in juju v.2.9

For more information, check the Juju 3.0 Release Notes.

Manual rollback

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 PostgreSQL cluster continues 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.

Minor rollback steps

  1. Prepare the Charmed PostgreSQL K8s application for the in-place rollback.
  2. Rollback. Perform the first charm rollback - only the first unit. The unit with the maximal ordinal will be chosen.
  3. Resume. Continue rollback to all other units if the first unit rolled back successfully.
  4. Check. Make sure the charm and cluster are in healthy state again.

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 88.

Step 1: Prepare

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

juju run postgresql-k8s/leader pre-upgrade-check

Step 2: Rollback

When using a charm from charmhub:

juju refresh postgresql-k8s --revision=88

When deploying from a local charm file, one must have the previous revision charm file and the postgresql-image resource, then run

juju refresh postgresql-k8s --path=./postgresql-k8s_ubuntu-22.04-amd64.charm \

…where postgresql-k8s_ubuntu-22.04-amd64.charm is the previous revision charm file. The reference for the resource for a given revision can be found at the metadata.yaml file in the charm repository.

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 PostgreSQL K8s revision.

Step 3: Resume

We still need to resume the upgrade on the remaining units, which is done with the resume-upgrade action.

juju run postgresql-k8s/leader resume-upgrade

This will roll out the pods in the remaining units, but to the same charm revision.

Step 4: Check

Future improvements are planned to check the state on pods/clusters on a low level. At the moment, check juju status to make sure the cluster state is OK.