How to upgrade between minor versions

Charm upgrades can include both upgrades of operator code (e.g. the revision used by the charm) and/or the workload version. Note that since the charm code pins a particular version of the workload, a charm upgrade may or may not involve also a workload version upgrade.

In general, the following guide only applies for in-place upgrades that involve (at most) minor version upgrades of Apache Kafka workload, e.g. between Apache Kafka 3.4.x to 3.5.x. Major workload upgrades are generally NOT SUPPORTED, and they should be carried out using full cluster-to-cluster migrations.

While upgrading an Apache Kafka cluster, do not perform any other major operations, including, but not limited to, the following:

1. Adding or removing units
2. Creating or destroying new relations
3. Changes in workload configuration
4. Upgrading other connected applications (e.g. Charmed Apache ZooKeeper)

The concurrency with other operations is not supported, and it can lead the cluster into inconsistent states.

Minor upgrade process overview

When performing an in-place upgrade process, the full process is composed of the following high-level steps:

1. Collect all necessary pre-upgrade information, necessary for a rollback (if ever needed)
2. Prepare the charm for the in-place upgrade, by running some preparatory tasks
3. Upgrade the charm and/or the workload. Once started, all units in a cluster will refresh the charm code and undergo a workload restart/update. The upgrade will be aborted if the unit upgrade has failed, requiring the admin user to roll back.
4. Post-upgrade checks to make sure all units are in the proper state and the cluster is healthy.

Step 1: Collect

The first step is to record the revisions of the running application, as a safety measure for a rollback action if needed. To accomplish this, simply run the juju status command and look for the revisions of the deployed Charmed Apache Kafka and Charmed Apache ZooKeeper applications. You can also retrieve this with the following command (that requires yq to be installed):

KAFKA_CHARM_REVISION=$(juju status --format json | yq .applications.<KAFKA_APP_NAME>.charm-rev)
ZOOKEEPER_CHARM_REVISION=$(juju status --format json | yq .applications.<ZOOKEEPER_APP_NAME>.charm-rev)

Please fill <KAFKA_APP_NAME> and <ZOOKEEPER_APP_NAME>} placeholder appropriately, e.g. kafka-k8s and zookeeper-k8s.

Step 2: Prepare

Before upgrading, the charm needs to perform some preparatory tasks to define the upgrade plan.
To do so, run the pre-upgrade-check action against the leader unit:

juju run kafka/leader pre-upgrade-check --format yaml

Make sure that the output of the action is successful.

The action will also configure the charm to minimize high-availability reduction and ensure a safe upgrade process. After successful execution, the charm is ready to be upgraded.

Step 3: Upgrade

Use the juju refresh command to trigger the charm upgrade process. Note that the upgrade can be performed against:

• selected channel/track, therefore upgrading to the latest revision published on that track:

  juju refresh kafka-k8s --channel 3/edge
  

• selected revision:

  juju refresh kafka-k8s --revision=<REVISION>
  

• a local charm file:

  juju refresh kafka-k8s --path ./kafka_ubuntu-22.04-amd64.charm
  

When issuing the commands, only the unit with the largest ordinal number will refresh (i.e. receive new charm content). A new pod will be rescheduled with the new image and/or the new charm revision. As the Pod starts, the upgrade charm event will be fired on the pod, running any custom logic required during the upgrade.

Once the unit is returned active and healthy, the charm will block the upgrade process, to provide a way for the user to manually check that the new version works properly and make sure everything is fine before rolling out the upgrade also on the remaining units.

If the manual checks are successful, the upgrade process can be resumed with

juju run kafka-k8s/leader resume-upgrade --format yaml  --wait=1m 

The upgrade process can be monitored using juju status command, where the message of the units will provide information about which units have been upgraded already, which unit is currently upgrading and which units are waiting for the upgrade to be triggered, as shown below:

...

App        Version  Status  Scale  Charm      Channel   Rev  Exposed  Message
kafka               active      3  kafka      3/stable  147  no

Unit          Workload  Agent  Address       Ports  Message
...
kafka/0       active    idle   10.193.41.131        Other units upgrading first...
kafka/1*      active    idle   10.193.41.109        Upgrading...
kafka/2       active    idle   10.193.41.221        Upgrade completed
...

Failing upgrade

Before upgrading the unit, the charm will check whether the upgrade can be performed, e.g. this may mean:

1. Checking that the upgrade from the previous charm revision and Apache Kafka version is allowed.
2. Checking that other external applications that Apache Kafka depends on (e.g. Apache ZooKeeper) are running the correct version.

Note that these checks are only possible after a refresh of the charm code, and therefore cannot be done upfront (e.g. during the pre-upgrade-checks action). If some of these checks fail, the upgrade will be aborted. When this happens, the workload may still be operating (as only the operator may have failed) but we recommend rolling back the upgrade as soon as possible.

To roll back the upgrade, re-run steps #2 and #3, using the revision taken in step 1:

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

juju refresh kafka-k8s --revision=${KAFKA_CHARM_REVISION}

We strongly recommend to also retrieve the full set of logs with juju debug-log, to extract insights on why the upgrade failed.

Combined upgrades

If Charmed Apache Kafka and Charmed Apache ZooKeeper both need to be upgraded, we recommend starting the upgrade from the Charmed Apache ZooKeeper. As outlined above, the two upgrades should NEVER be done concurrently.