Perform a minor upgrade

Example: PostgreSQL 14.8 → PostgreSQL 14.9
(including simple charm revision bump: from revision 99 to revision 102)

This guide is part of Charmed PostgreSQL K8s Upgrades. Please refer to this page for more information and an overview of the content.

Before upgrading

Before performing a minor PostgreSQL upgrade, there are some important considerations to take into account:

• Concurrency with other operations during the upgrade
• Backing up your data
• Service disruption

Concurrency with other operations

We strongly recommend to NOT perform any other extraordinary operations on Charmed PostgreSQL K8s cluster while upgrading.

Some examples are operations like (but not limited to) the following:

• Adding or removing units
• Creating or destroying new relations
• Changes in workload configuration
• Upgrading other connected/related/integrated applications simultaneously

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

Backups

Make sure to have a backup of your data when running any type of upgrade.

Guides on how to configure backups with S3-compatible storage can be found here.

Service disruption

It is recommended to deploy your application in conjunction with the Charmed PgBouncer K8s operator.

This will ensure minimal service disruption, if any.

Minor upgrade steps

Here is a summary of the steps to perform a minor upgrade.:

1. Collect all necessary pre-upgrade information. It will be necessary for a rollback, if needed. Do NOT skip this step, it is better to be safe than sorry!
2. (optional) Scale up. The new unit will be the first one to be updated, and it will simplify the rollback procedure a lot in case of an upgrade failure.
3. Prepare your Charmed PostgreSQL Juju application for the in-place upgrade. See the step description below for all technical details executed by charm here.
4. Upgrade - phase 1. Once started, only one unit in a cluster will be upgraded. In case of failure, the rollback is simple: remove the pod that was recently added in Step 2: Scale up.
5. Resume upgrade - phase 2. If the new pod is OK after the refresh, the upgrade can be resumed for all other units in the cluster. All units in a cluster will be executed sequentially: from biggest ordinal to the lowest one.
6. (optional) Consider rolling back in case of disaster. Please inform and include us in your case scenario troubleshooting to trace the source of the issue and prevent it in the future. Contact us!
7. (optional) Scale back. Remove no longer necessary K8s pod created in step 2 (if any).
8. Do a post-upgrade check. Make sure all units are in the proper state and the cluster is healthy.

Step 1: Collect

The first step is to record the revision of the running application as a safety measure for a rollback action. To accomplish this, simply run the juju status command and look for the deployed Charmed PostgreSQL revision in the command output, e.g.:

Model        Controller  Cloud/Region        Version  SLA          Timestamp
welcome-k8s  microk8s    microk8s/localhost  3.1.6    unsupported  12:23:03+02:00

App             Version  Status  Scale  Charm           Channel  Rev  Address         Exposed  Message
postgresql-k8s  14.9     active      3  postgresql-k8s  14/beta  145  10.152.183.166  no       

Unit               Workload  Agent  Address     Ports  Message
postgresql-k8s/0*  active    idle   10.1.12.12         Primary
postgresql-k8s/1   active    idle   10.1.12.19         
postgresql-k8s/2   active    idle   10.1.12.20     

For this example, the current revision is 145. Store it safely to use in case of a rollback!

Step 2: Scale-up (optional)

It is recommended to scale the application up by one unit before starting the upgrade process.

The new unit will be the first one to be updated, and it will assert that the upgrade is possible. In case of failure, having the extra unit will ease the rollback procedure without disrupting service. You can read more about this in the Minor rollback guide.

You can scale your application using the following command:

juju scale-application postgresql-k8s <current_units_count+1>

Wait for the new unit to be up and ready.

Step 3: Prepare

After the application has settled, it’s necessary to run the pre-upgrade-check action against the leader unit:

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

Make sure there are no errors in the Juju output.

This action will configure the charm to minimize the amount of primary switchover, among other preparations for the upgrade process. After successful execution, the charm is ready to be upgraded.

Step 4: Upgrade

Use the juju refresh command to trigger the charm upgrade process. If using juju version 3 or higher, it is necessary to add the --trust option.

Example with channel selection and juju 2.9.x:

juju refresh postgresql-k8s --channel 14/edge

Example with channel selection and juju 3.x:

juju refresh postgresql-k8s --channel 14/edge --trust

Example with specific revision selection:

juju refresh postgresql-k8s --revision=189

Important Notes

The upgrade will execute only on the highest ordinal unit.

For the running example postgresql-k8s/3, the juju status will look like:

Model        Controller  Cloud/Region        Version  SLA          Timestamp
welcome-k8s  microk8s    microk8s/localhost  3.1.6    unsupported  12:26:32+02:00

App             Version  Status   Scale  Charm           Channel  Rev  Address         Exposed  Message
postgresql-k8s  14.9     waiting      4  postgresql-k8s  14/edge  154  10.152.183.166  no       installing agent

Unit               Workload     Agent      Address     Ports  Message
postgresql-k8s/0*  waiting      idle       10.1.12.12         other units upgrading first...
postgresql-k8s/1   waiting      idle       10.1.12.19         other units upgrading first...
postgresql-k8s/2   waiting      idle       10.1.12.20         other units upgrading first...
postgresql-k8s/3   maintenance  executing  10.1.12.23         upgrading unit

Do NOT trigger rollback procedure during the running upgrade procedure. It is expected to have some status changes during the process: waiting, maintenance, active.

Make sure upgrade has failed/stopped and cannot be fixed/continued before triggering rollback!

Please be patient during huge installations. The unit should recover shortly after, but the time can vary depending on the amount of data written to the cluster while the unit was not part of it.

Step 5: Resume

After the unit is upgraded, the charm will set the unit upgrade state as completed. If deemed necessary, the user can further assert the success of the upgrade.

Given that the unit is healthy within the cluster, the next step is to resume the upgrade process by running:

juju run postgresql-k8s/leader resume-upgrade 

The resume-upgrade command will roll out the upgrade for the following unit, always from highest to lowest. For each successful upgraded unit, the process will roll out the next one automatically.

Sample juju status output:

Model        Controller  Cloud/Region        Version  SLA          Timestamp
welcome-k8s  microk8s    microk8s/localhost  3.1.6    unsupported  12:28:38+02:00

App             Version  Status   Scale  Charm           Channel  Rev  Address         Exposed  Message
postgresql-k8s  14.9     waiting      4  postgresql-k8s  14/edge  154  10.152.183.166  no       installing agent

Unit               Workload     Agent      Address     Ports  Message
postgresql-k8s/0*  waiting      executing  10.1.12.12         other units upgrading first...
postgresql-k8s/1   waiting      executing  10.1.12.19         other units upgrading first...
postgresql-k8s/2   maintenance  executing  10.1.12.24         (config-changed) upgrading unit
postgresql-k8s/3   maintenance  executing  10.1.12.23         upgrade completed

Step 6: Rollback (optional)

This step must be skipped if the upgrade went well!

Although 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. Please switch to the dedicated minor rollback guide for more information about this process.

Step 7: Scale back (optional)

If the application scale was changed for the upgrade procedure, it is now safe to scale it back to the desired unit count:

juju scale-application postgresql-k8s <unit_count>

Step 8: Check

Future improvements are planned to check the state of a pod/cluster on a low level.

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