Scale your Charmed PostgreSQL

This is part of the Charmed PostgreSQL Tutorial. Please refer to this page for more information and the overview of the content.

Adding and Removing units

Charmed PostgreSQL K8s operator uses PostgreSQL Patroni-based cluster for scaling. It provides features such as automatic membership management, fault tolerance, automatic failover, and so on. The charm uses Postgres’s Synchronous replication with Patroni.

! Disclaimer: this tutorial hosts replicas all on the same machine, this should not be done in a production environment. To enable high availability in a production environment, replicas should be hosted on different servers to maintain isolation.

Add cluster members (replicas)

You can add two replicas to your deployed PostgreSQL application by scaling it to three units using:

juju scale-application postgresql-k8s 3

You can now watch the scaling process in live using: juju status --watch 1s. It usually takes several minutes for new cluster members to be added. You’ll know that all three nodes are in sync when juju status reports Workload=active and Agent=idle:

Model     Controller  Cloud/Region        Version  SLA          Timestamp
tutorial  charm-dev   microk8s/localhost  2.9.42   unsupported  12:09:49+01:00

App             Version  Status  Scale  Charm           Channel    Rev  Address         Exposed  Message
postgresql-k8s           active      3  postgresql-k8s  14/stable  56   10.152.183.167  no

Unit               Workload  Agent  Address       Ports  Message
postgresql-k8s/0*  active    idle   10.1.188.206         Primary
postgresql-k8s/1   active    idle   10.1.188.209
postgresql-k8s/2   active    idle   10.1.188.210

Remove cluster members (replicas)

Removing a unit from the application, scales the replicas down. Before we scale down the replicas, list all the units with juju status, here you will see three units postgresql-k8s/0, postgresql-k8s/1, and postgresql-k8s/2. Each of these units hosts a PostgreSQL replica. To scale the application down to two units, enter:

juju scale-application postgresql-k8s 2

You’ll know that the replica was successfully removed when juju status --watch 1s reports:

Model     Controller  Cloud/Region        Version  SLA          Timestamp
tutorial  charm-dev   microk8s/localhost  2.9.42   unsupported  12:10:08+01:00

App             Version  Status  Scale  Charm           Channel    Rev  Address         Exposed  Message
postgresql-k8s           active      2  postgresql-k8s  14/stable  56   10.152.183.167  no

Unit               Workload  Agent  Address       Ports  Message
postgresql-k8s/0*  active    idle   10.1.188.206         Primary
postgresql-k8s/1   active    idle   10.1.188.209