Charmed PostgreSQL K8s Tutorial > 4. Scale replicas

Scale your replicas

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

In this section, you will learn to scale your Charmed PostgreSQL by adding or removing units (replicas).

Summary

• Add units
• Remove units

Add units

Currently, your deployment has only one juju unit, known in juju as the leader unit. You can think of this as the database primary instance. For each replica, a new unit is created. All units are members of the same database cluster.

To add two replicas to your deployed PostgreSQL application, use juju scale-application to scale it to three units:

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 units

Removing a unit from the application scales down the replicas.

Before we scale them down, list all the units with juju status. 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

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