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).
This tutorial hosts all replicas 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.
Summary
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
Note: Unlike machine models, Kubernetes models use juju scale-application instead of juju add-unit and juju remove-unit. For more information about juju’s scaling logic for kubernetes, check this post.
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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