Charmed PostgreSQL VM Tutorial > 3. Scale replicas
Scale your replicas
In this section, you will learn to scale your Charmed PostgreSQL by adding or removing juju units.
The Charmed PostgreSQL VM 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.
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, run
juju add-unit postgresql -n 2
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 overlord localhost/localhost 3.1.7 unsupported 10:16:44+01:00
App Version Status Scale Charm Channel Rev Exposed Message
postgresql active 3 postgresql 14/stable 281 no
Unit Workload Agent Machine Public address Ports Message
postgresql/0* active idle 0 10.89.49.129 Primary
postgresql/1 active idle 1 10.89.49.197
postgresql/2 active idle 2 10.89.49.175
Machine State Address Inst id Series AZ Message
0 started 10.89.49.129 juju-a8a31d-0 jammy Running
1 started 10.89.49.197 juju-a8a31d-1 jammy Running
2 started 10.89.49.175 juju-a8a31d-2 jammy Running
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/0, postgresql/1, and postgresql/2. Each of these units hosts a PostgreSQL replica.
To remove the replica hosted on the unit postgresql/2 enter:
juju remove-unit postgresql/2
You’ll know that the replica was successfully removed when juju status --watch 1s reports:
Model Controller Cloud/Region Version SLA Timestamp
tutorial overlord localhost/localhost 3.1.7 unsupported 10:17:14+01:00
App Version Status Scale Charm Channel Rev Exposed Message
postgresql active 2 postgresql 14/stable 281 no
Unit Workload Agent Machine Public address Ports Message
postgresql/0* active idle 0 10.89.49.129
postgresql/1 active idle 1 10.89.49.197
Machine State Address Inst id Series AZ Message
0 started 10.89.49.129 juju-a8a31d-0 jammy Running
1 started 10.89.49.197 juju-a8a31d-1 jammy Running
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