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.

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, 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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