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 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 with:

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  2.9.42   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 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/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  2.9.42   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