Charmed MySQL K8s Tutorial > 3. Scale your replicas

Scale your replicas

In this section, you will learn to scale your Charmed MySQL K8s by adding or removing juju units.

The Charmed MySQL K8s operator uses MySQL InnoDB Cluster for scaling. It is built on MySQL Group Replication, providing features such as automatic membership management, fault tolerance, and automatic failover.

An InnoDB Cluster usually runs in a single-primary mode, with one primary instance (read-write) and multiple secondary instances (read-only).

Summary

• Add replicas
• Remove replicas

Currently, your deployment has only one juju unit, known in juju as the leader unit. For each MySQL replica, a new juju unit (non-leader) is created. All units are members of the same database cluster.

Add replicas

You can add two replicas to your deployed MySQL K8s application by scaling it to a total of three units with juju scale-application:

juju scale-application mysql-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  overlord    microk8s/localhost  3.5.2   unsupported  22:48:57+01:00

App        Version    Status  Scale  Charm      Channel     Rev  Address         Exposed  Message
mysql-k8s  8.0.31     active      3  mysql-k8s  8.0/stable  36   10.152.183.234  no       

Unit          Workload  Agent  Address      Ports  Message
mysql-k8s/0*  active    idle   10.1.84.74          Unit is ready: Mode: RW
mysql-k8s/1   active    idle   10.1.84.127         Unit is ready: Mode: RO
mysql-k8s/2   active    idle   10.1.84.73          Unit is ready: Mode: RO

Remove replicas

Removing a unit from the application scales down the replicas.

Before we scale down, list all the units with juju status. You will see three units: mysql-k8s/0, mysql-k8s/1, and mysql-k8s/2. Each of these units hosts a MySQL K8s replica.

To scale the application down to two units, run:

juju scale-application mysql-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  overlord    microk8s/localhost  3.5.2   unsupported  22:48:57+01:00

App        Version    Status  Scale  Charm      Channel     Rev  Address         Exposed  Message
mysql-k8s  8.0.31     active      3  mysql-k8s  8.0/stable  36   10.152.183.234  no       

Unit          Workload  Agent  Address      Ports  Message
mysql-k8s/0*  active    idle   10.1.84.74          Unit is ready: Mode: RW
mysql-k8s/1   active    idle   10.1.84.127         Unit is ready: Mode: RO

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