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).
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.
Summary
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 2.9.38 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
The maximum number of Charmed MySQL units in a single Juju application is 9. This is a limitation of MySQL Group replication. Read more about all limitations in the official MySQL documentation.
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 2.9.38 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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