Scale your Charmed MySQL

This is part of the Charmed MySQL Tutorial. Please refer to this page for more information and the overview of the content.

Adding and Removing units

Charmed MySQL operator uses MySQL InnoDB Cluster for scaling. Being built on MySQL Group Replication, provides features such as automatic membership management, fault tolerance, automatic failover, and so on. An InnoDB Cluster usually runs in a single-primary mode, with one primary instance (read-write) and multiple secondary instances (read-only). The future versions on Charmed MySQL will take advantage of a multi-primary mode, where multiple instances are primaries. Users can even change the topology of the cluster while InnoDB Cluster is online, to ensure the highest possible availability.

! 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 MySQL application with:

juju add-unit mysql -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  3.1.6    unsupported  23:33:55+01:00

App    Version          Status  Scale  Charm  Channel     Rev  Exposed  Message
mysql  8.0.32-0ubun...  active      3  mysql  8.0/stable  147  no

Unit      Workload  Agent  Machine  Public address  Ports  Message
mysql/0*  active    idle   0        10.234.188.135         Primary
mysql/1   active    idle   1        10.234.188.214
mysql/2   active    idle   2        10.234.188.6

Machine  State    Address         Inst id        Series  AZ  Message
0        started  10.234.188.135  juju-ff9064-0  jammy       Running
1        started  10.234.188.214  juju-ff9064-1  jammy       Running
2        started  10.234.188.6    juju-ff9064-2  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 mysql/0, mysql/1, and mysql/2. Each of these units hosts a MySQL replica. To remove the replica hosted on the unit mysql/2 enter:

juju remove-unit mysql/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  3.1.6    unsupported  23:46:43+01:00

App    Version          Status  Scale  Charm  Channel     Rev  Exposed  Message
mysql  8.0.32-0ubun...  active      2  mysql  8.0/stable  147  no

Unit      Workload  Agent  Machine  Public address  Ports  Message
mysql/0*  active    idle   0        10.234.188.135         Primary
mysql/1   active    idle   1        10.234.188.214

Machine  State    Address         Inst id        Series  AZ  Message
0        started  10.234.188.135  juju-ff9064-0  jammy       Running
1        started  10.234.188.214  juju-ff9064-1  jammy       Running

Scaling limitations

Note: the maximum number of Charmed MySQL units in a single Juju application is 9. It is a limitation of MySQL Group replication, read more about all limitations here.