How to scale units

Replication in MySQL is the process of creating copies of the stored data. This provides redundancy, which means the application can provide self-healing capabilities in case one replica fails. In this context, each replica is equivalent to one juju unit.

This guide will show you how to establish and change the amount of juju units used to replicate your data.

Deploy MySQL with replicas

To deploy MySQL with multiple replicas, specify the number of desired units with the -n option.

juju deploy mysql -n <number_of_cluster_members>

It is recommended to use an odd number to prevent a [split-brain](https://en.wikipedia.org/wiki/Split-brain_(computing) scenario.

Primary vs. leader unit

The MySQL primary server unit may or may not be the same as the juju leader unit.

The juju leader unit is the represented in juju status by an asterisk (*) next to its name.

To retrieve the juju unit that corresponds to the MySQL primary, use the action get-primary on any of the units running mysql:

juju run mysql/leader get-primary

Similarly, the primary replica is displayed as a status message in juju status. However, one should note that this hook gets called on regular time intervals and the primary may be outdated if the status hook has not been called recently.

juju model-config update-status-hook-interval=<time(s/m/h)>

Scale replicas on an existing application

To scale up the cluster, use juju add-unit:

juju add-unit mysql --num-units <amount_of_units_to_add>

To scale down the cluster, use juju remove-unit:

juju remove-unit mysql/<unit_id_to_remove>