Deploying Controllers in an HA arrangement provides significant benefits. It allows you to minimize the downtime in the event of a server failure and take the primary Controller down for maintenance with minimal disruption. It fulfills requirements for backing up the Controller data since the secondary maintains an updated copy of the Controller data. The secondary can also be used to perform certain resource-intensive operations that are not advised to be performed on a live Controller, such as performing a cold backup of the data or accessing the database to perform long-running queries, say for troubleshooting or custom reporting purposes.

In HA mode, each Controller has its own MySQL database with a full set of the data generated by the Controller. The primary Controller has the master MySQL database, which replicates data to the secondary Controller's replica MySQL database. HA mode uses a MySQL Master-Master replication type of configuration. The individual machines in the Controller HA pair need to have an equivalent amount of disk space.

The following figure shows the deployment of an HA pair at a high level. In this scenario, the agents connect to the primary Controller through a proxy load balancer. The Controllers in an HA pair must be equivalent versions, and be in the same data center.

In the diagram, the MySQL instances are connected via a dedicated link for purposes of data replication. This is an optional but recommended measure for high volume environments. It should be a high capacity link and ideally a direct connection, without an intervening reverse proxy or firewall. See Load Balancer Requirements and Considerations on Set Up a High Availability Deployment for more information on the deployment environment.