Application monitors

If the monitor indicates that the application is already running, PowerHA SystemMirror will not start the second instance of the application. If the application monitors are not configured to PowerHA SystemMirror, then you may write an application start script that checks the state of the application before starting it.

Application monitors, configurable in PowerHA SystemMirror, are a critical piece of the PowerHA SystemMirror cluster configuration; they enable PowerHA SystemMirror to keep applications highly available. When PowerHA SystemMirror starts an application on a node, it also periodically monitors the application (using the monitor that you configure) to make sure that the application is up and running.

An erroneous application monitor may not detect a failed application. As a result, PowerHA SystemMirror would not recover it or may erroneously detect an application as failed, which may cause PowerHA SystemMirror to move the application to a takeover node, resulting in unnecessary downtime. To summarize, we highly recommend properly configured and tested application monitors for all applications that you want to keep highly available with the use of PowerHA SystemMirror. Use them as follows:

• Use a process monitor if the intent is to monitor whether the process(es) exist on the UNIX™ system.
• Use a custom monitor if the intent is to check the health of the application, for example, whether the database is still functioning by querying a database table.
• Use both process and custom monitors when needed.

During verification, PowerHA SystemMirror issues a warning if an application monitor is not configured.

For information on configuring an application monitor, see the section Configuring multiple application monitors.