Applications

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The purpose of a highly available system is to ensure that critical services are accessible to users. Applications usually need no modification to run in the PowerHA® SystemMirror® environment. Any application that can be successfully restarted after an unexpected shutdown is a candidate for PowerHA SystemMirror.

For example, all commercial DBMS products provide a checkpoint on the state of the disk in some sort of transaction journal. In the event of a server failure, the fallover server restarts the DBMS, which reestablishes database consistency and then resumes processing.

You can use AIX® Fast Connect to share resources with systems that are not running AIX as the operating system. If you configure Fast Connect as a SystemMirror resource, PowerHA SystemMirror keeps it highly available and recover from node and network interface failures. PowerHA SystemMirror also verifies the configuration of Fast Connect during cluster verification. Applications are managed by defining the application to PowerHA SystemMirror as an application controller resource. The application controller includes application start and stop scripts. PowerHA SystemMirror uses these scripts when the application needs to be brought online or offline on a particular node, to keep the application highly available.

Note: The start and stop scripts are the main points of control for PowerHA SystemMirror over an application. It is very important that the scripts you specify operate correctly to start and stop all aspects of the application. If the scripts fail to properly control the application, other parts of the application recovery might be affected. For example, if the stop script you use fails to completely stop the application and a process continues to access a disk, PowerHA SystemMirror will not be able to bring the volume group offline on the node that failed or recover it on the backup node.

Add your application controller to a PowerHA SystemMirror resource group only after you have thoroughly tested your application start and stop scripts.

The resource group that contains the application controller should also contain all the resources that the application depends on, including service IP addresses, volume groups, and file systems. Once such a resource group is created, PowerHA SystemMirror manages the entire resource group and, therefore, all the interdependent resources in it as a single entity. PowerHA SystemMirror coordinates the application recovery and manages the resources in the order that ensures activating all interdependent resources before other resources.

In addition, PowerHA SystemMirror includes application monitoring capability, whereby you can define a monitor to detect the unexpected termination of a process or to periodically poll the termination of an application and take automatic action upon detection of a problem.

You can configure multiple application monitors and associate them with one or more application controllers. By supporting multiple monitors per application, PowerHA SystemMirror can support more complex configurations. For example, you can configure one monitor for each instance of an Oracle parallel server in use. Or, you can configure a custom monitor to check the health of the database, and a process termination monitor to instantly detect termination of the database process.

You can also specify a mode for an application monitor. It can either track how the application is being run (running mode), or whether the application has started successfully (application startup mode). Using a monitor to watch the application startup is especially useful for complex cluster configurations.