Monitoring allows you to detect conditions of interest associated with the cluster nodes and their related resources, and then automatically take action when those conditions occur. The key elements in monitoring are conditions and responses. A condition identifies one or more resources that you want to monitor (for example, the /var file system), and the specific resource state you are interested in (for example, % full). A response specifies one or more actions to be taken when the condition is found to be true. Actions can include notification, running commands, and logging.
System resources that you can monitor are organized into general categories called resource classes. Examples of resource classes include Processor, File System, Physical Volume, and Ethernet Device. Each resource class includes individual system resources, and each of these resources also has a set of properties that you can monitor.
For a condition, you specify the monitored attribute of the resource in a logical expression that defines a threshold or state of the monitored resource. For example, /var > 90% full. When the logical expression is true (the threshold is reached or the state becomes true), an event is generated. The logical expression is the event expression of the condition. Event expressions are typically used to monitor potential problems and significant change in the system. Note that RSCT provides you with predefined conditions.
You may find it useful to set up a rearm expression. A rearm expression typically indicates when the monitored resource has returned to an acceptable state. When the rearm expression is met, monitoring for the condition resumes.
A response consists of one or more actions to be performed by the system when an event or rearm event occurs for a condition. After monitoring for the condition begins, the system evaluates the event expression to see if it is true. When the event expression becomes true, an event occurs that automatically notifies all of the associated event responses, which causes each event response to run its defined actions.
The Monitoring application allows you to use predefined responses, or you can create new responses and associate them with conditions as needed. You can associate multiple responses with one condition, or you can associate one response with multiple conditions.
For more detailed information about conditions, responses, and using expressions, see IBM RSCT for Linux: Guide and Reference.