Agents are multi-threaded processes that provide the logic to manage resources. VCS has one agent per resource type. The agent monitors all resources of that type; for example, a single IP agent manages all IP resources.
When the agent is started, it obtains the necessary configuration information from VCS. It then periodically monitors the resources, and updates VCS with the resource status.
The action to bring a resource online or take it offline differs significantly for each resource type. For example, bringing a disk group online requires importing the disk group. But, bringing a database online requires starting the database manager process and issuing the appropriate startup commands.
VCS monitors resources when they are online and offline to ensure they are not started on systems on which they are not supposed to run. For this reason, VCS starts the agent for any resource that is configured to run on a system when the cluster is started. If no resources of a particular type are configured, the agent is not started. For example, if there are no Oracle resources in your configuration, the Oracle agent is not started on the system.
Certain agents can identify when an application has been intentionally shut down outside of VCS control.
For agents that support this functionality, if an administrator intentionally shuts down an application outside of VCS control, VCS does not treat it as a fault. VCS sets the service group state as offline or partial, depending on the state of other resources in the service group.
This feature allows administrators to stop applications without causing a failover. The feature is available for V51 agents.
See also "VCS behavior for resources that support the intentional offline functionality" on page 372.
Agents carry out specific functions on resources. The functions an agent performs are called entry points. For details on any of the following agent functions, see the Veritas Cluster Server Agent Developer's Guide.
The retrieved information is stored in the resource attribute ResourceInfo. This function is invoked periodically by the agent framework when the resource type attribute InfoInterval is set to a non-zero value. The InfoInterval attribute indicates the period after which the info function must be invoked. For example, the Mount agent may use this function to indicate the space available on the file system.
Bundled agents are packaged with VCS. They include agents for Disk, Mount, IP, and various other resource types.
See the Veritas Cluster Server Bundled Agents Reference Guide.
Enterprise agents control third party applications and are licensed separately. These include agents for Oracle, Sybase, and DB2. Contact your sales representative for more information.
Custom agents are agents that customers or by Symantec consultants develop. Typically, agents are developed because the user requires control of an application that the current bundled or enterprise agents do not support.
See the Veritas Cluster Server Agent Developer's Guide.