Callback Coordination

When a callback is coordinated across the cluster, that callback is only executed on one instance of the polling notification in the cluster. For example, if the enableCallBack is coordinated, the first polling notification in the cluster that is enabled, executes the enableCallBack. When subsequent instances of that notification are enabled, the enableCallBack call is suppressed. If the callbacks were not coordinated, each instance executes the enableCallBack when the node instance is enabled.

The purpose of callback coordination is to prevent redundant updates to the backend associated with starting or stopping a notification. For example, when the disableCallBack is called on a polling notification for the Adapter for JDBC, a database trigger that gathers information for the notification is removed. Without coordination, all instances of that notification would be effectively disabled as soon as the first instance is disabled. With coordination, the disableCallBack is not executed until the last instance of that notification in the cluster is disabled.

From a design standpoint, it is important to segregate management of resources on the backend in separate callbacks from management of resources that are local to the notification instance. Callback coordination is configured so that related pairs of callbacks are either coordinated, or not. The following pairs of callbacks may be coordinated:

Coordinated Callback Pairs	Description
enableCallBack and disableCallBack	This occurs when the persistent node state is changed from disabled or changed to disabled. When coordinated, the first instance to go from disabled to enabled executes the enableCallBack method and the last instance to go from either suspended or enabled to disabled executes the disableCallBack method.
startupCallBack and shutdownCallBack	This occurs when the node becomes active or inactive for any reason, including when the node is enabled, disabled, suspended, or resumed (if the node is enabled, it also includes Integration Server and package startup or shutdown). When coordinated, the first instance to become active will execute the startupCallBack and the last instance to become inactive executes the shutdownCallBack.
suspendCallBack and resumeCallBack	This occurs when the node is suspended or resumed. When coordinated, the first instance to go from enabled to suspended executes the suspendCallBack and the last instance to go from either suspended to enabled executes the resumeCallBack.