List Structure Enhancements

With a coupling facility of CFLEVEL=3 or higher, a keyed list structure can optionally support event queues that are associated with sublist monitoring. (A sublist is a subset of a list — see Understanding List Entry Key Assignment for a description of a sublist.) The system uses the event queues to hold control objects called event monitor controls (EMCs), which contain information about the user and the sublist being monitored. Whenever a monitored sublist transitions from an empty to a nonempty state, an EMC is queued to the user's event queue. The system withdraws the EMC from the user's event queue when the sublist transitions from a nonempty to an empty state.

Figure 1 shows the optional parts of a keyed list structure allocated in a coupling facility of CFLEVEL=3 or higher.

Figure 1. Event Queues in a List Structure

The additional parts of the keyed list structure in a coupling facility of CFLEVEL=3 or higher are:

Event queue controls: Contain control information about the state of the event queue and monitoring data. There is an event queue and event queue controls associated with each user who has connected to the keyed list structure.
Event monitor controls: Contain information about the user and the sublist being monitored. There is an event monitor controls object for each user and sublist combination when the user has registered interest in monitoring a particular sublist. (For example, an EMC exists for User 1 and the sublist specified by List 1 and Key 5; another EMC exists for User 2 and the sublist specified by List 1 and Key 6.) An EMC object can reside in the list structure in association with a particular monitored sublist or on the monitoring user's event queue.

Figure 1 shows that for the sublist (List=1, Key=5) there are two EMCs allocated. When the sublist transitioned to a nonempty state, the EMCs were queued to the appropriate users' event queues. If the sublist transitioned to an empty state, the system would withdraw the EMCs from the users' event queues.

The figure also shows that for the sublist (List=1, Key=6), there are as yet no list entries. However, there are two EMCs allocated — for users 2 and 8. When the sublist transitions to a nonempty state, the system will queue these EMCs to the appropriate users' event queues.