How to Select a Notification Method


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The EventBroker supports a variety of methods for notifying subscribers of events, as shown in the following diagram.

Notification Methods Supported by the EventBroker

Whichever notification method you choose, the procedure for implementing it is the same: in your call to tpsubscribe(), specify an argument that refers to a structure of type TPEVCTL.

If the value of the argument is NULL, the EventBroker sends an unsolicited message to the subscriber. Two of these methods, having the notification sent to a service and having it sent to a queue in stable storage, cannot be requested directly by a client. Instead, a client must invoke a service routine to subscribe on its behalf.

For each subscription, you can select any of the following notification methods. The EventBroker can:

Notify the client—the EventBroker keeps track of events in which the client is interested and sends unsolicited notifications to the client when they occur. Some events are anonymously posted. A client can join an application, regardless of whether any other clients have subscribed, and post events to the EventBroker. The EventBroker matches these events against its database of subscriptions and sends an unsolicited notification to the appropriate clients. (See the definition of the T_EVENT_CLIENT class in the EVENT_MIB(5) entry in the File Formats, Data Descriptions, MIBs, and System Processes Reference.)
Invoke a service—if a subscriber wants event notifications to be sent to service calls, then the ctl parameter must point to a valid TPEVCTL structure. (See the definition of the T_EVENT_SERVICE class in the EVENT_MIB(5) entry in the File Formats, Data Descriptions, MIBs, and System Processes Reference.)
Enqueue messages to stable-storage queues—for subscriptions to stable-storage queues, a queue space, queue name, and correlation identifier are specified, in addition to values for eventexpr and filter, so that matching can be performed. The correlation identifier can be used to differentiate among several subscriptions characterized by the same event expression and filter rule, and destined for the same queue. (See the definition of the T_EVENT_QUEUE class in the EVENT_MIB(5) entry in the File Formats, Data Descriptions, MIBs, and System Processes Reference.)
Execute commands—using the T_EVENT_COMMAND class of the EVENT_MIB, subscribers can invoke an executable process. When a match is found, the data is used as the name of the executable process and any required options. (See the definition of the T_EVENT_COMMAND class in the EVENT_MIB(5) entry in the File Formats, Data Descriptions, MIBs, and System Processes Reference.)
Write messages to the user log (ULOG)—using the T_EVENT_USERLOG class of the EVENT_MIB, subscribers can write system USERLOG messages. When events are detected and matched, they are written to the USERLOG. (See the definition of the T_EVENT_USERLOG class in the EVENT_MIB(5) entry in the File Formats, Data Descriptions, MIBs, and System Processes Reference.)