Enqueue and Dequeue

A typical Apply process can dequeue both the captured events and the user-enqueued events. The Apply process handles all events containing LCR(s) automatically. It uses the specified rule conditions to apply directly to the database objects, or it may also use a user specified procedure to do the transaction processing. When there is no LCR in the event and there are only user messages, the Apply process reacts differently.  When it can rely only on user messages, the Apply process can call upon a user-specified procedure to process the messages.  These procedures are called Message Handlers.

User written applications can be developed and used to explicitly dequeue the user-enqueued events. These applications may or may not rely upon the Streams messaging client. It is important to remember that captured events can not be dequeued by a user application. Only the Apply process is able to dequeue the captured events. The only technical exception is when an Apply process calls a user procedure such as a message handler that enqueues an event explicitly.  This event is considered a user enqueued event and can be dequeued explicitly even though the event may have originally been a captured event.

The different types of queues and the various ways that events can be queued and dequeued are some of the exciting features of the Streams methodology.