Consumer agents listen for messages in one or more queues that are then dequeued
so users can use the contents. Notification of the existence of messages in the queue
can occur via the Oracle Call Interface (OCI; described in Chapter 1) callback registration
or through a listen call that can be used by applications to monitor for
messages in multiple queues.
Because messages are stored persistently in queues in the database, a number of message-
management features are available. End-to-end tracking is enabled since each
message carries its history with it, including location and state of the message, nodes
314 | Chapter 13: Oracle Distributed Databases and Distributed Data
visited, and previous recipients. Messages that don??™t reach subscribers within a
defined lifetime are moved to the exception queue, from which they can be traced.
Messages that successfully reach subscribers may be retained after consumption for
additional analysis, including enqueue and dequeue times. As messages may be
related (for example, one message might be caused by the successful execution of
two other messages), retaining the messages can be useful in tracking sequences.
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