Advanced Queuing - Oracle

Elements of Advanced Queuing
Rule
Rule-Based Subscribers
Transformation
Queue Monitor
directed toward specific recipients who may or may not be subscribers to the queue,
thereby overriding the subscriber list.
A rule is used to define one or more subscribers’ interest in subscribing to messages
that conform to that rule. The messages that meet the rule criterion are delivered to
the interested subscribers. A rule is specified as a boolean expression (one that
evaluates to true or false) using syntax similar to the WHERE clause of a SQL query.
The boolean expression can include conditions on the following:
Message properties (currently priority and correlation identifier)
User data properties (object payloads only)
Functions (as specified in the WHERE clause of a SQL query)
A rule-based subscriber is a subscriber with associated rules in the default recipient
list. If an associated rule evaluates to TRUE for a message, the message is sent to the
rule-based subscriber even if the message has no specified recipients.
A transformation defines a mapping from one Oracle data type to another. The
transformation is represented by a SQL function that takes the source data type as
input and returns an object of the target data type. A transformation can be
specified during enqueue, to transform the message to the correct type before
inserting it into the queue. It can be specified during dequeue to receive the
message in the desired format. If specified with a remote subscriber, the message
will be transformed before propagating it to the destination queue.
The queue monitor (QMNn) is a background process that monitors messages in
queues. It provides the mechanism for message delay, expiration, and retry delay.
The QMNn also performs garbage collection for the queue table and its indexes and
index-organized tables (IOTs). For example, the QMNn determines when all
subscribers of multiconsumer queues have received a message and subsequently
removes the message from the queue table and supporting indexes and IOTs.
1-24 Oracle9i Application Developer’s Guide - Advanced Queuing