However, in some situations deferred or disconnected execution may be preferred, or even required. This is referred to as disconnected or deferred communication between programs. In this model, programs in the role of producers place requests in a queue and then proceed with their work. Programs in the role of consumers retrieve requests from the queue and act on them. To protect against failure, there is a need for requests to be stored persistently and processed exactly once. This can be achieved by combining persistent queuing with transaction protection. Oracle8 provides a queuing technology that does not depend on the use of TP-monitors or any other evolving Message-Oriented Middleware infrastructure.

Queuing implements deferred execution of work. When a request for work is entered, queuing defers its processing until the requester has completed the task, process or transaction that created the request.

Oracle uses the following components to deliver AQ functionality:

Message

A message is the smallest unit of information inserted into and retrieved from a queue. It contains user data (payload) in the form of an object type, and control information (metadata) that can be used to specify message ordering, a time window for execution, message priority and so on. A message can reside in only one queue. Messages are created using the DBMS_AQ.ENQUEUE PL/SQL procedure. Messages can later be read by the same or another application using the DBMS_AQ.DEQUEUE PL/SQL procedure. The application developer is responsible for the application's use of the ENQUEUE and DEQUEUE procedure calls.

Queue

A queue is a repository of messages. There are two types of queues: user queues and exception queues. A user queue is for normal message processing. Messages that cannot be retrieved or processed from user queues are transferred to an exception queue. Queues are created, altered, started, stopped and dropped using the Oracle AQ administrative interface package DBMS_AQADM. There is no limit to the number of queues defined in the database.

Queue Tables

Queues are stored in queue tables. Each queue table is a database table and contains one or more queues. Each queue table contains a default exception queue. Each column of a queue table represents message queues and rows represent individual messages. You create queue tables using the DBMS_AQADM.CREATE_QUEUE_TABLE procedure.

Agents

An agent is a queue user. There are 2 types of agents: producers and consumers. A producer places messages in queues; this process is called enqueuing. A consumer retrieves messages; this process is called dequeuing. Any number of producers and consumers may access the queue at a given time. An agent is identified by its name, address and protocol. The address and protocol are reserved for future use. Agents insert messages into and retrieve messages from queues using the Oracle AQ operational interface procedures DBMS_AQ.ENQUEUE and DBMS_AQ.DEQUEUE.

Time Manager

The time manager is a background process that monitors the messages in the queue. It provides the mechanism for message expiration, retry and delay. A parameter called AQ_TM_PROCESSES should be specified in the init.ora parameter file if you want to perform time monitoring on queue messages. This parameter can only be set to values 0 or 1. If this parameter is set to 1, one time manager process will be created as a background process to monitor the messages.

Example:

#init<sid>.ora parameter file
AQ_TM_PROCESSES=1