WebSphere Message Brokers
File: aq01150_
Writer: Terry Cowling

Concept topic

This build: July 31, 2007 21:35:07

Publications

A publication is a piece of information about a specified topic that is available to a broker in a publish/subscribe system.

Typically, a broker distributes a publication that it receives to all applications that are connected to it and that have registered a subscription for the publication. The broker also distributes the publication to all other brokers connected to it, either directly or through a network of brokers that have subscribers for the publication.

Local publications

Publishers can restrict access to their publications to only those subscribers that are registered to the same broker as the publisher. This publication is known as a local publication. Local publications are not forwarded to other brokers.

Global publications

A publication whose distribution is not restricted to only those subscribers that are registered to the same broker as the publisher is known as global publication. A global publication is forwarded to all brokers, connected either directly or through a network of brokers, that have one or more subscribers for the publication.

Retained publications

Typically, a broker discards a publication after it has been sent. However, a publisher can specify (in the case of the Publish message, by specifying the RetainPub option) that it wants the broker to keep a copy of the publication, which is then called a retained publication.

If a retained publication has been published, new subscribers to that publication receive the publication without having to wait for it to be published again.

For example, a subscriber that registers a subscription for a stock price receives the latest published stock price immediately, and does not have to wait for the stock price to be republished.

A broker retains only one publication for each combination of topic and subscription point.

State and event information

Information being published can be categorized either as state information or as event information.

State information is information about the current state of something. The current price of stock or the current score in a soccer match are both examples of state information.

Event information is information about an individual event that occurs. A change in the price of stock or the scoring of a particular goal in a soccer match are both examples of event information.

When an event occurs, the current state information is no longer required and is superseded by new state information.

If a publication contains state information, it is often published as a retained publication. A new subscriber typically wants the current information immediately; the subscriber does not want to wait for an event that causes the information to be republished.

Related tasks
Publishing
Subscribing
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Copyright IBM Corporation 1999, 2007Copyright IBM Corporation 1999, 2007. All Rights Reserved.
This build: July 31, 2007 21:35:07

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