Publish/subscribe components

Publish/subscribe is the mechanism by which subscribers can receive information, in the form of messages, from publishers. The interactions between publishers and subscribers are controlled by queue managers, using standard IBM MQ facilities.

A typical publish/subscribe system has more than one publisher and more than one subscriber on many different topics, and often has more than one queue manager. An application can be both a publisher and a subscriber.

The provider of information is called a publisher. Publishers supply information about a subject, without needing to know anything about the applications that are interested in that information. Publishers generate this information in the form of messages, called publications that they want to publish and define the topic of these messages.

The consumer of the information is called a subscriber. Subscribers create subscriptions that describe the topic that the subscriber is interested in. Thus, the subscription determines which publications are forwarded to the subscriber. Subscribers can make multiple subscriptions and can receive information from many different publishers.

Published information is sent in an IBM MQ message, and the subject of the information is identified by its topic. The publisher specifies the topic when it publishes the information, and the subscriber specifies the topics about which it wants to receive publications. The subscriber is sent information about only those topics it subscribes to.

It is the existence of topics that allows the providers and consumers of information to be decoupled in publish/subscribe messaging by removing the need to include a specific destination in each message as is required in point-to-point messaging.

Interactions between publishers and subscribers are all controlled by a queue manager. The queue manager receives messages from publishers, and subscriptions from subscribers (to a range of topics). The queue manager's job is to route the published messages to the subscribers that have registered an interest in the topic of the messages.

Standard IBM MQ facilities are used to distribute messages, so the applications can use all the features that are available to existing IBM MQ applications. This means that we can use persistent messages to get once-only assured delivery, and that your messages can be part of a transactional unit-of-work to ensure that messages are delivered to the subscriber only if they are committed by the publisher.

• Publishers and publications
  In IBM MQ publish/subscribe a publisher is an application that makes information about a specified topic available to a queue manager in the form of a standard IBM MQ message called a publication. A publisher can publish information about more than one topic.
• Subscribers and subscriptions
  In IBM MQ publish/subscribe, a subscriber is an application that requests information about a specific topic from a queue manager in a publish/subscribe network. A subscriber can receive messages, about the same or different topics, from more than one publisher.
• Topics
  A topic is the subject of the information that is published in a publish/subscribe message.

Parent topic: Publish/subscribe messaging