Editing Publish–subscribe pattern (section)

==Message filtering==
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In the publish–subscribe model, subscribers typically receive only a subset of the total messages published. The process of selecting messages for reception and processing is called ''filtering''. There are two common forms of filtering: topic-based and content-based.

In a '''topic-based''' system, messages are published to "topics" or named logical channels. Subscribers in a topic-based system will receive all messages published to the topics to which they subscribe. The publisher is responsible for defining the topics to which subscribers can subscribe.

In a '''content-based''' system, messages are only delivered to a subscriber if the attributes or content of those messages matches constraints defined by the subscriber. The subscriber is responsible for classifying the messages.

Some systems support a '''hybrid''' of the two; publishers post messages to a topic while subscribers register content-based subscriptions to one or more topics.