The publish/subscribe pattern
The publish/subscribe pattern is a communication pattern alternative to the well-known request/reply pattern. Instead of a client (issuing a request) and a server (replying with a response to that request), a publish/subscribe architecture consists of publishers and subscribers.
Each publisher can emit messages. It is of no concern to the publisher who actually gets these messages. This is the concern of the subscribers; each subscriber can subscribe to a certain type of message and be notified whenever a publisher publishes a given type of message. In reverse, each subscriber does not concern itself with where a message actually came from.
The request/reply and the publish/subscribe communication patterns
In practice, many publish/subscribe architectures require a central infrastructure component—the message broker. Publishers publish messages at the message broker, and subscribers subscribe to messages at the message broker. One of the broker's main tasks then...
