Publish/subscribe pattern

Publish/subscribe (often abbreviated Pub/Sub) is probably the best known one-way messaging pattern. We should already be familiar with it, as it's nothing more than a distributed observer pattern. As in the case of observer, we have a set of subscribers registering their interest in receiving a specific category of messages. On the other side, the publisher produces messages that are distributed across all the relevant subscribers. The following figure shows the two main variations of the pub/sub pattern, the first peer-to-peer, the second using a broker to mediate the communication:

What makes pub/sub so special is the fact that the publisher doesn't know who the recipients of the messages are in advance. As we said, it's the subscriber which has to register its interest to receive a particular message, allowing the publisher to work with an unknown number of receivers. In other words, the two sides of the pub/sub pattern are loosely coupled, which makes this an...