Publish/Subscribe pattern
Publish/Subscribe (often abbreviated to 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. Figure 13.7 shows the two main variants of the Pub/Sub pattern; the first is based on a peer-to-peer architecture, and the second uses a broker to mediate the communication:
Figure 13.7: Publish/Subscribe messaging pattern
What makes Pub/Sub so special is the fact that the publisher doesn't know in advance who the recipients of the messages are. As we said, it's the subscriber that has to register its interest to receive a particular message, allowing the publisher to...
