Explain Publish/Subscribe Messaging Domain.

In a publish/subscribe (pub/sub) product or application, clients(publishers as well as subscribers) address messages to a topic, which functions similar to a bulletin board. Both publishers and subscribers are generally anonymous and can dynamically publish or subscribe to the content hierarchy. The system takes care of distributing the messages arriving from a topic's multiple publishers to its multiple subscribers. Topics retain messages only as long as it takes to distribute them to current subscribers.

Pub/sub messaging has the following characteristics.

Each message can have multiple consumers.
Publishers and subscribers have a timing dependency. A client that subscribes to a topic can consume only messages published after the client has created a subscription, and the subscriber must continue to be active in order for it to consume messages.

Pub-Sub Messaging Domain(Image Source:java.sun.com)