Event Message

Event handling in Java should be familiar to you by now; but on the off chance that it isn't, here's the primer: an object can register itself with another object as an event listener. Once the listener has been registered, the target has a handle to the listener and can use callback methods to notify the listening object that a particular event has occurred. This process allows programmers to easily modify runtime behavior by plugging new listeners in at appropriate places. Swing is full of listeners, as is the Servlet API (ServletContextListener, etc.). The Gang of Four identified this behavior as Observer, in which the actor is the object or set of objects performing the main actions, and the observer is the listener.

An event message extends the Observer model to a set of distributed applications. The basic informational content of an event message is implicit in the fact that the message has been sent. Event messages can be sent from one system to another to provide notification of lifecycle events within an application, or to announce the status of particular activities. Applications for this pattern include enterprise monitoring (see the discussion of the control bus later in this chapter) and centralized logging.

An important characteristic of event messages is that they do not require a reply. In fact, one could go so far as to say there is no requirement that event messages be received, either. In some cases this is true, and in others it isn't: whether events must be received should influence your choice of message transport.

In JMS, an event message can be implemented with the user-defined headers of a message. The benefit of this approach is simplicity: applications only need to agree on header names, so there is no need to define a format for the message data. Performance improves, too, since JMS and most MOM packages provide built-in functionality to filter messages based on their headers.