Using the event-driven architecture pattern, you can create systems for processing the events that shape your everyday business environment. An event can be something as simple as an electrical component being switched on or off, or more complicated, such as a bid being made in an auction house for the painting of a great master. An event represents something that has happened, and it may or may not require some follow-up action to be taken. An event can also represent something that was expected to happen but has failed to happen.

We all experience an event driven world every day. We walk through an airport and hear announcements of planes arriving and departing (these are events). The announcements (events) are emitted even if nobody is listening. If, however, the announcement is for my plane, then I will accept it, and start taking an action. My action may be to run to the gate, while someone else's activity might be to walk and get a snack. This is the basis of event-driven architecture: events are emitted and listeners can either take action on them or ignore them. The action I take is totally self-contained and does not rely on the activity of another person.

The term event-driven indicates that when an event happens, it can have a significance which requires some follow-up action to be taken. An event can be noticed by several observers or listeners, and each observer can react to the event differently. For one observer, an event might represent some critical status which requires immediate action. For another observer the same event might not be relevant at all.

The significance of a single event is sometimes only visible when viewed in the context of other events that together form a pattern. For example, if cash is withdrawn at a cash machine in the city center, this is not unusual, but if cash is withdrawn at many different cash machines on the same day throughout the city using the same card, this might raise the suspicion that the card is stolen.

If we change the focus from everyday events that we observe in the world around us to events that can have an influence on the way a company does its business, we can see that events such as the following could trigger a component in a predefined workflow:

The existence of an event can be the trigger for processes, such as the invocation of a service, the initiation of a business process, or the publication of relevant information. Event Routing picks up on these ideas and provides a set of concepts for dealing with events at all stages throughout the processing chain.