Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions (Addison-Wesley Signature Series (Fowler))

by Hohpe Gregor

The event can be packaged as a Message (66) so that messaging will reliably communicate the event to the observers (subscribers). Then, the event channel is a Message Channel (60). But how will a messaging channel properly communicate the event to all of the subscribers? Each subscriber needs to be notified of a particular event once but should not be notified repeatedly of the same event. The event cannot be considered consumed until all of the subscribers have been notified, but once they have, the event can be considered consumed and should disappear from the channel. Yet, having the subscribers coordinate to determine when a message is consumed violates the decoupling of the Observer pattern. Concurrent consumers should not compete but should be able to share the event message.