An Event Notification Service is implemented as a Broker or set of of brokers, which in reality probably consist of ESB's (Enterprise Service Bus) with Message Broker and Event Broker functionality. An ESB is acting as a mediator between Event Producers and Event Consumers and between services. While the ESB generally does message routing, transport protocol conversion, message format transformation and business event handling, the Event Broker functionality executes the topic/content-based routing of the event notification messages. Read Beth Gold-Bernstein' article about ESB and read about the SEMEASY project, which among others has the objective "...to make event-driven development easier by adding event-broker software in the Enterprise Services Bus.". See Jack van Hoof's article were he describes an 'Exchange Area' which is realized by an ESB. An example of a commercial broker is IBM:s WebSphere Message Broker.


Read more about EDA:

The Inaugural International Conference on Distributed Event-Based Systems (DEBS) conference has been held from June 20th-22nd, 2007, in Toronte Canada.

See the article "Peter Pietzuch, Gero Muhl, and Ludger Fiege, "Distributed Event-Based Systems: An Emerging Community," IEEE Distributed Systems Online, vol. 8, no. 2, 2007, art. no. 0702-o2002."

Peter R. Pietzuch, Gero Mühl and Ludger Fiege are also the authors of the book:
Distributed Event-Based Systems
Gero Mühl, Ludger Fiege, Peter R. Pietzuch
ISBN 3-540-32651-0

is a synchronous command-and-control type of pattern.
If a service has to be invoked by a service request, a Service Activator (ServiceActivator Pattern), which is put in front of a service, receives the service request from the service consumer, invokes the service and optionally sends the service response back to the service consumer.
Service Activator > Service
An ESB acts as a mediator between a service consumer and a service provider.

Service Consumer > Enterprise Service Bus > Service Provider

The following operations are performed by the SOA components 'Service Provider', 'Service Consumer' and 'Service Registry':
1) Service Provider publishes a service at a Service Registry
2) Service Consumer discovers service at a Service Registry
3) Service Consumer binds to the Service Provider and invokes a service request
4) Depending on the service, a response is sent back to the Service Consumer.

The Service Registry has to be implemented with the the corresponding scope (public/private).

Jack van Hoof recommends to "strip the applications down to core business rules implementations by externalizing shared areas for exchange, access and persistency."

Jack van Hoof describes "a shared external access area" to bring applications together at the user interface layer, "a shared external exchange area" to bring applications together at the business logic layer and "a shared external persistency area" to bring applications together at the data layer.

In his recapitulation he writes:
"
Integration at the data layer: the glue is shared databases, DBMS, data warehouse
Integration at the business logic layer : the glue is ESB, SOA, EDA
Integration at the user interface layer (server side): the glue is portlets in portals
Integration at the user interface layer (client side): the glue is mashups
"

At the client side Mashups integrate applications in the browser with techniques based on JavaScript and AJAX.
At the server side portals integrate applications with help of portlets.

The ESB (Enterprise Service Bus), has a central role in integrating enterprise applications. It acts as the mediator between service consumers and providers in SOA as well as mediator between event emitter and event handler in EDA. Several commercial implementations are existing and open source implementations are evolving as well like Fuse Open Source ESB and Mule Open Source ESB.