Complex event processing takes SOA to a new level through the introduction of decoupled services - a significant step beyond loosely coupled services. CEP enables the gathering of data from and about any services running in the enterprise. It also allows business logic to be applied to that data in order to derive insight and enable appropriate real-time response to changing conditions. In the context of SOA, the power of the event-driven model is that it allows greater flexibility since services are entirely decoupled, unaware of who is producing the events they operate on or consuming the events they produce. It also allows for better insight into current conditions and the ability to instantly respond as events occur.

So how does this work?

In a "traditional" service oriented architecture based on a request-response paradigm, one has many distributed components, most of them providing a service to other components. These are continually on "stand-by," waiting for a service request which contains additional data. The service processes the data and returns the result to the requesting application. This approach has proven far superior to monolithic applications for most business needs by allowing for reuse along with enabling business agility - an individual service can be used by multiple applications, and a service can be modified without affecting any other services.

For all its strengths, there are still significant limitations. SOA requires applications to know what services are available and how to interact with them. It also means that nothing happens unless or until an application invokes a service request. With the request-response paradigm, every service must know which other services need to be informed about what has taken place. One of the implications of this is that in order to add a new function, existing services have to be modified.