In two recent blogs - Mark Tsimelzon from Coral8 argued for CEP server vs. embedded CEP libraries, while Paul Vincent from TIBCO argued for the need for infrastructure stack outside the
Since I avoid making product evaluations in this Blog, I'll talk about the principle.
Conceptually we have a model of "event producer" that produces the events, "event consumer" that consumes the event, and the "event processor" which processes the events and stands in the middle. There are two questions about envelopes:
(1). Is "event processor" an embedded capabilities inside applications or an server.
(2). If "event processor" is a server, is it independent server or part of a larger middleware ?
The answer, as anything is not binary (zero or one), there are cases in which there is a need to have event processing as embedded capabilities (e.g. inside pervasive devices), but Mark is right that the big majority of "event processors" are tend towards the server.
The second question is more interesting --- we have today both stand-alone server and servers that are part of a larger middleware, the rational behind being part of a larger middleware, stems from the fact that event processing is not isolated and has various relations with different applications in the enterprise, it is true that the loose-coupling nature of event driven architectures eases the task of separating it from the applications, but still the integration is the most costly part of building event processing applications, and means to ease the integration has already built into application integration middleware, and if the event processing server is a stand-alone one, there is a need to re-invent this integration, as Paul Vincent rightly say: every $ a CEP vendor spends on middleware integration is a $ less on interesting CEP functionality.
Furthermore, there are some event processing infrastructure and functions (pub/sub, routing - for simple event processing, and ESB mediations like - enrichment, transformation etc, that are already there). Thus, it seems that the ROI will be higher if event processors will be implemented on top of a "middleware stack".
An interesting observation is that from the point of view of application integration middlewares - event processing is becoming a key feature, and there are already some predictions that the standard event processing programming model (which is still not there!), will be the basis for application servers of the future, e.g. Gartner's XTP that I have once discussed and should discuss more.