Since my previous post on this topic has triggered some discussion, comments and questions - I am returning to the issue of "event processing and business rules". One observation I got today is that if we'll some of the "event processing typical applications", example: Paul Vincent's post about EDA and CEP in Insurance are considered by the business rule people also as typical applications for business rule engine (and the insurance area has strong presence of business rules). To answer that I'll cite another CEP blogger, whom I don't have the honor to know personally, who used the eternal-wisdom title: When all you have is a hammer everything looks like a nail, while the post talks about SQL, this is true also for business rules. In fact, if some programming tool has enough expressive power, one can, somehow, express everything using this tool, which does not say a lot, since everything can be expressed nicely by a Turing machine (this wikipedia link also points at some freeware implementation of Turing machine, so we have a single programming tool to do everything...). However, there are things that are naturally fit into a certain programming paradigm and things that look rather awkward. Of course, there are some cases in which one can equally implement it within multiple paradigms, this is typically one of two cases - it may be very simple, or there is not (yet) a specific paradigm to express it nicely, so multiple paradigms are awkward in the same way.
It is not surprising that the same phrase about hammer and nails is also going in the opposite directions- I have seen cases where people who had CEP engines, "force" them to do things that are not natural for this paradigm, by creating artificially events and patterns.
And one comment about ECA rules (Event-Condition-Action) --- the contribution of the glorious "active databases" community to the computing industry. Complex Event Processing deals with the E part - taking events, apply some function, and at the end deriving more events. Each derived event that arrive to the "edge of the event processing network", i.e. sent to a consumer, can then conditionally trigger an action. Thus, from architecture point of view, ECA rules are really part of the consumer logic, and not part of the "event processing network". More - later.