Monday, September 7, 2009

On Event Processing Patterns

This is an illustration that has been created by my of my former colleagues to the AMiT team, Tali Yatzkar, when she attended a "presentation course" as an excercise in the course to explain what is an event processing pattern (we did not use this term at that time), this is the original picture, it is animated (the animation is not presevered when copying from file to picture) and the geometric shapes in the left-hand side of the picture are moving. The idea is simple, there are patterns that designate the relationship between a set of events, e.g. a conjuction: event E1 and event E2 both hoccur in the same context (e.g. relate to the same person within 2 hours). This rather simple idea is the jewel of the crown in event processing systems, and the basis of what David Luckham called: Complex Event Processing. It is also what makes a composite event in active database terminology (I have discussed in the past the subtle differences between those term definition). This illustration in some variations has a life of its own, and we saw it in presentations of some other companies and people, I even once had to comment on a Slideshare presentation when it was attributed to (see my comment to this presentation). Anyway, besides giving Tali her due credit, I am writing about event processing patterns, since one of the chapters we complete now for the second-third review of the EPIA book deals with the notion of event processing pattern as a major abstraction. As all abstractions in our meta-language, a specific languages may implement a certain pattern as a language primitive, or implement it through a combination of language primitives. Those interested in the formal definition will need to read the book since the formal definition require definitions of several terms, so I'll give some a less formal definition here -- pattern is really a function that takes a collection (or stream) of input events that satisfy some filtering assertions (e.g. they have to be within context, and have certain other patterns) and returns zero or more "matching sets", which include a collection of individual events that collectively satisfy the pattern. Let's take a couple of examples:

The first example: Bid example: There is a bid for some auction that has been provided on an auction site. The idea is to select a single winner. The input events are acution offering events and bid events. The bid events are partitioned according to the auction offering they are refering to, and are also filterred out according to time (each auction is open for a certain amount of time only) and according to threshold condition (has to be no less than a minimal price).The matching set in this case consists of a single bid event per auction offering. The matching pattern here is - "relative max", which means that any event that we are looking for the event with the relative (to the other input events) maximal value of some attribute (in this case the bid amount). Note that the "relative max" pattern does not necessarily provide a single bidder, thus we also need a "synonyms policy" to determine what happens when we have multiple events of the same type that match the criteria. In this case we take the fairness criterion of FCFS, and the synonyms policy will be -"first", meaning the first bidder that offerred the maximal price. In our meta-language this looks like:
Pattern name = Bidder selection; Pattern type = relative max; Input events = (Auction offering, Bid); Context = (segment = by auction offering, temporal = auction offering is open); Filtering assertion = (Bid.Price >= Auction Offering.Minimal Price); policies = (cardinality = single, synonyms = first)

Note that in these three and a quarter lines we have expressed logic that is quite complex, and this is the magic of patterns. As an exercise to the reader, write the equivalent logic in Java, and then change it so that it will chose all bidders that have provided the relative maximum for a second round of bids.

The second example is a sequence example, this figure is being taken from the EPIA book; the example is looking at the case in which a patient is released from the hospital and then admitted again within 48 hours with the same complain that brought this patient to the hospital in the first time.

Here we are looking for a sequence (the order is important, of course), of the patient release event, and the patient admission event for the same patient with the same complain within 48 hours. The definition in our meta-language will be roughly:

Pattern name = Repeating admission, Filter type = sequence, input events = (Patient Release, Patient Admission), Context = (segment = by patient and complain) temporal = Patient release + 48 hours).

This pattern creates a matching set which consists of a pair of events of types patient release and patient admission).

Note that the pattern return the selected events, and the EPA can derive new events as the function of these selected event.

Here we saw two type of patterns: relative max, which is a set oriented pattern, and sequence which is event oriented patterns. I'll provide the list of patterns collected so far in one of the future postings.

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