Sunday, April 20, 2008
On Event Pattern Semantics
Today is Passover, while I am far from being religious, there are several traditions we keep, one of them is to have a family dinner in Passover-eve, and reading (at least part of) the Haggadah, so I've looked at the internet to find some fancy Haggadah in English, and here is the result.
The call for EPTS founding members
is also progressing - by now more than 20 compnies either signed or indicated that they are in internal approval process, and intend to sign as EPTS members, in addition to about 20 individual members. We excpect this number to grow towards the deadline, and call anybody who has not joined and wish to contribute to the emerging EP community to join.
Moving to today's topic: Tom Puzak has posted on the CEP interest group a message about nine features the CEP engine should have. This discussion is useful, since there is no agreed upon "CEP manifesto", a definition what are the functions that should be supported by "CEP engines", and we are going to need one, sooner or later.
Since I am working on a tutorial for the DEBS conference which will talk about event pattern semantics as a major theme, here is a sneak preview about the type of semantic decisions that are needed, this is in addition to the semantics of the specific pattern (conjunction, disjunction, absernce, sequence...).
1. In which context this particular pattern is relevant. Context can be temporal (within working hours, 1 hour from the power break), spatial (within the headquarter building), semantic (only for platinum customers or state-oriented ( while it is rainining) - or combinations of all the various dimensions (I have written before about the notion of context).
2. Is an event participate in the same pattern in a single context or in multiple contexts ? this can happen when there there is overlap among contexts.
3. Is the action / notification about the fact that the pattern has been detected should execute immediately or in a deferred mode (example: at the end of the temporal context).
4. Within a context - is the pattern existential (i.e. we are looking for a single pattern per context) or can there be multiple instances >
5. Using quantifiers on synonims - Taking the example from Tom Puzak's message: we are looking for a message of A, B within 60 secondes (temporal context), and the actual flowing events are: A1 A2 B1 A3 B2 B3 - we may want the cartesian product, but typically this is not what we really wish - thus, we can use quantifiers to select among the A and B events. Quantifiers can be according to order - firts, last, each or according to content of attributes (or both).
6. Can a single event particpate in more than one pattern within the same context ?
7. Should newer synonim kill older sysnonims ?
This are just titles - and in the DEBS tutorial I'll explain each with examples and show how they impact the pattern detection behavior.
Bottom line -- tune up the semantics of a pattern consists of several decisions, if these decisions are not supported in the language, and the application does not conform with the default, results in hacking around... more - later.