Monday, November 26, 2007

Non events again - much ado about nothing

Shakespeare fans (and theater fans in general) love the wonderful play : Much Ado about Nothing.
I recalled the title of the play when seeing that different people this week have written various things about the issue of "non events". Tim Bass thinks that this is just a time-based event, there have been various answers to David Luckham's question on the CEP forum. I actually did not think that I'll write second posting on this topic, since I don't think that this topic is that important, it is just one of various basic patterns. However, I'll try to provide more formal discussion on what is called "non-event event".

Let's defer the name discussion for a while - and try to define what it is:

This pattern is a function of two things (as many patterns are): an event E, and a context C, for simplicity let's assume that this is a temporal context only - i.e. a time window.

The definition:
  • Forall t: t is a time-stamp and t is an element of C, the event e does not occur in C.
We can of course enable a condition - saying "event E that satisfies predicate P", but this does not change the principle.

The first question is - does the fact that an event E does not occur in a time-stamp t is an event - i.e. is a "negative event" an event by itself - the answer is NO - the fact that an event does not happen in a certain time-stamp is not an event, since an event is something that happens and not something that does not happen.

The second question is - is the definition of pattern shown above is an event ? the answer is - like any other pattern, the detection of a pattern may create a derived event. The semantics is that the detection of this pattern may be interpreted as a situation that requires some reaction, and in this case, we chose to derive an event that designates that this situation has been reported, this does not change the answer to the first question - the negative event at any time-stamp is still not an event!

Now - to the question of WHEN such a derived event is reported. In general, a derived event may be reported immediately when the pattern is detected, or in a deferred mode (this term is taken from active databases). There are patterns that only make sense in deferred mode - this pattern is one of them - since it is only make sense to talk about the fact that an event did not happen during a time-interval - at the end of this time-interval. Deferred report of derived events typically occurs at the end of a temporal context, but it can also have something like temporal coherence condition, such as: report within 2 hours. The reporting of the derived event is time-based, but this is true not only for this pattern, but to any derived event that is reported in deferred mode.

Last but not least -- the name : non-event event is a bad name, since it has a flavor of negative events as noted in question 1. The names - "absent pattern", "not pattern", and "time-out pattern" are used for it. Time-out can be a good name - but need to think if it general enough.

Back to other topics -- in the next post.

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