A CEP Quiz: streams and relations

Last week, we were training some of Oracle’s top consulting and partners on CEP.

The training was realized in Reading, UK (near London). Beautiful weather, contrary to what I was told is the common English predicament for October.

At the end, we gave a quiz, which I am reproducing here:

Consider an input channel C1 of the event type E1, defined as having a single property called p1 of type Integer.

An example of events of type E1 are { p1 = 1 } and { p1 = 2 }.

This input channel C1 is connected to a processor P, which is then connected to another (output) channel C2, whose output is sent to cache CACHE. Assume CACHE is keyed on p1.

Next, consider three CQL queries as follows which reside on processor P:

1. SELECT * FROM C1[NOW]
2. ISTREAM (SELECT * FROM C1[NOW])
3. DSTREAM (SELECT * FROM C1[NOW])

Finally, send a single event e1 = { p1 = 1 } to S1.

The question is: what should be the content of the cache at the end for each one of these three queries?

To answer this, a couple of points need to be observed.

First, as I have mentioned in the past, CEP deals with two main concepts: that of a stream and that of a relation.

A stream is container of events, which is unbounded, and only supports inserts. Why only inserts? Well, because there is no such thing as a stream delete, think about it, how could we delete an event that has already happened?!

Whereas a relation is a container of events that is bounded by a certain number of events. A relation supports inserts, deletes, and updates.

Second, remember that a cache is treated like a table, or more precisely, like a relation, and therefore supports insert, delete, and update operations. In the case the query outputs a stream, then the events inserted into the stream are mapped to inserts (or puts) into the cache. If the query outputs a relation, then inserts into the relation are likewise mapped into puts into the cache, however a delete on the relation, becomes a remove of an entry in the cache.

Third, keep in mind that the operations ISTREAM (i.e. insert stream) and DSTREAM (i.e. delete stream) convert relations to streams. The former converts relation inserts into stream inserts, but ignores the relation updates and deletes. The latter converts relation deletes into stream inserts, and ignores relation inserts and updates (in reality, things are a bit more complicated, but let’s ignore the details for the time being).

Fourth, we want the answer as if time has moved on from ‘now’. For all purpose, say we measuring time in seconds, and we sent event e1 at time 1 second and want the answer at time 2 seconds.

I will post the answer in a follow up post next week.

The crucial point of this exercise is to understand the difference between two of the most important CEP concepts: that of a STREAM and RELATION, and how they relate to each other.

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