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From Frank Dekervel <ker...@gmail.com>
Subject more complex patterns for CEP (was: CEP two transitions to the same state)
Date Fri, 16 Sep 2016 20:04:06 GMT

i did some more analysis wrt the problem i'm facing and the flink CEP api.

In order to complete the problem i'm facing using flink CEP i would need 3
additions to the API (i think). I tried to understand the NFA logic, and i
think 2 of them should be doable without fundamental changes.

First is to add a "negative" pattern (notFollowedBy / notNext):

Reason is the flow below: i have a start and a termination event, and an
optional "failure" event in between. i want all succesful termination
events, so i want to express there should not be a failure event between
the start and the termination event. Note that there is no "success" event
in this case on which i could match.

[image: Inline image 1]

To implement, upon checking whether a transition would be possible, one
would first need to check if it was not already dead-ended by a
notFollowedBy / notNext. This would add a bit of complexity to the logic
(when seeing if a transition is valid for a state, first check if on this
state there was not already a match made to an notFollowedBy/notNext state.
in that case one would reject the match)

Second is to allow the filterfunction to inspect the partial match made, so
one would be able to filter based on the already-matched event. Reason is
the following (hypothetical) example where we would match arrivals of a
trains in a station. We cannot keyBy train (because the "occupied" events
of the station don't have train information), neither can we keyBy station
(as the start of the sequence is outside the station), so we need to add an
additional condition for the second event: the train number must equal the
train number of the first one. And in the third event, the station number
should equal the station number of the second one.

I think this could be accomplished by overloading the where function with a
second filterfunction variant that takes 2 parameters: the event considered
+ the partial match (as a Map<String,T> with T the class of the event)

[image: Inline image 2]

Third one is - i think - more difficult to accomplish, and that's more
complex graphs i asked in my original e-mail (eg two states having 2
transitions ending in the same state).
The problem here is that it allows one to construct cyclic states, and the
PatternStream takes a Map<String,T> as input, which cannot express a state
occuring twice, neither the order (which event was the first and which
event was the second). In the problem i'm trying to solve cyclic states are
not necessary, but i can imagine usecases exist.

[image: Inline image 3]
I think the NFA implementation would already allow such scenario's but the
nfacompiler and the CEP api would need changing.

I wonder if the problem i'm facing is exotic (so a custom CEP would be more
logic) or it is just something that should be implemented in the flink CEP.
I'm relatively new to CEP, so i cannot compare which other
systems/implementations. I'd like to try implementing the changes myself
(at least the first two) after taking some advice here ...


On Wed, Sep 14, 2016 at 5:22 PM, Frank Dekervel <kervel@gmail.com> wrote:

> Hello,
> I'm trying to model a FSM using the flink CEP patterns. However, there is
> something i can't figure out as all the documentation examples are linear
> (either you go to the single possible next state, either no match).
> Suppose that two transitions lead from one state to two different states.
> I guess this is doable by just defining multiple followedBy/next on the
> same state.
> But what about two different states that can end up in the same state (in
> the order / delivery example: suppose there are two different delivery
> methods, having a separate starting state but resulting in the same end
> state). It is possible to deduplicate the "delivered" state but this would
> lead to difficult to manage patterns when things get more complex.
> Thanks!
> greetings,
> Frank

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