Hi,
We are also looking for negation (absence of an event) functionality in Flink
CEP. Something like notFollowedBy/notNext that detects the following patterns
will be great additions to Flink CEP (other CEP frameworks support negation):1.
Occurrence of an event (that matches specific criteria) followed by absence of
an event (that matches specific criteria) followed by another event (that
matches specific different criteria)2. Occurrence of an event (that matches
specific criteria) followed by absence of an event (that matches specific
criteria) for a specific period
Could this be implemented?
Thank you.
- LF
From: Frank Dekervel <[email protected]>
To: [email protected]
Sent: Friday, September 16, 2016 1:04 PM
Subject: more complex patterns for CEP (was: CEP two transitions to the same
state)
Hello,
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.
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)
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.
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 ...
thanks!greetings,Frank
On Wed, Sep 14, 2016 at 5:22 PM, Frank Dekervel <[email protected]> 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
