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From <lg...@yahoo.com>
Subject Re: more complex patterns for CEP (was: CEP two transitions to the same state)
Date Fri, 07 Oct 2016 22:42:33 GMT
hi Till,
Thanks for the detailed response.
I'm looking forward to seeing these features implemented in Flink. Can anyone provide timelines
for the 3 tickets that you mentioned in your response?  - LF

      From: Till Rohrmann <trohrmann@apache.org>
 To: user@flink.apache.org 
 Sent: Tuesday, September 20, 2016 7:13 AM
 Subject: Re: more complex patterns for CEP (was: CEP two transitions to the same state)
Hi Frank,
thanks for sharing your analysis. It indeed pinpoints some of the current CEP library's shortcomings.
Let me address your points:
1. Lack of not operator
The functionality to express events which must not occur in a pattern is missing. We've currently
a JIRA [1] which addresses exactly this. For the notFollowedBy operator, we should discard
all patterns where we've seen a matching event for the not state. I think it could be implemented
like a special terminal state where we prune the partial pattern.
For the notNext operator, we could think about keeping the event which has not matched the
notNext state and return it as part of the fully matched pattern. Alternatively, we could
simply forget about it once we've assured that it does not match.
2. Allow functions to access fields of previous events
This hasn't been implemented yet because it is a quite expensive operation. Before calling
the filter function you always have to reconstruct the current partial pattern and then give
it to the filter function. But I agree that the user should be allowed to use such a functionality
(and then pay the price for it in terms of efficiency). Giving access to the partially matched
fields via a Map would be a way to solve the problem on the API level.
I think that almost all functionality for this feature is already in place. We simply would
have to check the filter condition whether they require access to previous events and then
compute the partial pattern.
3. Support for recursive patterns
The underlying SharedBuffer implementation should allow recursive event patterns. Once we
have support for branching CEP patterns [2] which allow to connect different states this should
also be possible with some minor changes.
However, a more interesting way to specify recursive CEP patterns is to use regular expression
syntax (Kleene star, bounded occurrences) to express recursive parts of a pattern. I think
this makes specifying such a pattern easier and more intuitive for the user. We've also a
JIRA issue to track the process there [3] and Ivan is already working on this.
If you want to get involved in Flink's CEP development, then feel free to take over any free
JIRA issue or create one yourself :-)
[1] https://issues.apache.org/jira/browse/FLINK-3320
[2] https://issues.apache.org/jira/browse/FLINK-4641[3] https://issues.apache.org/jira/browse/FLINK-3318
On Fri, Sep 16, 2016 at 10:04 PM, Frank Dekervel <kervel@gmail.com> wrote:

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 ...

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

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.

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