Thanks, Till. I will wait for your response.
- LF
From: Till Rohrmann <trohrm...@apache.org>
To: user@flink.apache.org; lg...@yahoo.com
Sent: Tuesday, October 11, 2016 2:49 AM
Subject: Re: more complex patterns for CEP (was: CEP two transitions to the
same state)
The timeline is hard to predict to be honest. It depends a little bit on how
fast the community can proceed with these things. At the moment I'm personally
involved in other issues and, thus, cannot work on the CEP library. I hope to
get back to it soon.
Cheers,Till
On Sat, Oct 8, 2016 at 12:42 AM, <lg...@yahoo.com> wrote:
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 <trohrm...@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
Cheers,Till
On Fri, Sep 16, 2016 at 10:04 PM, Frank Dekervel <ker...@gmail.com> wrote:
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 <ker...@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
