One large WindowFunction vs. several smaller ones

[Maarten Hamal <ha...@gmail.com>]

Hi

I'm pretty new to Flink and stream processing in general but I'm writing a
thesis about it and I have a small question regarding WindowFunctions (WF).
I have one large WF that does essentially two things: counting how much
events there are in a window and calculating the difference between this
and the previous window. And in the end I need the output of both
calculations.

I'm in doubt if I should use one large WF that does both in combination
with a simple count-WF to get the result of the count
OR a small count-WF and a small diff-WF so that I can run the count-WF
first and supply the result to both a sink and to the diff-WF.

Graphically these are the two options:
[image: Inline afbeelding 1]

If I think about it every option has positive and negative sides:
The blue option (Count-WF + Diff-WF):

   - + has more modularity and every WF does exactly one thing
   - - I need to do an extra KeyBy and Window operation for the Diff-WF
   with the same values as for the Count-WF (performance hit)

While the red option (Count-WF + Large WF):

   - + might have better performance
   - - has less modularity


So I would like the opinion of someone who has more experience with Flink
that I have (so almost everybody here).

Best regards,
Maarten

Re: One large WindowFunction vs. several smaller ones

[Nico Kruber <ni...@data-artisans.com>]

Hi Maarten,
If the Count-WF is counting the number of events per window and the Diff-WF is 
just comparing this number to the output of the previous window, then you do 
not need a WindowFunction for the Diff-WF afterall:

Just use your Count-WF and plug in a stateful map (also see [1]) afterwards 
which stores the previous value, compares it, and emits (for example) a tuple 
with {key, count, diff}, or have them separate as you wish.


Nico

[1] https://ci.apache.org/projects/flink/flink-docs-release-1.3/dev/stream/
state.html
On Sunday, 13 August 2017 01:47:13 CEST Maarten Hamal wrote:
> Hi
> 
> I'm pretty new to Flink and stream processing in general but I'm writing a
> thesis about it and I have a small question regarding WindowFunctions (WF).
> I have one large WF that does essentially two things: counting how much
> events there are in a window and calculating the difference between this
> and the previous window. And in the end I need the output of both
> calculations.
> 
> I'm in doubt if I should use one large WF that does both in combination
> with a simple count-WF to get the result of the count
> OR a small count-WF and a small diff-WF so that I can run the count-WF
> first and supply the result to both a sink and to the diff-WF.
> 
> Graphically these are the two options:
> [image: Inline afbeelding 1]
> 
> If I think about it every option has positive and negative sides:
> The blue option (Count-WF + Diff-WF):
> 
>    - + has more modularity and every WF does exactly one thing
>    - - I need to do an extra KeyBy and Window operation for the Diff-WF
>    with the same values as for the Count-WF (performance hit)
> 
> While the red option (Count-WF + Large WF):
> 
>    - + might have better performance
>    - - has less modularity
> 
> 
> So I would like the opinion of someone who has more experience with Flink
> that I have (so almost everybody here).
> 
> Best regards,
> Maarten