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From Fabian Hueske <fhue...@gmail.com>
Subject Re: Interpretation of Trigger and Eviction on a window
Date Fri, 27 Nov 2015 14:20:37 GMT
Hi Nirmalya,

it is correct that the evictor is called BEFORE the window function is
applied because this is required to support certain types of sliding
If you want to remove all elements from the window after the window
function was applied, you need a trigger that purges the window. The
CountTrigger is not doing this, how ever you can wrap it in a


Best, Fabian

2015-11-27 13:25 GMT+01:00 Nirmalya Sengupta <sengupta.nirmalya@gmail.com>:

> Hello there.
> I have just started exploring Apache Flink, and it has immediately got me
> excited. Because I am a beginner, my questions may be a bit naive. Please
> bear with me.
> I refer to this particular sentence from Flink 0.10.0 Guide
> <https://ci.apache.org/projects/flink/flink-docs-release-0.10/apis/streaming_guide.html#windows-on-unkeyed-data-streams>
> :
> ' *After the trigger fires, and before the function (e.g., sum, count) is
> applied to the window contents, an optional Evictor removes some elements
> from the beginning of the window before the remaining elements are passed
> on to the function* '
> I am a bit confused with the assertion that elements are evicted *before*
> the function is applied. Let me elaborate what my understanding is.
> Let us say that my window has a 'count' trigger of 10 elements, with some
> time-pane of 2 seconds (assumption: rate of ingestion is fast enough for at
> least 10 elements to arrive within 2 seconds).
> windowedStream.*trigger*(CountTrigger.of(10)).*evictor*(CounEvictor.of(10)).sum(_._1)
> // summation of the 2nd field of a tuple
> Now, till the time 9 elements have gathered in the window, the trigger is
> dormant. After the 10th element  enters the window-pane, the trigger is
> fired. At this point in time, all these 10 elements should be passed to the
> _sum_ function so that correct summated value is generated and **only
> then** the evictor is allowed to take out all the 10 elements leaving the
> window-pane empty. The window's element count is set to zero and  it awaits
> the arrival of the next element.
> However, what the documents seems to suggest is that the evictor will be
> able to take out _some_ (how many?) elements from the _beginning_ of the
> window, before the _sum_ function can see the elements. Isn't this
> counterintuitive or am I missing something obvious here?
> Will keenly wait to hear from you.
> -- Nirmalya
> --
> Software Technologist
> http://www.linkedin.com/in/nirmalyasengupta
> "If you have built castles in the air, your work need not be lost. That is
> where they should be.
> Now put the foundation under them."

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