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From Aljoscha Krettek <aljos...@apache.org>
Subject Re: [DISCUSS] Allowed Lateness in Flink
Date Wed, 06 Jul 2016 13:19:15 GMT
Hi,
I cleaned up the document a bit and added sections to address comments on
the doc:
https://docs.google.com/document/d/1Xp-YBf87vLTduYSivgqWVEMjYUmkA-hyb4muX3KRl08/edit?usp=sharing
(I
also marked proposed features that are already implemented as [done].)

The main thing that remains to be figured out is how we deal with purging,
i.e. whether the trigger can decide to purge a window or whether the
WindowOperator should do this and also what happens when window state is
garbage collected. The original proposal was to reduce the current set of
trigger results from (CONTINUE, FIRE, PURGE, FIRE_AND_PURGE) to (CONTINUE,
FIRE) and have a global flag in the WindowOperator that says whether firing
windows should be purged (DISCARDING) or kept for a bit, until the allowed
lateness expires (ACCUMULATING). Based on comments by Elias I added a
section that sketches an alternative where the triggers are in charge of
purging and also decide what should happen in case of window cleanup.

One thing we should also keep in mind is how we can make the windowing API
easy to use for people that don't need all the bells and whistles of custom
triggers, allowed lateness and so on. This is partially covered by the
proposal to add composite triggers but I feel we can go further there.

In the future, it might be good to to discussions directly on the ML and
then change the document accordingly. This way everyone can follow the
discussion on the ML. I also feel that Google Doc comments often don't give
enough space for expressing more complex opinions.

Cheers,
Aljoscha


On Mon, 30 May 2016 at 11:23 Aljoscha Krettek <aljoscha@apache.org> wrote:

> Thanks for the feedback! :-) I already read the comments on the file.
>
> On Mon, 30 May 2016 at 11:10 Gyula Fóra <gyula.fora@gmail.com> wrote:
>
>> Thanks Aljoscha :) I added some comments that might seem relevant from the
>> users point of view.
>>
>> Gyula
>>
>> Aljoscha Krettek <aljoscha@apache.org> ezt írta (időpont: 2016. máj. 30.,
>> H, 10:33):
>>
>> > Hi,
>> > I created a new doc specifically about the interplay of lateness and
>> > window state garbage collection:
>> >
>> https://docs.google.com/document/d/1vgukdDiUco0KX4f7tlDJgHWaRVIU-KorItWgnBapq_8/edit?usp=sharing
>> >
>> > There is still some stuff that needs to be figured out, both in the new
>> > doc and the existing doc. For example, we need to decide whether to make
>> > accumulating/discarding behavior global for a window operation or
>> > controllable by triggers. Initially, I suggested to make
>> > accumulating/discarding a global setting for the window operation
>> because
>> > we can get away with keeping less state if we know that we always
>> discard
>> > when firing. Please take a look at the new doc to see what I'm talking
>> > about there.
>> >
>> > Feedback very welcome!
>> >
>> > Cheers,
>> > Aljoscha
>> >
>> > On Tue, 26 Apr 2016 at 16:45 Aljoscha Krettek <aljoscha@apache.org>
>> wrote:
>> >
>> >> Hi Max,
>> >> thanks for the Feedback and suggestions! I'll try and address each
>> >> paragraph separately.
>> >>
>> >> I'm afraid deciding based on the "StreamTimeCharacteristic is not
>> >> possible since a user can use processing-time windows in their job even
>> >> though the set the characteristic to event-time. Enabling event time
>> does
>> >> not disable processing time, it just enables an additional feature.
>> (IMHO,
>> >> the handling of the StreamTimeCharacteristic is still somewhat
>> problematic.)
>> >>
>> >> Making the decision based purely on the class of the WindowAssigner is
>> >> also not possible since we don't know in advance which WindowAssigners
>> the
>> >> users will write and what time characteristic they will use.
>> >>
>> >> Regarding the third proposition. Removing 'System.currentTimeMillis()'
>> is
>> >> very desirable and part of my proposal. However, it is still meant as
>> being
>> >> separate from "event-time" since a Trigger/WindowAssigner might need
>> both.
>> >> For example, a Trigger might want to do early triggering a few
>> >> (processing-time) seconds after the first elements arrive and proper
>> >> triggering once the watermark for the end of the window arrives.
>> >>
>> >> These are good ideas but I'm afraid we still don't have a good
>> solution.
>> >> This whole processing time/event time business is just very tricky.
>> >>
>> >> Cheers,
>> >> Aljoscha
>> >>
>> >> On Tue, 26 Apr 2016 at 16:26 Maximilian Michels <mxm@apache.org>
>> wrote:
>> >>
>> >>> Hi Aljoscha,
>> >>>
>> >>> Thank you for the detailed design document.
>> >>>
>> >>> Wouldn't it be ok to allow these new concepts regardless of the time
>> >>> semantics? For Event Time and Ingestion Time "Lateness" and
>> >>> "Accumulating/Discarding" make sense. If the user chooses Processing
>> >>> time then these can be ignored during translation of the StreamGraph
>> >>> (possibly with a warning).
>> >>>
>> >>> Detecting when these concepts make sense should be possible by
>> >>> checking the "Stream Charateristics" of the ExecutionEnvironment or
>> >>> the involved classes (e.g. SlidingProcessingTimeWindows) in the
>> >>> StreamGraph. If the users uses a custom WindowAssigner then the user
>> >>> has to take care that it is used correctly. I don't like the
>> >>> "isEventTime()" method. Even with the additional method, users could
>> >>> return 'true' there although they meant 'false', right? So this does
>> >>> not really solve the problem that it is hard to distinguish Event Time
>> >>> and Processing Time semantics in Flink.
>> >>>
>> >>> Another approach that I could think of is getting rid of
>> >>> 'System.currentTimeMillis()' and only allow to get time via a special
>> >>> interface that WindowAssigners implement. Then we could determine what
>> >>> time is assigned and also verify that it is actually used (in contrast
>> >>> to the isEventTime() method). Would that be an option or would it
>> >>> break the API?
>> >>>
>> >>> Cheers,
>> >>> Max
>> >>>
>> >>> On Tue, Apr 5, 2016 at 12:29 PM, Aljoscha Krettek <
>> aljoscha@apache.org>
>> >>> wrote:
>> >>> > By the way. The way I see to fixing this is extending WindowAssigner
>> >>> with
>> >>> > an "isEventTime()" method and then allow accumulating/lateness
in
>> the
>> >>> > WindowOperator only if this is true.
>> >>> >
>> >>> > But it seems a but hacky because it special cases event-time. But
>> then
>> >>> > again, maybe we need to special case it ...
>> >>> >
>> >>> > On Tue, 5 Apr 2016 at 12:23 Aljoscha Krettek <aljoscha@apache.org>
>> >>> wrote:
>> >>> >
>> >>> >> Hi Folks,
>> >>> >> as part of my effort to improve the windowing in Flink [1]
I also
>> >>> thought
>> >>> >> about lateness, accumulating/discarding and window cleanup.
I have
>> >>> some
>> >>> >> ideas on this but I would love to get feedback from the community
>> as I
>> >>> >> think that these things are important for everyone doing event-time
>> >>> >> windowing on Flink.
>> >>> >>
>> >>> >> The basic problem is this: Some elements can arrive behind
the
>> >>> watermark
>> >>> >> if the watermark is not 100 % correct (which it is not, in
most
>> >>> cases, I
>> >>> >> would assume). We need to provide API that allows to specify
what
>> >>> happens
>> >>> >> when these late elements arrive. There are two main knobs for
the
>> >>> user here:
>> >>> >>
>> >>> >> - Allowed Lateness: How late can an element be before it is
>> completely
>> >>> >> ignored, i.e. simply discarded
>> >>> >>
>> >>> >> - Accumulating/Discarding Fired Windows: When we fire a window,
do
>> we
>> >>> >> purge the contents or do we keep it around until the watermark
>> passes
>> >>> the
>> >>> >> end of end window plus the allowed lateness? If we keep the
window
>> a
>> >>> late
>> >>> >> element will be added to the window and the window will be
emitted
>> >>> again.
>> >>> >> If don't keep the window then the late element will essentially
>> >>> trigger
>> >>> >> emission of a one-element window.
>> >>> >>
>> >>> >> This is somewhat straightforward to implement: If accumulating
set
>> a
>> >>> timer
>> >>> >> for the end of the window plus the allowed lateness. Cleanup
the
>> >>> window
>> >>> >> when that fires (basically). All in event-time with watermarks.
>> >>> >>
>> >>> >>  My problem is only this: what should happen if the user specifies
>> >>> some
>> >>> >> allowed lateness and/or accumulating mode but uses processing-time
>> >>> >> windowing. For processing-time windows these don't make sense
>> because
>> >>> >> elements cannot can be late by definition. The problem is,
that we
>> >>> cannot
>> >>> >> figure out, by looking at a WindowAssigner or the Windows that
it
>> >>> assigns
>> >>> >> to elements whether these windows are in event-time or
>> processing-time
>> >>> >> domain. At the API level this is also not easily visible, since
a
>> user
>> >>> >> might have set the "stream-time-characteristic" to event-time
but
>> >>> still use
>> >>> >> a processing-time window (plus trigger) in the program.
>> >>> >>
>> >>> >> Any ideas for solving this are extremely welcome. :-)
>> >>> >>
>> >>> >> Cheers,
>> >>> >> Aljoscha
>> >>> >>
>> >>> >> [1]
>> >>> >>
>> >>>
>> https://docs.google.com/document/d/1Xp-YBf87vLTduYSivgqWVEMjYUmkA-hyb4muX3KRl08/edit#heading=h.psfzjlv68tp
>> >>> >>
>> >>>
>> >>
>>
>

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