On Tue, Apr 27, 2021 at 11:25 AM Jan Lukavský <[email protected]> wrote: > > > Are you asking for a way to ignore early triggers on side input mapping, > > and only map to on-time triggered values for the window? > > No, that could for sure be done before applying the View transform. I'd like > a know if it would be possible to create mode of the matching which would be > deterministic. One possibility to make it deterministic seems to be, that > main input elements would be pushed back until side input watermark 'catches > up' with main input. Whenever the side input watermark would be delayed after > the main input watermark, elements would start to be pushed back again. Not > sure if I'm explaining it using the right words. The side input watermark can > be controlled using timer in an upstream transform, so this defines which > elements in main input would be matched onto which pane of the side input.
Perhaps I'm not following the request correctly, but this is exactly how side inputs work by default. It is only when one explicitly requests a non-deterministic trigger upstream of the side input (e.g. one that may fire multiple times or ahead of the watermark) that one sees a side input with multiple variations or data in the side input before the watermark of the side input is caught up to the main input. > On 4/27/21 8:03 PM, Reuven Lax wrote: > > > > On Tue, Apr 27, 2021 at 10:53 AM Jan Lukavský <[email protected]> wrote: >> >> > If using early triggers, then the side input loads the latest trigger for >> > that window. >> >> Does not the the word 'latest' imply processing time matching? The watermark >> of main input might be arbitrarily delayed from the watermark of side input. >> If we consider GlobalWindows on both sides, than "latest" trigger in side >> input looks exactly as processing time matching. Yes, between different >> windows, the matching is in event time. But within the same window (ignoring >> the window mapping for now), the matching looks like processing time, right? > > > Not really. Think of each trigger of a window as a refinement - so the latest > trigger for that window is our best approximation of the "correct" value for > that window. For this reason, window panes are indexed by an integer > (pane_index), not by a timestamp. The idea here is that the main input > element maps to the side input for the best, most-recent knowledge of the > window. > > Are you asking for a way to ignore early triggers on side input mapping, and > only map to on-time triggered values for the window? > >> >> If we look at the SimplePushbackSideInputDoFnRunner used by runners exactly >> for the side input matching, there is no testing of side input watermark to >> determine if an element should be 'pushed back' or processed. Element is >> processed if, and only if, all side inputs for particular window are ready. >> >> On 4/27/21 7:24 PM, Reuven Lax wrote: >> >> The windows shouldn't need to match - at least if the FlinkRunner >> implementation is correct. By default, the side-input's WindowFn should be >> used to map the main input's timestamp into a window, and that window is >> used to determine which version of the side input to load. A custom WindowFn >> can be used to to even more - e.g. if you want the main input element to map >> to the _previous_ window in the side input (you would do this by overloading >> getDefaultWindowMappingFn). >> >> If using early triggers, then the side input loads the latest trigger for >> that window. This is still an event-time mapping - for example two >> main-input events in two different windows will still map the the side input >> for the matching window. However if the side input PCollection is triggered, >> than the latest trigger for each window's side input will be loaded. >> >> It's possible that the FlinkRunner implementation is incomplete, in which >> case it should be fixed. >> >> On Tue, Apr 27, 2021 at 9:36 AM Jan Lukavský <[email protected]> wrote: >>> >>> It seems to me, that this is true only with matching windows on both sides >>> and default trigger of the side input. Then it will (due to condition a)) >>> work as if the matching happenned in event time. But when using any early >>> triggers then it will work in processing time. At least, that is my >>> understanding from studying the code in FlinkRunner. >>> >>> On 4/27/21 4:05 PM, Robert Burke wrote: >>> >>> I thought the matching happened with elements in the matching window, in >>> Event time, not in Processing time. >>> >>> Granted, I'm not that familiar with this area myself, but one key part of >>> Beam is nearly everything is Event time by default, not Processing time. >>> >>> On Tue, Apr 27, 2021, 12:43 AM Jan Lukavský <[email protected]> wrote: >>>> >>>> Hi, >>>> >>>> I have a question about matching side inputs to main input. First a >>>> recap, to make sure I understand the current state correctly: >>>> >>>> a) elements from main input are pushed back (stored in state) until a >>>> first side input pane arrives (that might be on time, or early) >>>> >>>> b) after that, elements from the main input are matched to the current >>>> side input view - the view is updated as new data arrives, but is >>>> matched to the main input elements in processing time >>>> >>>> If this is the current state, my question is, would it be possible to >>>> add a new mode of matching of side inputs? Something like >>>> >>>> ParDo.of(new MyDoFn()).withSideInput("name", myView, >>>> TimeDomain.EVENT_TIME) >>>> >>>> the semantics would be that the elements from the main PCollection would >>>> be stored into state as pairs with the value of the current main input >>>> watermark and on update of side-input watermark only main input elements >>>> with associated input watermark less than that of the side input would >>>> be matched with the side input and sent downstream. Although this >>>> approach is necessarily more expensive and introducing additional >>>> latency than processing time matching, there are situations when >>>> processing time matching is inapropriate for correctness reasons. >>>> >>>> WDYT? >>>> >>>> Jan >>>>
