The default timestamp combiner used to be earliest as well. On Fri, Feb 12, 2021 at 12:10 AM Reuven Lax <re...@google.com> wrote:
> IIRC, this was introduced because at the time users complained that > sliding windows were virtually unusable for reasonably-sized windows. > However this was before we allowed customizing the timestamp combiner, so > maybe this is less of a problem now? > > On Thu, Feb 11, 2021 at 10:53 PM Robert Bradshaw <rober...@google.com> > wrote: > >> On Wed, Feb 10, 2021 at 8:03 PM Kenneth Knowles <k...@apache.org> wrote: >> >>> >>> >>> On Wed, Feb 10, 2021 at 2:24 PM Alex Amato <ajam...@google.com> wrote: >>> >>>> >>>> >>>> On Wed, Feb 10, 2021 at 12:14 PM Kenneth Knowles <k...@apache.org> >>>> wrote: >>>> >>>>> On a PR (https://github.com/apache/beam/pull/13927) we got into a >>>>> discussion of a very old and strange feature of Beam that I think we >>>>> should >>>>> revisit. >>>>> >>>>> The WindowFn has the ability to shift timestamps forward in order to >>>>> unblock downstream watermarks. Why? Specifically in this situation: >>>>> >>>>> - aggregation/GBK with overlapping windows like SlidingWindows >>>>> - timestamp combiner of the aggregated outputs is EARLIEST of the >>>>> inputs >>>>> - there is another downstream aggregation/GBK >>>>> >>>>> The output watermark of the upstream aggregation is held to the >>>>> minimum of the inputs. When an output is emitted, we desire the output to >>>>> flow through the rest of the pipeline without delay. However, the >>>>> downstream aggregation can (and often will) be delayed by the window size >>>>> because of *watermark holds in other later windows that are not >>>>> released until those windows output.* >>>>> >>>> Could you describe this a bit more? Why would later windows hold up the >>>> watermark for upstream steps. (Is it due to some subtlety? Such as tracking >>>> the watermark for each stage, rather than for each step?) >>>> >>> >>> It does not have to do with stages/fusion (a runner-specific concept) >>> but is a necessity of watermarks being per-PCollection. >>> >>> Suppose: >>> >>> - Default triggering >>> - Timestamp combiner EARLIEST >>> - 60s windows sliding every 10s >>> - An element with timestamp 42 >>> - Aggregation (A) with downstream aggregation (B) >>> >>> Here is what happens: >>> >>> - The element falls into [-10, 50) and [0, 60) and [10, 70) and [20, >>> 80) and [30, 90) and [40, 100) >>> - For each of those windows the output watermark hold is set to 42 (the >>> element's timestamp) >>> - At time 50 the aggregation (A) over the first window is emitted; the >>> other windows remain buffered and held >>> - The element arrives at aggregation (B) and is buffered because the >>> input watermark (which is the held output watermark from A) is still 42, >>> even though no other data will arrive for that window (WLOG if elements >>> from other keys are shuffled in) >>> - The input watermark for aggregation (B) does not advance past 42 >>> until the [40, 100) window is fired and releases its watermark hold >>> >>> It is, indeed, subtle. To me, anyhow. I was wrong - it is not delayed by >>> the window size, but by the difference in end-of-window timestamps to all >>> assigned windows (window size minus slide?) >>> >>> So to avoid this, what actually happens in Java today is that the >>> watermark hold, and output timestamp, is set not to 42 but altered to 50 to >>> not overlap the prior window. Timestamp of 50 is very nonintuitive since >>> you asked for the EARLIEST of input timestamps. EARLIEST combiner plays an >>> important role in CoGBK based joins in SQL, where the iterables are >>> re-exploded with timestamps that may be the minimum of input elements. This >>> shifting may actually break SQL... >>> >>> This predated our switch away from "delta from watermark" late data >>> dropping to "window expiry" data dropping. So maybe there is some new way >>> to set a hold that does not make data late or droppable but still use the >>> EARLIEST timestamp. That is my question, for which I have not figured out >>> the answer. >>> >> >> This is, indeed, a very tough question... >> >> I'd say this is generally a problem with EARLIEST and non-aligned >> windows. E.g. for sessions, a long key can hold up the watermark for all >> others. Here we "know" what the hold up is, and can adjust for it. But I >> don't think doing this adjustment is the right thing. It would certainly >> seem to mess up the timestamp of the outputs from a join. And it's possible >> that the values get re-windowed in which case this element should get >> joined with itself from a later window (which I'll admit is a bit odd, but >> maybe a reflection that multiple-windowing, like multi-firing triggering, >> is non-local). >> >> Logicaly, the reason we want [-10 50) window for B to fire shortly after >> the input watermark for A passes 50 because no non-late data coming out of >> A could influence it. In some sense, the "watermark" for the [-10, 50) >> windows has indeed passed, but not that for later windows. I don't think >> the beam model requires that we have a single watermark, just that we fire >> triggers/timers once we have seen all the on-time data that we think we >> could, and a runner could be smart about this. >> >> We may want to keep the ability to shift timestamps for WindowFns, but I >> think we shouldn't be doing so for the default sliding windows. Correctness >> (of output timestamps) over latency unless one asks otherwise. >> >> >>> Kenn >>> >>> >>>> >>>>> To avoid this problem, element x in window w will have its timestamp >>>>> shifted to not overlap with any earlier windows. It is a weird behavior. >>>>> It >>>>> fixes the watermark hold problem but introduces a strange output with a >>>>> mysterious timestamp that is hard to justify. >>>>> >>>>> Any other ideas? >>>>> >>>>> Kenn >>>>> >>>>
