> Yes, outputWithTimestamp should likely be restricted to min(elements seen so far).
Am I understanding correctly that in terms of options for immediate fixes, given that some runners such as Flink have only ad hoc bundles, the most feasible way to enforce that "min(elements seen so far)" restriction would be to enforce that watermark updates may only happen at bundle boundaries? Are there other immediate term options? - Evan On Tue, Mar 29, 2022 at 4:11 AM Jan Lukavský <je...@seznam.cz> wrote: > > There's another interesting API, which is being discussed for the > > internal variant of Dataflow, which is that rather than allowing one > > to fabricate timestamps (or windows) ex nihilo one would instead need > > ot ask for a "timestamped" or "windowed" element in the Process > > method, from which one could construct a new timestamped/windowed > > element (with a new value, but the same timestamp/window/paneinfo) > > that could then be safely emitted. I'm curious how constraining this > > would be. > > I'm not sure I follow. Do you suggest that - for the case of in-memory > batching - one would store a TimestampedValue in the buffer and when > flushing the buffer one would say "I'm emitting this value, that was > created based on this input element"? That seems to work fine, though I > suppose this is probably not the main motivation for such API. :) > > On 3/28/22 20:54, Robert Bradshaw wrote: > > On Mon, Mar 28, 2022 at 11:45 AM Jan Lukavský <je...@seznam.cz> wrote: > >> On 3/28/22 20:17, Reuven Lax wrote: > >> > >> On Mon, Mar 28, 2022 at 11:08 AM Robert Bradshaw <rober...@google.com> > wrote: > >>> On Mon, Mar 28, 2022 at 11:04 AM Reuven Lax <re...@google.com> wrote: > >>>> On Mon, Mar 28, 2022 at 10:59 AM Evan Galpin <evan.gal...@gmail.com> > wrote: > >>>>> I don't believe that the issue is Flink specific but rather that > Flink is one example of many potential examples. Enforcing that watermark > updates can only happen at bundle boundaries would ensure that any data > buffered while processing a single bundle in a DoFn could be output > ON_TIME, especially without any need for a TimerSpec to explicitly hold the > watermark for that purpose. This is in reference to data buffered within a > single bundle, and not cross-bundle buffering such as in the case of > GroupIntoBatches. > >>>> > >>>> Any in-flight data (i.e. data being processed that is not yet > committed back to the runner) must hold up the output watermark. Since in > the Beam model all records in a bundle are somewhat atomic (e.g. if the > bundle succeeds, none of of them should be replayed in a proper > exactly-once runner), I think this implicitly means that any elements in an > in-flight bundle must hold up the watermark. This doesn't mean that the > watermark can't advance while the bundle is in flight -just that it can't > advance past any of the timestamps outstanding in the bundle. > >>> Yes. The difficulty is that we don't have much visibility into > >>> "timestamps outstanding in the bundle" so we have to take > >>> min(timestamps of input elements in the bundle) which is not that > >>> different from only having watermark updates at bundle boundaries. > >> > >> Exactly. > >> > >> Agree, this works exactly the same. The requirement is not to not > update the watermark, but not to update it past any on-time element in the > bundle. Not updating the watermark at all is one solution, computing > min(timestamps in bundle) works the same. Unfortunately, Flink does not > construct bundles in advance, it is more an ad-hoc concept. Therefore the > only way to hold the watermark is not to update it, because the timestamps > of elements that will be part of the bundle are not known. > >> > >> Two more questions: > >> > >> a) it seems that we are missing some @ValidatesRunner tests for this, > right? > >> > >> b) should we relax the restriction of not allowing > outputWithTimestamp() output element before the current element? I think it > should be "before lowest element in the current bundle" or "before output > watermark, if not already late, or not droppable if late (uh, this gets a > little complicated :))". Not allowing outputting element with timestamp > lower than the current element seems to be just a "safety-first" solution > to the problem discussed here and is too restrictive. It could be > worked-around using getAllowedTimestampSkew(), but that can cause errors. > > Yes, outputWithTimestamp should likely be restricted to min(elements > > seen so far). > > > > There's another interesting API, which is being discussed for the > > internal variant of Dataflow, which is that rather than allowing one > > to fabricate timestamps (or windows) ex nihilo one would instead need > > ot ask for a "timestamped" or "windowed" element in the Process > > method, from which one could construct a new timestamped/windowed > > element (with a new value, but the same timestamp/window/paneinfo) > > that could then be safely emitted. I'm curious how constraining this > > would be. > > > >>>>> Take for example a PCollection with 1 second Fixed windowing. The > PCollection holds payload bodies for an external API to which requests will > be made. A hypothetical runner creates a bundle with Element A and Element > B where Element A belongs to the window [0:00:01, 0:00:02) and Element B > belongs to the window [0:00:02, 0:00:03). Assume that the DoFn is going to > buffer all elements in a bundle so as to generate fewer round-trip requests > to the external API, and then output the corresponding responses. The > following is a high-level order of events that could result in data being > labelled as LATE: > >>>>> > >>>>> 1. Watermark is 0:00:00 > >>>>> 2. DoFn receives the bundle containing both Element A and Element B > >>>>> 3. Element A is processed by the DoFn, buffering in-memory and > returning/completing > >>>>> 4. Watermark is (maybe) updated after having processed the element; > let's assume in this example it is in fact updated to 0:00:02 > >>>>> 5. Element B (from the same bundle) is processed by the DoFn > >>>>> 6. It's the end of the bundle, so now the in-memory buffered > entities are used to make a request to external API > >>>>> 7. The API responses are gathered and intended to be output to the > same window from which the corresponding element with request data > originated (Element A and Element B carried this data) > >>>>> 8. The response data associated with the request payload found in > Element A is output with the timestamp of Element A i.e. something in the > range of Element A's window [0:00:01, 0:00:02) > >>>>> 9. The data in the prior step is considered LATE, strictly as a > result of updating the watermark to 0:00:02 in Step 4 above > >>>>> > >>>>> If Step 4 was moved to be the last step in the process (i.e. at the > bundle boundary) this issue would be avoided. I would also argue that > updating the watermark only after receiving a response for an input Element > is a more accurate depiction of having completed processing for the > element. All that said, I could buy the argument that the above > description might represent an anti-pattern of sorts where response data > should actually be output with a timestamp corresponding to its receipt > rather than the timestamp of its corresponding input element carrying the > request body. > >>>>> > >>>>> - Evan > >>>>> > >>>>> On Mon, Mar 28, 2022 at 11:07 AM Reuven Lax <re...@google.com> > wrote: > >>>>>> I agree with you that changing on-time elements to late elements is > incorrect, however I don't quite understand why doing things on bundle > boundaries helps. Is this specific to Flink? > >>>>>> > >>>>>> > >>>>>> > >>>>>> On Mon, Mar 28, 2022 at 1:07 AM Jan Lukavský <je...@seznam.cz> > wrote: > >>>>>>> Hi Robert, > >>>>>>> > >>>>>>> I had the same impression that holding the watermark between > bundles is > >>>>>>> actually not part of the computational model. Now the question is - > >>>>>>> should it be? > >>>>>>> > >>>>>>> As you said, buffering and emitting in-memory buffered data means > >>>>>>> possibly outputting data that arrived as ON_TIME, but is outputted > as > >>>>>>> LATE (or droppable, which is even worse). My understanding is that > this > >>>>>>> is why there is the (deprecated) getAllowedTimestampSkew() method > of > >>>>>>> DoFn, but that only bypasses the check, does not solve the issue > (which > >>>>>>> is why it is deprecated, I suppose). I strongly believe that > outputting > >>>>>>> elements that switch from ON_TIME to LATE is a correctness bug, > because > >>>>>>> it has the potential to violate causality (which is strongly > >>>>>>> counter-intuitive in our universe :)). For some pipelines it can > >>>>>>> definitely cause incorrect outputs. > >>>>>>> > >>>>>>> If we could ensure the output watermark gets updated only between > >>>>>>> @FinishBundle and @StartBundle call then this problem would go > away. I > >>>>>>> looked into the code of FlinkRunner and it seems to me that we > could > >>>>>>> quite easily ensure this by not outputting watermark when a bundle > is > >>>>>>> open and output it once it finishes. I didn't dig into that too > deep, so > >>>>>>> I don't know if there would be any caveats, the question is > apparently, > >>>>>>> if we could make these guarantees for other runners as well and if > we > >>>>>>> could sensibly create a @ValidatesRunner test. > >>>>>>> > >>>>>>> WDYT? > >>>>>>> > >>>>>>> Jan > >>>>>>> > >>>>>>> On 3/25/22 23:06, Robert Bradshaw wrote: > >>>>>>>> I do not think there is a hard and fast rule about updating > watermarks > >>>>>>>> only at bundle boundaries. This seems perfectly legal for a pure > 1:1 > >>>>>>>> mapping DoFn. The issue is that DoFns are allowed to buffer data > and > >>>>>>>> emit them in a later process (or finishBundle). If the watermark > has > >>>>>>>> moved on, that may result in late data. We don't really have a > way for > >>>>>>>> a DoFn to declare *it's* output watermark (i.e. "I promise not to > emit > >>>>>>>> any data before this timestamp.") > >>>>>>>> > >>>>>>>> On Thu, Mar 24, 2022 at 8:10 AM Evan Galpin <egal...@apache.org> > wrote: > >>>>>>>>> Thanks for starting this thread Jan, I'm keen to hear thoughts > and outcomes! I thought I would mention that answers to the questions > posed here will help to unblock a 2.38.0 release blocker[1]. > >>>>>>>>> > >>>>>>>>> [1] https://issues.apache.org/jira/browse/BEAM-14064 > >>>>>>>>> > >>>>>>>>> On Thu, Mar 24, 2022 at 5:28 AM Jan Lukavský <je...@seznam.cz> > wrote: > >>>>>>>>>> Hi, > >>>>>>>>>> > >>>>>>>>>> this is follow-up thread started from [1]. In the thread there > is mentioned multiple times that (in stateless ParDo), the output watermark > is allowed to advance only on bundle boundaries [2]. Essentially that would > mean that anything in between calls to @StartBundle and @FinishBundle would > be processed in single instant in (output) event-time. This makes perfect > sense. > >>>>>>>>>> > >>>>>>>>>> The issue is that it seems that not all runners actually > implement this behavior. FlinkRunner for instance does not have a "natural" > concept of bundles and those are created in a more ad-hoc way to adhere > with the DoFn life-cycle (see [3]). Watermark updates and elements are > completely interleaved without any synchronization with bundle "open" or > "close". If watermark updates are allowed to happen only on boundaries of > bundles, then this seems to break this contract. > >>>>>>>>>> > >>>>>>>>>> The question therefore is - should we consider FlinkRunner as > non-compliant with this aspect of the Apache Beam model or is this an > "optional" part that runners are free to implement at will? In the case of > the former, do we miss some @ValidatesRunner tests for this? > >>>>>>>>>> > >>>>>>>>>> Jan > >>>>>>>>>> > >>>>>>>>>> [1] > https://lists.apache.org/thread/mtwtno2o88lx3zl12jlz7o5w1lcgm2db > >>>>>>>>>> > >>>>>>>>>> [2] > https://lists.apache.org/thread/7foy455spg43xo77zhrs62gc1m383t50 > >>>>>>>>>> > >>>>>>>>>> [3] > https://github.com/apache/beam/blob/14862ccbdf2879574b6ce49149bdd7c9bf197322/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/DoFnOperator.java#L786 > >>>>>>>>>> > >>>>>>>>>> >
