On Wed, May 4, 2022 at 11:03 AM Steven Wu <stevenz...@gmail.com> wrote: > Any opinion on different timestamp for source alignment (vs Flink application > watermark)? For Iceberg source, we might want to enforce alignment on kafka > timestamp but Flink application watermark may use event time field from > payload.
I imagine that more generally the question is alignment based on the iceberg partition/file metadata vs. individual rows? I think that should work as long as there is a guarantee for out of orderness within the split? Thomas > > Thanks, > Steven > > On Wed, May 4, 2022 at 7:02 AM Becket Qin <becket....@gmail.com> wrote: >> >> Hey Piotr, >> >> I think the mechanism FLIP-182 provided is a reasonable default one, which >> ensures the watermarks are only drifted by an upper bound. However, >> admittedly there are also other strategies for different purposes. >> >> In the Iceberg case, I am not sure if a static strictly allowed watermark >> drift is desired. The source might just want to finish reading the assigned >> splits as fast as possible. And it is OK to have a drift of "one split", >> instead of a fixed time period. >> >> As another example, if there are some fast readers whose splits are always >> throttled, while the other slow readers are struggling to keep up with the >> rest of the splits, the split enumerator may decide to reassign the slow >> splits so all the readers have something to read. This would need the >> SplitEnumerator to be aware of the watermark progress on each reader. So it >> seems useful to expose the WatermarkAlignmentEvent information to the >> SplitEnumerator as well. >> >> Thanks, >> >> Jiangjie (Becket) Qin >> >> >> >> On Tue, May 3, 2022 at 7:58 PM Piotr Nowojski <pnowoj...@apache.org> wrote: >> >> > Hi Steven, >> > >> > Isn't this redundant to FLIP-182 and FLIP-217? Can not Iceberg just emit >> > all splits and let FLIP-182/FLIP-217 handle the watermark alignment and >> > block the splits that are too much into the future? I can see this being an >> > issue if the existence of too many blocked splits is occupying too many >> > resources. >> > >> > If that's the case, indeed SourceCoordinator/SplitEnumerator would have to >> > decide on some basis how many and which splits to assign in what order. But >> > in that case I'm not sure how much you could use from FLIP-182 and >> > FLIP-217. They seem somehow orthogonal to me, operating on different >> > levels. FLIP-182 and FLIP-217 are working with whatever splits have already >> > been generated and assigned. You could leverage FLIP-182 and FLIP-217 and >> > take care of only the problem to limit the number of parallel active >> > splits. And here I'm not sure if it would be worth generalising a solution >> > across different connectors. >> > >> > Regarding the global watermark, I made a related comment sometime ago >> > about it [1]. It sounds to me like you also need to solve this problem, >> > otherwise Iceberg users will encounter late records in case of some race >> > conditions between assigning new splits and completions of older. >> > >> > Best, >> > Piotrek >> > >> > [1] >> > https://issues.apache.org/jira/browse/FLINK-21871?focusedCommentId=17495545&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-17495545 >> > >> > pon., 2 maj 2022 o 04:26 Steven Wu <stevenz...@gmail.com> napisał(a): >> > >> >> add dev@ group to the thread as Thomas suggested >> >> >> >> Arvid, >> >> >> >> The scenario 3 (Dynamic assignment + temporary no split) in the FLIP-180 >> >> (idleness) can happen to Iceberg source alignment, as readers can be >> >> temporarily starved due to the holdback by the enumerator when assigning >> >> new splits upon request. >> >> >> >> Totally agree that we should decouple this discussion with the FLIP-217, >> >> which addresses the split level watermark alignment problem as a follow-up >> >> of FLIP-182 >> >> >> >> Becket, >> >> >> >> Yes, currently Iceberg source implemented the alignment leveraging the >> >> dynamic split assignment from FLIP-27 design. Basically, the enumerator >> >> can >> >> hold back split assignments to readers when necessary. Everything are >> >> centralized in the enumerator: (1) watermark extraction and aggregation >> >> (2) >> >> alignment decision and execution >> >> >> >> The motivation of this discussion is to see if Iceberg source can leverage >> >> some of the watermark alignment solutions (like FLIP-182) from Flink >> >> framework. E.g., as mentioned in the doc, Iceberg source can potentially >> >> leverage the FLIP-182 framework to do the watermark extraction and >> >> aggregation. For the alignment decision and execution, we can keep them in >> >> the centralized enumerator. >> >> >> >> Thanks, >> >> Steven >> >> >> >> On Thu, Apr 28, 2022 at 2:05 AM Becket Qin <becket....@gmail.com> wrote: >> >> >> >> > Hi Steven, >> >> > >> >> > Thanks for pulling me into this thread. I think the timestamp >> >> > alignment use case here is a good example of what FLIP-27 was designed >> >> for. >> >> > >> >> > Technically speaking, Iceberg source can already implement the timestamp >> >> > alignment in the Flink new source even without FLIP-182. However, I >> >> > understand the rationale here because timestamp alignment is also >> >> trying to >> >> > orchestrate the consumption of splits. However, it looks like FLIP-182 >> >> was >> >> > not designed in a way that it can be easily extended for other use >> >> cases. >> >> > It may probably worth thinking of a more general mechanism to answer the >> >> > following questions: >> >> > >> >> > 1. What information whose source of truth is the Flink framework should >> >> be >> >> > exposed to the SplitEnumerator and SourceReader? And how? >> >> > 2. What control actions in the Flink framework are worth exposing to the >> >> > SplitEnumerators and SourceReaders? And how? >> >> > >> >> > In the context of timestamp alignment, the first question is more >> >> > relevant. For example, instead of hardcode the ReportWatermarkEvent >> >> > handling logic in the SourceCoordinator, should we expose this to the >> >> > SplitEnumerator? So basically there will be some information, such as >> >> > subtask local watermark, whose source of truth is Flink runtime, but >> >> useful >> >> > to the user provided pluggables. >> >> > >> >> > I think there are a few control flow patterns to make a complete design: >> >> > >> >> > a. Framework space information (e.g. watermark) --> User space >> >> Pluggables >> >> > (e.g. SplitEnumerator) --> User space Actions (e.g. Pause reading a >> >> split). >> >> > b. Framework space information (e.g. task failure) --> User space >> >> > pluggables (e.g. SplitEnumerator) --> Framework space actions (e.g. exit >> >> > the job) >> >> > c. User space information (e.g. a custom workload metric) --> User space >> >> > pluggables (e.g. SplitEnumerator) --> User space actions (e.g. rebalance >> >> > the workload across the source readers). >> >> > d. Use space information (e.g. a custom stopping event in the stream) >> >> --> >> >> > User space pluggables (e.g. SplitEnumerator) --> Framework space actions >> >> > (e.g. stop the job). >> >> > >> >> > So basically for any user provided pluggables, the input information may >> >> > either come from another user provided logic or from the framework, and >> >> > after receiving that information, the pluggable may either want the >> >> > framework or another pluggable to take an action. So this gives the >> >> above 4 >> >> > combinations. >> >> > >> >> > In our case, when the pluggables are SplitEnumerator and SourceReader, >> >> the >> >> > control flows that only involve user space actions are fully supported. >> >> But >> >> > it seems that when it comes to control flows involving framework space >> >> > information, some of the information has not been exposed to the >> >> pluggable, >> >> > and some framework actions might also be missing. >> >> > >> >> > Thanks, >> >> > >> >> > Jiangjie (Becket) Qin >> >> > >> >> > On Thu, Apr 28, 2022 at 3:44 PM Arvid Heise <ar...@apache.org> wrote: >> >> > >> >> >> Hi folks, >> >> >> >> >> >> quick input from my side. I think this is from the implementation >> >> >> perspective what Piotr and I had in mind for a global min watermark >> >> that >> >> >> helps in idleness cases. See also point 3 in >> >> >> >> >> https://cwiki.apache.org/confluence/display/FLINK/FLIP-180%3A+Adjust+StreamStatus+and+Idleness+definition >> >> >> . >> >> >> >> >> >> Basically, we would like to empower source enumerators to determine the >> >> >> global min watermark for all source readers factoring in even future >> >> >> splits. Not all sources can supply that information (think of a general >> >> >> file source) but most should be able to. Basically, Flink should know >> >> for a >> >> >> given source at a given point in time what the min watermark across all >> >> >> source subtasks is. >> >> >> >> >> >> Here is some background: >> >> >> In the context of idleness, we can deterministically advance the >> >> >> watermark. In the pre-FLIP-27 era, we had heuristic approaches in >> >> sources >> >> >> to switch to idleness and thus allow watermarks to increase in cases >> >> where >> >> >> fewer splits than source tasks are available. However, for sources with >> >> >> dynamic split discovery that actually yields incorrect results. Think >> >> of a >> >> >> Kinesis consumer where a shard is split. Then a previously idle source >> >> >> subtask may receive a new split with time t0 as the lowest timestamp. >> >> Since >> >> >> the source subtask did not participate in the global watermark >> >> generation >> >> >> (because it was idle), the previously emitted watermark may be past t0 >> >> and >> >> >> thus results in late records potentially being discarded. A rerun of >> >> the >> >> >> same pipeline on historic data would not render the source subtask >> >> idle and >> >> >> not result in late records. The solution was to not render source >> >> subtasks >> >> >> automatically idle by the framework if there are no spits. That leads >> >> to >> >> >> confusion for Kafka users with static topic subscription where #splits >> >> < >> >> >> #parallelism stalls pipelines because the watermark is not advancing. >> >> Here, >> >> >> your sketched solution can be transferred to KafkaSource to let Flink >> >> know >> >> >> that min global watermark on a static assignment is determined by the >> >> >> slowest partition. Hence, all idle readers emit that min global >> >> watermark >> >> >> and the user sees progress. >> >> >> This whole idea is related to FLIP-182 watermark alignment but I'd go >> >> >> with another FLIP as the goal is quite different even though the >> >> >> implementation overlaps. >> >> >> >> >> >> Now Iceberg seems to use the same information to actually pause the >> >> >> consumption of files and create some kind of orderness guarantees as >> >> far as >> >> >> I understood. This probably can be applied to any source with dynamic >> >> split >> >> >> discovery. However, I wouldn't mix up the concepts and hence I >> >> appreciate >> >> >> you not chiming into the FLIP-182 and ff. threads. The goal of >> >> FLIP-182 is >> >> >> to pause readers while consuming a split, while your approach pauses >> >> >> readers before processing another split. So it feels more closely >> >> related >> >> >> to the global min watermark - so it could either be part of that FLIP >> >> or a >> >> >> FLIP of its own. Afaik API changes should actually happen only on the >> >> >> enumerator side both for your ideas and for global min watermark. >> >> >> >> >> >> Best, >> >> >> >> >> >> Arvid >> >> >> >> >> >> On Wed, Apr 27, 2022 at 7:31 PM Thomas Weise <t...@apache.org> wrote: >> >> >> >> >> >>> Hi Steven, >> >> >>> >> >> >>> Would it be better to bring this as a separate thread related to >> >> Iceberg >> >> >>> source to the dev@ list? I think this could benefit from broader >> >> input? >> >> >>> >> >> >>> Thanks >> >> >>> >> >> >>> On Wed, Apr 27, 2022 at 9:36 AM Steven Wu <stevenz...@gmail.com> >> >> wrote: >> >> >>> >> >> >>>> + Becket and Sebastian >> >> >>>> >> >> >>>> It is also related to the split level watermark alignment discussion >> >> >>>> thread. Because it is already very long, I don't want to further >> >> complicate >> >> >>>> the ongoing discussion there. But I can move the discussion to that >> >> >>>> existing thread if that is preferred. >> >> >>>> >> >> >>>> >> >> >>>> On Tue, Apr 26, 2022 at 10:03 PM Steven Wu <stevenz...@gmail.com> >> >> >>>> wrote: >> >> >>>> >> >> >>>>> Hi all, >> >> >>>>> >> >> >>>>> We are thinking about how to align with the Flink community and >> >> >>>>> leverage the FLIP-182 watermark alignment in the Iceberg source. I >> >> put some >> >> >>>>> context in this google doc. Would love to get hear your thoughts on >> >> this. >> >> >>>>> >> >> >>>>> >> >> >>>>> >> >> https://docs.google.com/document/d/1zfwF8e5LszazcOzmUAOeOtpM9v8dKEPlY_BRFSmI3us/edit# >> >> >>>>> >> >> >>>>> Thanks, >> >> >>>>> Steven >> >> >>>>> >> >> >>>> >> >> >> >
