Hi Becket, > why do we need to change the behavior of addSplitsBack()? Should it remain the same?
How does the enumerator get the splits from ReaderRegistrationEvent and then reassign it? You have given a advice before: > 1. Put all the reader information in the SplitEnumerator context. 2. notify the enumerator about the new reader registration. 3. let the split enumerator get whatever information it wants from the context and do its job. However, each time a source task fails over, the ConcurrentMap<Integer, ConcurrentMap<Integer, ReaderInfo>> registeredReaders will remove this reader infos. When the source task is registered again, it will be added again. *Thus, registeredReaders cannot know whether is registered before. * Therefore, registeredReaders enumerator#addReader does not distinguish the following situations: However, each time one source task is failover. The `ConcurrentMap<Integer, ConcurrentMap<Integer, ReaderInfo>> registeredReaders` will remove this source. When source Task is registered again, enumerator#addReader not distinguished three situations: 1. The Reader is registered when the global restart. In this case, redistribution the split from the infos. (take off all the splits from ReaderInfo). 2. The Reader is registered when a partial failover(before the first successful checkpoint). In this case, ignore the split from the infos. (leave alone all the splits from ReaderInfo). 3. The Reader is registered when a partial failover(after the first successful checkpoint).In this case, we need assign the split to same reader again. (take off all the splits from ReaderInfo but assigned to it again). we still need the enumerator to distinguish them (using pendingSplitAssignment & assignedSplitAssignment. However, it is redundant to maintain split assigned information both in the enumerator and the enumerator context. I think if we change the behavior of addSplitsBack, it will be more simple. Just let the enumerator to handle these split based on pendingSplitAssignment & assignedSplitments. What do you think? Best, Hongshun On Fri, Oct 10, 2025 at 12:55 PM Becket Qin <becket....@gmail.com> wrote: > Hi Hongshun, > > Thanks for updating the FLIP. A quick question: why do we need to change > the behavior of addSplitsBack()? Should it remain the same? > > Regarding the case of restart with changed subscription. I think the only > correct behavior is removing obsolete splits without any warning / > exception. It is OK to add an info level logging if we want to. It is a > clear intention if the user has explicitly changed subscription and > restarted the job. There is no need to add a config to double confirm. > > Regards, > > Jiangjie (Becket) Qin > > On Thu, Oct 9, 2025 at 7:28 PM Hongshun Wang <loserwang1...@gmail.com> > wrote: > >> Hi Leonard, >> >> If the SplitEnumerator received all splits after a restart, it becomes >> straightforward to clear and un-assign the unmatched splits(checking >> whether matches the source options). However, a key question arises: *should >> automatically discard obsolete splits, or explicitly notify the user via an >> exception?* >> >> We provided a option `scan.partition-unsubscribe.strategy`: >> 1. If Strict, throws an exception when encountering removed splits. >> 2. If Lenient, automatically removes obsolete splits silently. >> >> What Do you think? >> >> Best, >> Hongshun >> >> On Thu, Oct 9, 2025 at 9:37 PM Leonard Xu <xbjt...@gmail.com> wrote: >> >>> Thanks hongshun for the updating and pretty detailed analysis for edge >>> cases, the updated FLIP looks good to me now. >>> >>> Only last implementation details about scenario in motivation section: >>> >>> *Restart with Changed subscription: During restart, if source options >>> remove a topic or table. The splits which have already assigned can not be >>> removed.* >>> >>> Could you clarify how we resolve this in Kafka connector ? >>> >>> Best, >>> Leonard >>> >>> >>> >>> 2025 10月 9 19:48，Hongshun Wang <loserwang1...@gmail.com> 写道： >>> >>> Hi devs, >>> If there are no further suggestions, I will start the voting tomorrow。 >>> >>> Best, >>> Hongshun >>> >>> On Fri, Sep 26, 2025 at 7:48 PM Hongshun Wang <loserwang1...@gmail.com> >>> wrote: >>> >>>> Hi Becket and Leonard, >>>> >>>> I have updated the content of this FLIP. The key point is that: >>>> >>>> When the split enumerator receives a split, *these splits must have >>>> already existed in pendingSplitAssignment or assignedSplitments*. >>>> >>>> - If the split is in pendingSplitAssignments, ignore it. >>>> - If the split is in assignedSplitAssignments but has a different >>>> taskId, ignore it (this indicates it was already assigned to >>>> another task). >>>> - If the split is in assignedSplitAssignments and shares the same >>>> taskId, move the assignment from assignedSplitments to >>>> pendingSplitAssignment >>>> to re-assign again. >>>> >>>> >>>> For better understanding why use these strategies. I added some >>>> examples and pictures to show it. >>>> >>>> Would you like to help me check whether there are still some problems? >>>> >>>> Best >>>> Hongshun >>>> >>>> >>>> >>>> On Fri, Sep 26, 2025 at 5:08 PM Leonard Xu <xbjt...@gmail.com> wrote: >>>> >>>>> Thanks Becket and Hongshun for the insightful discussion. >>>>> >>>>> The underlying implementation and communication mechanisms of Flink >>>>> Source indeed involve many intricate details, we discussed the issue of >>>>> splits re-assignment in specific scenarios, but fortunately, the final >>>>> decision turned out to be pretty clear. >>>>> >>>>> +1 to Becket’s proposal to keeps the framework cleaner and more >>>>> flexible. >>>>> +1 to Hongshun’s point to provide comprehensive guidance for connector >>>>> developers. >>>>> >>>>> >>>>> Best, >>>>> Leonard >>>>> >>>>> >>>>> >>>>> 2025 9月 26 16:30，Hongshun Wang <loserwang1...@gmail.com> 写道： >>>>> >>>>> Hi Becket, >>>>> >>>>> I Got it. You’re suggesting we should not handle this in the source >>>>> framework but instead let the split enumerator manage these three >>>>> scenarios. >>>>> >>>>> Let me explain why I originally favored handling it in the framework: >>>>> I'm concerned that connector developers might overlook certain edge cases >>>>> (after all, we even payed extensive discussions to fully clarify the >>>>> logic) >>>>> >>>>> However, your point keeps the framework cleaner and more flexible. >>>>> Thus, I will take it. >>>>> >>>>> Perhaps, in this FLIP, we should focus on providing comprehensive >>>>> guidance for connector developers: explain how to implement a split >>>>> enumerator, including the underlying challenges and their solutions. >>>>> >>>>> >>>>> Additionally, we can use the Kafka connector as a reference >>>>> implementation to demonstrate the practical steps. This way, developers >>>>> who >>>>> want to implement similar connectors can directly reference this example. >>>>> >>>>> >>>>> Best, >>>>> Hongshun >>>>> >>>>> >>>>> >>>>> On Fri, Sep 26, 2025 at 1:27 PM Becket Qin <becket....@gmail.com> >>>>> wrote: >>>>> >>>>>> It would be good to not expose runtime details to the source >>>>>> implementation if possible. >>>>>> >>>>>> Today, the split enumerator implementations are expected to track the >>>>>> split assignment. >>>>>> >>>>>> Assuming the split enumerator implementation keeps a split assignment >>>>>> map, that means the enumerator should already know whether a split is >>>>>> assigned or unassigned. So it can handle the three scenarios you >>>>>> mentioned. >>>>>> >>>>>> The split is reported by a reader during a global restoration. >>>>>>> >>>>>> The split enumerator should have just been restored / created. If the >>>>>> enumerator expects a full reassignment of splits up on global recovery, >>>>>> there should be no assigned splits to that reader in the split assignment >>>>>> mapping. >>>>>> >>>>>> The split is reported by a reader during a partial failure recovery. >>>>>>> >>>>>> In this case, when SplitEnumerator.addReader() is invoked, the split >>>>>> assignment map in the enumerator implementation should already have some >>>>>> split assignments for the reader. Therefore it is a partial failover. If >>>>>> the source supports split reassignment on recovery, the enumerator can >>>>>> assign splits that are different from the reported assignment of that >>>>>> reader in the SplitEnumeratorContext, or it can also assign the same >>>>>> splits. In any case, the enumerator knows that this is a partial recovery >>>>>> because the assignment map is non-empty. >>>>>> >>>>>> The split is not reported by a reader, but is assigned after the last >>>>>>> successful checkpoint and was never acknowledged. >>>>>> >>>>>> This is actually one of the step in the partial failure recover. >>>>>> SplitEnumerator.addSplitsBack() will be called first before >>>>>> SplitReader.addReader() is called for the recovered reader. When the >>>>>> SplitEnumerator.addSplitsBack() is invoked, it is for sure a partial >>>>>> recovery. And the enumerator should remove these splits from the split >>>>>> assignment map as if they were never assigned. >>>>>> >>>>>> I think this should work, right? >>>>>> >>>>>> Thanks, >>>>>> >>>>>> Jiangjie (Becket) Qin >>>>>> >>>>>> On Thu, Sep 25, 2025 at 8:34 PM Hongshun Wang < >>>>>> loserwang1...@gmail.com> wrote: >>>>>> >>>>>>> Hi Becket and Leonard, >>>>>>> >>>>>>> Thanks for your advice. >>>>>>> >>>>>>> > put all the reader information in the SplitEnumerator context >>>>>>> I have a concern: the current registeredReaders in* >>>>>>> SourceCoordinatorContext will be removed after subtaskResetor execution >>>>>>> on >>>>>>> failure*.However, this approach has merit. >>>>>>> >>>>>>> One more situation I found my previous design does not cover: >>>>>>> >>>>>>> 1. Initial state: Reader A reports splits (1, 2). >>>>>>> 2. Enumerator action: Assigns split 1 to Reader A, and split 2 >>>>>>> to Reader B. >>>>>>> 3. Failure scenario: Reader A fails before checkpointing. Since >>>>>>> this is a partial failure, only Reader A restarts. >>>>>>> 4. Recovery issue: Upon recovery, Reader A re-reports split (1). >>>>>>> >>>>>>> In my previous design, the enumerator will ignore Reader A's >>>>>>> re-registration which will cause data loss. >>>>>>> >>>>>>> Thus, when the enumerator receives a split, the split may originate >>>>>>> from three scenarios: >>>>>>> >>>>>>> 1. The split is reported by a reader during a global restoration. >>>>>>> 2. The split is reported by a reader during a partial failure >>>>>>> recovery. >>>>>>> 3. The split is not reported by a reader, but is assigned after >>>>>>> the last successful checkpoint and was never acknowledged. >>>>>>> >>>>>>> In the first scenario (global restore), the split should >>>>>>> be re-distributed. For the latter two scenarios (partial failover and >>>>>>> post-checkpoint assignment), we need to reassign the split to >>>>>>> its originally assigned subtask. >>>>>>> >>>>>>> By implementing a method in the SplitEnumerator context to track >>>>>>> each assigned split's status, the system can correctly identify and >>>>>>> resolve >>>>>>> split ownership in all three scenarios.*What about adding a >>>>>>> `SplitRecoveryType splitRecoveryType(Split split)` in >>>>>>> SplitEnumeratorContext.* SplitRecoveryTypeis a enum including >>>>>>> `UNASSIGNED`、`GLOBAL_RESTORE`、`PARTIAL_FAILOVER` and >>>>>>> `POST_CHECKPOINT_ASSIGNMENT`. >>>>>>> >>>>>>> What do you think? Are there any details or scenarios I haven't >>>>>>> considered? Looking forward to your advice. >>>>>>> >>>>>>> Best, >>>>>>> Hongshun >>>>>>> >>>>>>> On Thu, Sep 11, 2025 at 12:41 AM Becket Qin <becket....@gmail.com> >>>>>>> wrote: >>>>>>> >>>>>>>> Thanks for the explanation, Hongshun. >>>>>>>> >>>>>>>> Current pattern of handling new reader registration following: >>>>>>>> 1. put all the reader information in the SplitEnumerator context >>>>>>>> 2. notify the enumerator about the new reader registration. >>>>>>>> 3. Let the split enumerator get whatever information it wants from >>>>>>>> the >>>>>>>> context and do its job. >>>>>>>> This pattern decouples the information passing and the reader >>>>>>>> registration >>>>>>>> notification. This makes the API extensible - we can add more >>>>>>>> information >>>>>>>> (e.g. reported assigned splits in our case) about the reader to the >>>>>>>> context >>>>>>>> without introducing new methods. >>>>>>>> >>>>>>>> Introducing a new method of addSplitBackOnRecovery() is redundant >>>>>>>> to the >>>>>>>> above pattern. Do we really need it? >>>>>>>> >>>>>>>> Thanks, >>>>>>>> >>>>>>>> Jiangjie (Becket) Qin >>>>>>>> >>>>>>>> On Mon, Sep 8, 2025 at 8:18 PM Hongshun Wang < >>>>>>>> loserwang1...@gmail.com> >>>>>>>> wrote: >>>>>>>> >>>>>>>> > Hi Becket, >>>>>>>> > >>>>>>>> > > I am curious what would the enumerator do differently for the >>>>>>>> splits >>>>>>>> > added via addSplitsBackOnRecovery() V.S. addSplitsBack()? >>>>>>>> > >>>>>>>> > In this FLIP, there are two distinct scenarios in which the >>>>>>>> enumerator >>>>>>>> > receives splits being added back: >>>>>>>> > 1. Job-level restore: The job is restored, splits from reader’s >>>>>>>> state are >>>>>>>> > reported by ReaderRegistrationEvent. >>>>>>>> > 2. Reader-level restart: a reader is started but not the whole >>>>>>>> job, >>>>>>>> > splits assigned to it after the last successful checkpoint. This >>>>>>>> is what >>>>>>>> > addSplitsBack used to do. >>>>>>>> > >>>>>>>> > >>>>>>>> > In these two situations, the enumerator will choose different >>>>>>>> strategies. >>>>>>>> > 1. Job-level restore: the splits should be redistributed across >>>>>>>> readers >>>>>>>> > according to the current partitioner strategy. >>>>>>>> > 2. Reader-level restart: the splits should be reassigned directly >>>>>>>> back to >>>>>>>> > the same reader they were originally assigned to, preserving >>>>>>>> locality and >>>>>>>> > avoiding unnecessary redistribution >>>>>>>> > >>>>>>>> > Therefore, the enumerator must clearly distinguish between these >>>>>>>> two >>>>>>>> > scenarios.I used to deprecate the former >>>>>>>> addSplitsBack(List<SplitT> >>>>>>>> > splits, int subtaskId) but add a new addSplitsBack(List<SplitT> >>>>>>>> > splits, int subtaskId, >>>>>>>> > boolean reportedByReader). >>>>>>>> > Leonard suggest to use another method addSplitsBackOnRecovery but >>>>>>>> not >>>>>>>> > influenced currently addSplitsBack. >>>>>>>> > >>>>>>>> > Best >>>>>>>> > Hongshun >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>> > On 2025/09/08 17:20:31 Becket Qin wrote: >>>>>>>> > > Hi Leonard, >>>>>>>> > > >>>>>>>> > > >>>>>>>> > > > Could we introduce a new method like addSplitsBackOnRecovery >>>>>>>> with >>>>>>>> > default >>>>>>>> > > > implementation. In this way, we can provide better backward >>>>>>>> > compatibility >>>>>>>> > > > and also makes it easier for developers to understand. >>>>>>>> > > >>>>>>>> > > >>>>>>>> > > I am curious what would the enumerator do differently for the >>>>>>>> splits >>>>>>>> > added >>>>>>>> > > via addSplitsBackOnRecovery() V.S. addSplitsBack()? Today, >>>>>>>> > addSplitsBack() >>>>>>>> > > is also only called upon recovery. So the new method seems >>>>>>>> confusing. One >>>>>>>> > > thing worth clarifying is if the Source implements >>>>>>>> > > SupportSplitReassignmentOnRecovery, upon recovery, should the >>>>>>>> splits >>>>>>>> > > reported by the readers also be added back to the >>>>>>>> SplitEnumerator via the >>>>>>>> > > addSplitsBack() call? Or should the SplitEnumerator explicitly >>>>>>>> query the >>>>>>>> > > registered reader information via the SplitEnumeratorContext to >>>>>>>> get the >>>>>>>> > > originally assigned splits when addReader() is invoked? I was >>>>>>>> assuming >>>>>>>> > the >>>>>>>> > > latter in the beginning, so the behavior of addSplitsBack() >>>>>>>> remains >>>>>>>> > > unchanged, but I am not opposed in doing the former. >>>>>>>> > > >>>>>>>> > > Also, can you elaborate on the backwards compatibility issue >>>>>>>> you see if >>>>>>>> > we >>>>>>>> > > do not have a separate addSplitsBackOnRecovery() method? Even >>>>>>>> without >>>>>>>> > this >>>>>>>> > > new method, the behavior remains exactly the same unless the >>>>>>>> end users >>>>>>>> > > implement the mix-in interface of >>>>>>>> "SupportSplitReassignmentOnRecovery", >>>>>>>> > > right? >>>>>>>> > > >>>>>>>> > > Thanks, >>>>>>>> > > >>>>>>>> > > Jiangjie (Becket) Qin >>>>>>>> > > >>>>>>>> > > On Mon, Sep 8, 2025 at 1:48 AM Hongshun Wang <lo...@gmail.com> >>>>>>>> > > wrote: >>>>>>>> > > >>>>>>>> > > > Hi devs, >>>>>>>> > > > >>>>>>>> > > > It has been quite some time since this FLIP[1] was first >>>>>>>> proposed. >>>>>>>> > Thank >>>>>>>> > > > you for your valuable feedback—based on your suggestions, the >>>>>>>> FLIP has >>>>>>>> > > > undergone several rounds of revisions. >>>>>>>> > > > >>>>>>>> > > > Any more advice is welcome and appreciated. If there are no >>>>>>>> further >>>>>>>> > > > concerns, I plan to start the vote tomorrow. >>>>>>>> > > > >>>>>>>> > > > Best >>>>>>>> > > > Hongshun >>>>>>>> > > > >>>>>>>> > > > [1] >>>>>>>> > > > >>>>>>>> > >>>>>>>> https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=373886480 >>>>>>>> > > > >>>>>>>> > > > On Mon, Sep 8, 2025 at 4:42 PM Hongshun Wang <lo...@gmail.com >>>>>>>> > >>>>>>>> > > > wrote: >>>>>>>> > > > >>>>>>>> > > > > Hi Leonard, >>>>>>>> > > > > Thanks for your advice. It makes sense and I have modified >>>>>>>> it. >>>>>>>> > > > > >>>>>>>> > > > > Best, >>>>>>>> > > > > Hongshun >>>>>>>> > > > > >>>>>>>> > > > > On Mon, Sep 8, 2025 at 11:40 AM Leonard Xu <xb...@gmail.com> >>>>>>>> wrote: >>>>>>>> > > > > >>>>>>>> > > > >> Thanks Hongshun and Becket for the deep discussion. >>>>>>>> > > > >> >>>>>>>> > > > >> I only have one comment for one API design: >>>>>>>> > > > >> >>>>>>>> > > > >> > Deprecate the old addSplitsBack method, use a >>>>>>>> addSplitsBack with >>>>>>>> > > > >> param isReportedByReader instead. Because, The enumerator >>>>>>>> can apply >>>>>>>> > > > >> different reassignment policies based on the context. >>>>>>>> > > > >> >>>>>>>> > > > >> Could we introduce a new method like >>>>>>>> *addSplitsBackOnRecovery* with >>>>>>>> > > > default >>>>>>>> > > > >> implementation. In this way, we can provide better backward >>>>>>>> > > > >> compatibility and also makes it easier for developers to >>>>>>>> understand. >>>>>>>> > > > >> >>>>>>>> > > > >> Best, >>>>>>>> > > > >> Leonard >>>>>>>> > > > >> >>>>>>>> > > > >> >>>>>>>> > > > >> >>>>>>>> > > > >> 2025 9月 3 20:26，Hongshun Wang <lo...@gmail.com> 写道： >>>>>>>> > > > >> >>>>>>>> > > > >> Hi Becket, >>>>>>>> > > > >> >>>>>>>> > > > >> I think that's a great idea! I have added the >>>>>>>> > > > >> SupportSplitReassignmentOnRecovery interface in this FLIP. >>>>>>>> If a >>>>>>>> > Source >>>>>>>> > > > >> implements this interface indicates that the source >>>>>>>> operator needs >>>>>>>> > to >>>>>>>> > > > >> report splits to the enumerator and receive >>>>>>>> reassignment.[1] >>>>>>>> > > > >> >>>>>>>> > > > >> Best, >>>>>>>> > > > >> Hongshun >>>>>>>> > > > >> >>>>>>>> > > > >> [1] >>>>>>>> > > > >> >>>>>>>> > > > >>>>>>>> > >>>>>>>> > >>>>>>>> https://cwiki.apache.org/confluence/display/FLINK/FLIP-537%3A+Enumerator+with+Global+Split+Assignment+Distribution+for+Balanced+Split+assignment >>>>>>>> > > > >> >>>>>>>> > > > >> On Thu, Aug 21, 2025 at 12:09 PM Becket Qin < >>>>>>>> be...@gmail.com> >>>>>>>> > > > wrote: >>>>>>>> > > > >> >>>>>>>> > > > >>> Hi Hongshun, >>>>>>>> > > > >>> >>>>>>>> > > > >>> I think the convention for such optional features in >>>>>>>> Source is via >>>>>>>> > > > >>> mix-in interfaces. So instead of adding a method to the >>>>>>>> > SourceReader, >>>>>>>> > > > maybe >>>>>>>> > > > >>> we should introduce an interface >>>>>>>> SupportSplitReassingmentOnRecovery >>>>>>>> > > > with >>>>>>>> > > > >>> this method. If a Source implementation implements that >>>>>>>> interface, >>>>>>>> > > > then the >>>>>>>> > > > >>> SourceOperator will check the desired behavior and act >>>>>>>> accordingly. >>>>>>>> > > > >>> >>>>>>>> > > > >>> Thanks, >>>>>>>> > > > >>> >>>>>>>> > > > >>> Jiangjie (Becket) Qin >>>>>>>> > > > >>> >>>>>>>> > > > >>> On Wed, Aug 20, 2025 at 8:52 PM Hongshun Wang < >>>>>>>> > loserwang1...@gmail.com >>>>>>>> > > > > >>>>>>>> > > > >>> wrote: >>>>>>>> > > > >>> >>>>>>>> > > > >>>> Hi de vs, >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> Would anyone like to discuss this FLIP? I'd appreciate >>>>>>>> your >>>>>>>> > feedback >>>>>>>> > > > >>>> and suggestions. >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> Best, >>>>>>>> > > > >>>> Hongshun >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> 2025年8月13日 14:23，Hongshun Wang <lo...@gmail.com> 写道： >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> Hi Becket, >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> Thank you for your detailed feedback. The new contract >>>>>>>> makes good >>>>>>>> > > > sense >>>>>>>> > > > >>>> to me and effectively addresses the issues I encountered >>>>>>>> at the >>>>>>>> > > > beginning >>>>>>>> > > > >>>> of the design. >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> That said, I recommend not reporting splits by default, >>>>>>>> primarily >>>>>>>> > for >>>>>>>> > > > >>>> compatibility and practical reasons: >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> > For these reasons, we do not expect the Split objects >>>>>>>> to be >>>>>>>> > huge, >>>>>>>> > > > >>>> and we are not trying to design for huge Split objects >>>>>>>> either as >>>>>>>> > they >>>>>>>> > > > will >>>>>>>> > > > >>>> have problems even today. >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> 1. >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> Not all existing connector match this rule >>>>>>>> > > > >>>> For example, in mysql cdc connector, a binlog split >>>>>>>> may contain >>>>>>>> > > > >>>> hundreds (or even more) snapshot split completion >>>>>>>> records. This >>>>>>>> > > > state is >>>>>>>> > > > >>>> large and is currently transmitted incrementally >>>>>>>> through >>>>>>>> > multiple >>>>>>>> > > > >>>> BinlogSplitMetaEvent messages. Since the binlog >>>>>>>> reader operates >>>>>>>> > > > >>>> with single parallelism, reporting the full split >>>>>>>> state on >>>>>>>> > recovery >>>>>>>> > > > >>>> could be inefficient or even infeasible. >>>>>>>> > > > >>>> For such sources, it would be better to provide a >>>>>>>> mechanism to >>>>>>>> > skip >>>>>>>> > > > >>>> split reporting during restart until they redesign >>>>>>>> and reduce >>>>>>>> > the >>>>>>>> > > > >>>> split size. >>>>>>>> > > > >>>> 2. >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> Not all enumerators maintain unassigned splits in >>>>>>>> state. >>>>>>>> > > > >>>> Some SplitEnumerator(such as kafka connector) >>>>>>>> implementations >>>>>>>> > do >>>>>>>> > > > >>>> not track or persistently manage unassigned splits. >>>>>>>> Requiring >>>>>>>> > them >>>>>>>> > > > to >>>>>>>> > > > >>>> handle re-registration would add unnecessary >>>>>>>> complexity. Even >>>>>>>> > > > though we >>>>>>>> > > > >>>> maybe implements in kafka connector, currently, kafka >>>>>>>> connector >>>>>>>> > is >>>>>>>> > > > decouple >>>>>>>> > > > >>>> with flink version, we also need to make sure the >>>>>>>> elder version >>>>>>>> > is >>>>>>>> > > > >>>> compatible. >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> ------------------------------ >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> To address these concerns, I propose introducing a new >>>>>>>> method: >>>>>>>> > boolean >>>>>>>> > > > >>>> SourceReader#shouldReassignSplitsOnRecovery() with a >>>>>>>> default >>>>>>>> > > > >>>> implementation returning false. This allows source >>>>>>>> readers to opt >>>>>>>> > in >>>>>>>> > > > >>>> to split reassignment only when necessary. Since the new >>>>>>>> contract >>>>>>>> > > > already >>>>>>>> > > > >>>> places the responsibility for split assignment on the >>>>>>>> enumerator, >>>>>>>> > not >>>>>>>> > > > >>>> reporting splits by default is a safe and clean default >>>>>>>> behavior. >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> ------------------------------ >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> I’ve updated the implementation and the FIP >>>>>>>> accordingly[1]. It >>>>>>>> > quite a >>>>>>>> > > > >>>> big change. In particular, for the Kafka connector, we >>>>>>>> can now use >>>>>>>> > a >>>>>>>> > > > >>>> pluggable SplitPartitioner to support different split >>>>>>>> assignment >>>>>>>> > > > >>>> strategies (e.g., default, round-robin). >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> Could you please review it when you have a chance? >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> Best, >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> Hongshun >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> [1] >>>>>>>> > > > >>>> >>>>>>>> > > > >>>>>>>> > >>>>>>>> > >>>>>>>> https://cwiki.apache.org/confluence/display/FLINK/FLIP-537%3A+Enumerator+with+Global+Split+Assignment+Distribution+for+Balanced+Split+assignment >>>>>>>> > > > >>>> >>>>>>>> > > > >>>> On Sat, Aug 9, 2025 at 3:03 AM Becket Qin < >>>>>>>> be...@gmail.com> >>>>>>>> > > > wrote: >>>>>>>> > > > >>>> >>>>>>>> > > > >>>>> Hi Hongshun, >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> I am not too concerned about the transmission cost. >>>>>>>> Because the >>>>>>>> > full >>>>>>>> > > > >>>>> split transmission has to happen in the initial >>>>>>>> assignment phase >>>>>>>> > > > already. >>>>>>>> > > > >>>>> And in the future, we probably want to also introduce >>>>>>>> some kind >>>>>>>> > of >>>>>>>> > > > workload >>>>>>>> > > > >>>>> balance across source readers, e.g. based on the >>>>>>>> per-split >>>>>>>> > > > throughput or >>>>>>>> > > > >>>>> the per-source-reader workload in heterogeneous >>>>>>>> clusters. For >>>>>>>> > these >>>>>>>> > > > >>>>> reasons, we do not expect the Split objects to be huge, >>>>>>>> and we >>>>>>>> > are >>>>>>>> > > > not >>>>>>>> > > > >>>>> trying to design for huge Split objects either as they >>>>>>>> will have >>>>>>>> > > > problems >>>>>>>> > > > >>>>> even today. >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> Good point on the potential split loss, please see the >>>>>>>> reply >>>>>>>> > below: >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> Scenario 2: >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>>> 1. Reader A reports splits (1 and 2), and Reader B >>>>>>>> reports (3 >>>>>>>> > and 4) >>>>>>>> > > > >>>>>> upon restart. >>>>>>>> > > > >>>>>> 2. Before the enumerator receives all reports and >>>>>>>> performs >>>>>>>> > > > >>>>>> reassignment, a checkpoint is triggered. >>>>>>>> > > > >>>>>> 3. Since no splits have been reassigned yet, both >>>>>>>> readers have >>>>>>>> > empty >>>>>>>> > > > >>>>>> states. >>>>>>>> > > > >>>>>> 4. When restarting from this checkpoint, all four >>>>>>>> splits are >>>>>>>> > lost. >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> The reader registration happens in the >>>>>>>> SourceOperator.open(), >>>>>>>> > which >>>>>>>> > > > >>>>> means the task is still in the initializing state, >>>>>>>> therefore the >>>>>>>> > > > checkpoint >>>>>>>> > > > >>>>> should not be triggered until the enumerator receives >>>>>>>> all the >>>>>>>> > split >>>>>>>> > > > reports. >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> There is a nuance here. Today, the RPC call from the TM >>>>>>>> to the JM >>>>>>>> > is >>>>>>>> > > > >>>>> async. So it is possible that the SourceOpertor.open() >>>>>>>> has >>>>>>>> > returned, >>>>>>>> > > > but >>>>>>>> > > > >>>>> the enumerator has not received the split reports. >>>>>>>> However, >>>>>>>> > because >>>>>>>> > > > the >>>>>>>> > > > >>>>> task status update RPC call goes to the same channel as >>>>>>>> the split >>>>>>>> > > > reports >>>>>>>> > > > >>>>> call, so the task status RPC call will happen after the >>>>>>>> split >>>>>>>> > > > reports call >>>>>>>> > > > >>>>> on the JM side. Therefore, on the JM side, the >>>>>>>> SourceCoordinator >>>>>>>> > will >>>>>>>> > > > >>>>> always first receive the split reports, then receive the >>>>>>>> > checkpoint >>>>>>>> > > > request. >>>>>>>> > > > >>>>> This "happen before" relationship is kind of important >>>>>>>> to >>>>>>>> > guarantee >>>>>>>> > > > >>>>> the consistent state between enumerator and readers. >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> Scenario 1: >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>>> 1. Upon restart, Reader A reports assigned splits (1 >>>>>>>> and 2), and >>>>>>>> > > > >>>>>> Reader B reports (3 and 4). >>>>>>>> > > > >>>>>> 2. The enumerator receives these reports but only >>>>>>>> reassigns >>>>>>>> > splits 1 >>>>>>>> > > > >>>>>> and 2 — not 3 and 4. >>>>>>>> > > > >>>>>> 3. A checkpoint or savepoint is then triggered. Only >>>>>>>> splits 1 >>>>>>>> > and 2 >>>>>>>> > > > >>>>>> are recorded in the reader states; splits 3 and 4 are >>>>>>>> not >>>>>>>> > persisted. >>>>>>>> > > > >>>>>> 4. If the job is later restarted from this checkpoint, >>>>>>>> splits 3 >>>>>>>> > and >>>>>>>> > > > 4 >>>>>>>> > > > >>>>>> will be permanently lost. >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> This scenario is possible. One solution is to let the >>>>>>>> enumerator >>>>>>>> > > > >>>>> implementation handle this. That means if the >>>>>>>> enumerator relies >>>>>>>> > on >>>>>>>> > > > the >>>>>>>> > > > >>>>> initial split reports from the source readers, it >>>>>>>> should maintain >>>>>>>> > > > these >>>>>>>> > > > >>>>> reports by itself. In the above example, the enumerator >>>>>>>> will need >>>>>>>> > to >>>>>>>> > > > >>>>> remember that 3 and 4 are not assigned and put it into >>>>>>>> its own >>>>>>>> > state. >>>>>>>> > > > >>>>> The current contract is that anything assigned to the >>>>>>>> > SourceReaders >>>>>>>> > > > >>>>> are completely owned by the SourceReaders. Enumerators >>>>>>>> can >>>>>>>> > remember >>>>>>>> > > > the >>>>>>>> > > > >>>>> assignments but cannot change them, even when the >>>>>>>> source reader >>>>>>>> > > > recovers / >>>>>>>> > > > >>>>> restarts. >>>>>>>> > > > >>>>> With this FLIP, the contract becomes that the source >>>>>>>> readers will >>>>>>>> > > > >>>>> return the ownership of the splits to the enumerator. >>>>>>>> So the >>>>>>>> > > > enumerator is >>>>>>>> > > > >>>>> responsible for maintaining these splits, until they >>>>>>>> are assigned >>>>>>>> > to >>>>>>>> > > > a >>>>>>>> > > > >>>>> source reader again. >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> There are other cases where there may be conflict >>>>>>>> information >>>>>>>> > between >>>>>>>> > > > >>>>> reader and enumerator. For example, consider the >>>>>>>> following >>>>>>>> > sequence: >>>>>>>> > > > >>>>> 1. reader A reports splits (1 and 2) up on restart. >>>>>>>> > > > >>>>> 2. enumerator receives the report and assigns both 1 >>>>>>>> and 2 to >>>>>>>> > reader >>>>>>>> > > > B. >>>>>>>> > > > >>>>> 3. reader A failed before checkpointing. And this is a >>>>>>>> partial >>>>>>>> > > > >>>>> failure, so only reader A restarts. >>>>>>>> > > > >>>>> 4. When reader A recovers, it will again report splits >>>>>>>> (1 and 2) >>>>>>>> > to >>>>>>>> > > > >>>>> the enumerator. >>>>>>>> > > > >>>>> 5. The enumerator should ignore this report because it >>>>>>>> has >>>>>>>> > > > >>>>> assigned splits (1 and 2) to reader B. >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> So with the new contract, the enumerator should be the >>>>>>>> source of >>>>>>>> > > > truth >>>>>>>> > > > >>>>> for split ownership. >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> Thanks, >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> Jiangjie (Becket) Qin >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>> On Fri, Aug 8, 2025 at 12:58 AM Hongshun Wang < >>>>>>>> > > > loserwang1...@gmail.com> >>>>>>>> > > > >>>>> wrote: >>>>>>>> > > > >>>>> >>>>>>>> > > > >>>>>> Hi Becket, >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> I did consider this approach at the beginning (and it >>>>>>>> was also >>>>>>>> > > > >>>>>> mentioned in this FLIP), since it would allow more >>>>>>>> flexibility >>>>>>>> > in >>>>>>>> > > > >>>>>> reassigning all splits. However, there are a few >>>>>>>> potential >>>>>>>> > issues. >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> 1. High Transmission Cost >>>>>>>> > > > >>>>>> If we pass the full split objects (rather than just >>>>>>>> split IDs), >>>>>>>> > the >>>>>>>> > > > >>>>>> data size could be significant, leading to high >>>>>>>> overhead during >>>>>>>> > > > >>>>>> transmission — especially when many splits are >>>>>>>> involved. >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> 2. Risk of Split Loss >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> Risk of split loss exists unless we have a mechanism >>>>>>>> to make >>>>>>>> > sure >>>>>>>> > > > >>>>>> only can checkpoint after all the splits are >>>>>>>> reassigned. >>>>>>>> > > > >>>>>> There are scenarios where splits could be lost due to >>>>>>>> > inconsistent >>>>>>>> > > > >>>>>> state handling during recovery: >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> Scenario 1: >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> 1. Upon restart, Reader A reports assigned splits >>>>>>>> (1 and 2), >>>>>>>> > and >>>>>>>> > > > >>>>>> Reader B reports (3 and 4). >>>>>>>> > > > >>>>>> 2. The enumerator receives these reports but only >>>>>>>> reassigns >>>>>>>> > > > >>>>>> splits 1 and 2 — not 3 and 4. >>>>>>>> > > > >>>>>> 3. A checkpoint or savepoint is then triggered. >>>>>>>> Only splits 1 >>>>>>>> > and >>>>>>>> > > > >>>>>> 2 are recorded in the reader states; splits 3 and 4 >>>>>>>> are not >>>>>>>> > > > persisted. >>>>>>>> > > > >>>>>> 4. If the job is later restarted from this >>>>>>>> checkpoint, splits >>>>>>>> > 3 >>>>>>>> > > > >>>>>> and 4 will be permanently lost. >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> Scenario 2: >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> 1. Reader A reports splits (1 and 2), and Reader B >>>>>>>> reports (3 >>>>>>>> > and >>>>>>>> > > > >>>>>> 4) upon restart. >>>>>>>> > > > >>>>>> 2. Before the enumerator receives all reports and >>>>>>>> performs >>>>>>>> > > > >>>>>> reassignment, a checkpoint is triggered. >>>>>>>> > > > >>>>>> 3. Since no splits have been reassigned yet, both >>>>>>>> readers >>>>>>>> > have >>>>>>>> > > > >>>>>> empty states. >>>>>>>> > > > >>>>>> 4. When restarting from this checkpoint, all four >>>>>>>> splits are >>>>>>>> > > > lost. >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> Let me know if you have thoughts on how we might >>>>>>>> mitigate these >>>>>>>> > > > risks! >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> Best >>>>>>>> > > > >>>>>> Hongshun >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>> On Fri, Aug 8, 2025 at 1:46 AM Becket Qin < >>>>>>>> be...@gmail.com> >>>>>>>> > > > >>>>>> wrote: >>>>>>>> > > > >>>>>> >>>>>>>> > > > >>>>>>> Hi Hongshun, >>>>>>>> > > > >>>>>>> >>>>>>>> > > > >>>>>>> The steps sound reasonable to me in general. In terms >>>>>>>> of the >>>>>>>> > > > updated >>>>>>>> > > > >>>>>>> FLIP wiki, it would be good to see if we can keep the >>>>>>>> protocol >>>>>>>> > > > simple. One >>>>>>>> > > > >>>>>>> alternative way to achieve this behavior is following: >>>>>>>> > > > >>>>>>> >>>>>>>> > > > >>>>>>> 1. Upon SourceOperator startup, the SourceOperator >>>>>>>> sends >>>>>>>> > > > >>>>>>> ReaderRegistrationEvent with the currently assigned >>>>>>>> splits to >>>>>>>> > the >>>>>>>> > > > >>>>>>> enumerator. It does not add these splits to the >>>>>>>> SourceReader. >>>>>>>> > > > >>>>>>> 2. The enumerator will always use the >>>>>>>> > > > >>>>>>> SourceEnumeratorContext.assignSplits() to assign the >>>>>>>> splits. >>>>>>>> > (not >>>>>>>> > > > via the >>>>>>>> > > > >>>>>>> response of the SourceRegistrationEvent, this allows >>>>>>>> async >>>>>>>> > split >>>>>>>> > > > assignment >>>>>>>> > > > >>>>>>> in case the enumerator wants to wait until all the >>>>>>>> readers are >>>>>>>> > > > registered) >>>>>>>> > > > >>>>>>> 3. The SourceOperator will only call >>>>>>>> SourceReader.addSplits() >>>>>>>> > when >>>>>>>> > > > >>>>>>> it receives the AddSplitEvent from the enumerator. >>>>>>>> > > > >>>>>>> >>>>>>>> > > > >>>>>>> This protocol has a few benefits: >>>>>>>> > > > >>>>>>> 1. it basically allows arbitrary split reassignment >>>>>>>> upon >>>>>>>> > restart >>>>>>>> > > > >>>>>>> 2. simplicity: there is only one way to assign splits. >>>>>>>> > > > >>>>>>> >>>>>>>> > > > >>>>>>> So we only need one interface change: >>>>>>>> > > > >>>>>>> - add the initially assigned splits to ReaderInfo so >>>>>>>> the >>>>>>>> > Enumerator >>>>>>>> > > > >>>>>>> can access it. >>>>>>>> > > > >>>>>>> and one behavior change: >>>>>>>> > > > >>>>>>> - The SourceOperator should stop assigning splits to >>>>>>>> the from >>>>>>>> > state >>>>>>>> > > > >>>>>>> restoration, but >>>>>>>> > [message truncated...] >>>>>>>> > >>>>>>>> >>>>>>> >>>>> >>>
