a) "MySourceReader implements SourceReader, WithSplitsAlignment", along with "MySplitReader implements SplitReader, WithSplitsAlignment", or b) "MySourceReader implements AlignedSourceReader" and "MySplitReader implements AlignedSplitReader", or c) "MySourceReader implements SourceReader" and "MySplitReader implements SplitReader".
I think the latest proposal according to Dawid would be: d) "MySourceReader implements SourceReader" and "MySplitReader implements AlignedSplitReader". I am fine with this API, although personally speaking I think it is simpler to just add a new method to the split reader with default impl.I think that is a good idea to have it aligned as much as possible. I'd be +1 for your option c). We can merge AlignedSplitReader with SplitReader. We will update the FLIP shortly.
Best, Dawid On 25/04/2022 12:43, Becket Qin wrote:
Thanks for the comment, Jark. 3. Interface/Method Name.Can the interface be used to align other things in the future? For example, align read speed, I have seen users requesting global rate limits. This feature may also need an interface like this. If we don't plan to extend this interface to support align other things, I suggest explicitly declaring the purpose of the methods, such as `alignWatermarksForSplits` instead of `alignSplits`.This is a good point. Naming wise, it would usually be more extensible to just describe what the method actually does, instead of assuming the purpose of doing this. For example, in this case, pauseOrResumeSplits() would be more extensible because this can be used for any kind of flow control, be it watermark alignment or simple rate limiting. 4. Interface or Method.I don't have a strong opinion on this. I think they have their own advantages. In Flink SQL, we heavily use Interfaces for extending abilities (SupportsXxxx) for TableSource/TableSink, and I prefer Interfaces rather than methods in this case. When you have a bunch of abilities and each ability has more than one method, Interfaces can help to organize them and make users clear which methods need to implement when you want to have an ability.I am OK with decorative interfaces if this is a general design pattern in the other components in Flink. But it looks like the current API proposal is not symmetric. The current proposal is essentially "MySourceReader implements SourceReader, WithSplitsAlignment", along with "MySplitReader implements AlignedSplitsReader". Should we make the API symmetric? I'd consider any one of the following as symmetric. a) "MySourceReader implements SourceReader, WithSplitsAlignment", along with "MySplitReader implements SplitReader, WithSplitsAlignment", or b) "MySourceReader implements AlignedSourceReader" and "MySplitReader implements AlignedSplitReader", or c) "MySourceReader implements SourceReader" and "MySplitReader implements SplitReader". I think the latest proposal according to Dawid would be: d) "MySourceReader implements SourceReader" and "MySplitReader implements AlignedSplitReader". I am fine with this API, although personally speaking I think it is simpler to just add a new method to the split reader with default impl. @Dawid Wysakowicz<dwysakow...@apache.org>, thanks for the reply. Having said that, as I don't have a preference and I agree most of thesources will support the alignment I am fine following your suggestion to have the SourceReader extending from WithWatermarksSplitsAlignment, but would put the "supportsXXX" there, not in the Source to keep the two methods together.One benefit of having the "supportsXXX" in Source is that this allows some compile time check. For example, if a user enabled watermark alignment while it is not supported by the Source, an exception can be thrown at compile time. It seems in general useful. That said, I agree that API cleanliness wise it is better to put the two methods together. Thanks, Jiangjie (Becket) Qin On Mon, Apr 25, 2022 at 5:56 PM Jark Wu<imj...@gmail.com> wrote:Thank Dawid for the reminder on FLIP-182. Sorry I did miss it. I don't have other concerns then. Best, Jark On Mon, 25 Apr 2022 at 15:40, Dawid Wysakowicz<dwysakow...@apache.org> wrote:@Jark: 1. Will the framework always align with watermarks when the source implements the interface? I'm afraid not every case needs watermark alignment even if Kafka implements the interface, and this will affect the throughput somehow. I agree with Becket we may need a `supportSplitsAlignment()` method for users to configure the source to enable/disable the alignment. 2. How does the framework calculate maxDesiredWatermark? I think the algorithm of maxDesiredWatermark will greatly affectthroughputif the reader is constantly switching between pause and resume. Can users configure the alignment offset? This is covered in the previous FLIP[1] which has been alreadyimplementedin 1.15. In short, it must be enabled with the watermark strategy which also configures drift and update interval. If we don't plan to extend this interface to support align other things,Isuggest explicitly declaring the purpose of the methods, such as `alignWatermarksForSplits` instead of `alignSplits`. Sure let's rename it. @Becket: I understand your point. On the other hand putting all methods, even with "supportsXXX" methods for enabling certain features, makes the entry threshold for writing a new source higher. Instead of focusing on thebasicand required properties of the Source, the person implementing a source must bother with and need to figure out what all of the extra featuresareabout and how to deal with them. It makes it also harder to organize methods in coupled groups as Jark said. Having said that, as I don't have a preference and I agree most of the sources will support the alignment I am fine following your suggestion to have the SourceReader extending from WithWatermarksSplitsAlignment, but would put the "supportsXXX" there, not in the Source to keep the two methods together. Lastly, I agree it is really unfortunate the "alignSplits" methods differ slightly for SourceReader and SpitReader. The reason for that is SourceReaderBase deals only with SplitIds, whereas SplitReader needs the actual splits to pause them. I found the discrepancy acceptable for the sake of simplifying changes significantly, especially as they wouldhighlylikely impact performance as we would have to perform additional lookups. Moreover the SplitReader is a secondary interface. Best, Dawid [1]https://cwiki.apache.org/confluence/x/hQYBCw On 24/04/2022 17:15, Jark Wu wrote: Thanks for the effort, Dawid and Sebastian! I just have some minor questions (maybe I missed something). 1. Will the framework always align with watermarks when the source implements the interface? I'm afraid not every case needs watermark alignment even if Kafka implements the interface, and this will affect the throughput somehow. I agree with Becket we may need a `supportSplitsAlignment()` method for users to configure the source to enable/disable the alignment. 2. How does the framework calculate maxDesiredWatermark? I think the algorithm of maxDesiredWatermark will greatly affectthroughputif the reader is constantly switching between pause and resume. Can users configure the alignment offset? 3. Interface/Method Name. Can the interface be used to align other things in the future? Forexample,align read speed, I have seen users requesting global rate limits. This feature may also need an interface like this. If we don't plan to extend this interface to support align other things,Isuggest explicitly declaring the purpose of the methods, such as `alignWatermarksForSplits` instead of `alignSplits`. 4. Interface or Method. I don't have a strong opinion on this. I think they have their own advantages. In Flink SQL, we heavily use Interfaces for extending abilities (SupportsXxxx) for TableSource/TableSink, and I prefer Interfaces rather than methods in this case. When you have a bunch of abilities and each ability has more than one method, Interfaces can help to organize them and make users clear which methods need to implement when you want to have an ability. Best, Jark On Sun, 24 Apr 2022 at 18:13, Becket Qin<becket....@gmail.com> <becket....@gmail.com> wrote:Hi Dawid, Thanks for the explanation. Apologies that I somehow misread a bunch of "align" and thought they were "assign". Regarding 1, by default implementation, I was thinking of the defaultno-opimplementation. I am a little worried about the proliferation ofdecorativeinterfaces. I think the most important thing about interfaces is thattheyare easy to understand. In this case, I prefer adding new method to the existing interface for the following reasons: a) I feel the biggest drawback of decorative interfaces is whichinterfacethey can decorate and which combinations of multiple decorativeinterfacesare valid. In the current FLIP, the withSplitsAlignment interface is only applicable to the SourceReader which means it can't decorate any other interface. From an interface design perspective, a natural question iswhynot let "AlignedSplitReader" extend "withSplitsAlignment"? And it is also natural to assume that a split reader implementing both SplitReader and WithSplitAlignment would work, because a source reader implementing SourceReader and withSplitsAlignment works. So why isn't there aninterfaceof AlignedSourceReader? In the future, if there is a new feature added (e.g. sorted or pre-partitioned data aware), are we going to createanotherinterface of SplitReader such as SortedSplitReader orPrePartitionedAware?Can they be combined? So I think the additional decorative interface like withSplitsAlignment actually increases the understanding cost of users because they have to know what decorative interfaces are there, which interface they can decorate and which combinations of the decorative interfaces are valid and which are not. Ideally we want to avoid that. To be clear, I am not opposing having an interface of withSplitsAlignment,itis completely OK to have it as an internal interface and let SourceReader and SplitReader both extend it. b) Adding a new method to the SourceReader with a default implementationofno-op would help avoid logic branching in the source logic, especially given that we agree that the vast majority of the SourceReader implementations, if not all, would just extend from the SourceReaderBase. That means adding a new method to the interface would effectively givethesame user experience, but simpler. c) A related design principle that may be worth discussing is how do weletthe Source implementations tell Flink what capability is supported andwhatis not. Personally speaking I feel the most intuitive place to me is intheSource itself, because that is the entrance of the entire Sourceconnectorlogic. Based on the above thoughts, I am wondering if the following interface would be easier to understand by the users. - Change "withSplitsAlignment" to internal interface, let bothSourceReaderand SplitReader extend from it, with a default no-op implementation. - Add a new method "boolean supportSplitsAlignment()" to the Source interface, with a default implementation returning false. Sources thathaveimplemented the alignment logic can change this to return true, and override the alignSplits() methods in the SourceReader / SplitReader if needed. - In the future, if a new optional feature is going to be added to the Source, and that feature requires the awareness from Flink, we can addmoresuch methods to the Source. What do you think? Thanks, Jiangjie (Becket) Qin On Fri, Apr 22, 2022 at 4:05 PM Dawid Wysakowicz<dwysakow...@apache.org><dwysakow...@apache.org>wrote: @Konstantin: As part of this FLIP, the `AlignedSplitReader` interface (aka the stop & resume behavior) will be implemented for Kafka and Pulsar only, correct? Correct, as far as I know though, those are the only sources which consume concurrently from multiple splits and thus alignment applies. @Thomas: I wonder if "supporting" split alignment in SourceReaderBase and then doing nothing if the split reader does not implement AlignedSplitReader could be misleading? Perhaps WithSplitsAlignment can instead be added to the specific source reader (i.e. KafkaSourceReader) to make it explicit that the source actually supports it. I understand your concern. Hmm, I think we could actually do that. Given the actual implementation of the SourceReaderBase#alignSplits is rather short (just a forward to the corresponding method of SplitFetcher), we could reimplement it in the actual source implementations. This solution has the downside though. Authors of new sources would have to do two things: extend from AlignedSplitReader and implement WithSplitsAssignment, instead of just extending AlignedSplitReader. I would be fine with such a tradeoff though. What others think? @Steven: For this part from the motivation section, is it accurate? Let's assume one source task consumes from 3 partitions and one of the partition is significantly slower. In this situation, watermark for this source task won't hold back as it is reading recent data from other two Kafka partitions. As a result, it won't hold back the overall watermark. I thought the problem is that we may have late data for this slow partition. It will hold back the watermark. Watermark of an operator is the minimum of watermarks of all splits[1] I have another question about the restart. Say split alignment is triggered. checkpoint is completed. job failed and restored from the last checkpoint. because alignment decision is not checkpointed, initially alignment won't be enforced until we get a cycle of watermark aggregation and propagation, right? Not saying this corner is a problem. Just want to understand it more. Your understanding is correct. @Becket: 1. I think watermark alignment is sort of a general use case, so should we just add the related methods to SourceReader directly instead of introducing the new interface of WithSplitAssignment? We can provide default implementations, so backwards compatibility won't be an issue. I don't think we can provide a default implementation. How would we do that? Would it be just a no-op? Is it better than having an opt-in interface? The default implementation would have to be added exclusively in a *Public* SourceReader interface. By the way notice SourceReaderBase does extend from WithSplitsAlignment, so effectively all implementations do handle the alignment case. To be honest I think it is impossible to implement the SourceReader interface directly by end users. 2. As you mentioned, the SplitReader interface probably also needs some change to support throttling at the split granularity. Can you add that interface change into the public interface section as well? It has been added from the beginning. See *AlignedSplitReader.* 3. Nit, can we avoid using the method name assignSplits here, given that it is not actually changing the split assignments? It seems something like pauseOrResumeSplits(), or adjustSplitsThrottling() is more accurate. The method's called *alignSplits*, not assign. Do you still prefer a different name for that? Personally, I am open for suggestions here. Best, Dawid [1]https://nightlies.apache.org/flink/flink-docs-master/docs/dev/datastream/sources/#watermark-generationOn 22/04/2022 05:59, Becket Qin wrote: Thanks for driving the effort, Sebastion. I think the motivation makes a lot of sense. Just a few suggestions / questions. 1. I think watermark alignment is sort of a general use case, so should we just add the related methods to SourceReader directly instead of introducing the new interface of WithSplitAssignment? We can provide default implementations, so backwards compatibility won't be an issue. 2. As you mentioned, the SplitReader interface probably also needs some change to support throttling at the split granularity. Can you add that interface change into the public interface section as well? 3. Nit, can we avoid using the method name assignSplits here, given that it is not actually changing the split assignments? It seems something like pauseOrResumeSplits(), or adjustSplitsThrottling() is more accurate. Thanks, Jiangjie (Becket) Qin On Thu, Apr 21, 2022 at 11:39 PM Steven Wu<stevenz...@gmail.com> <stevenz...@gmail.com> <stevenz...@gmail.com> wrote: However, a single source operator may read data from multiple splits/partitions, e.g., multiple Kafka partitions, such that even with watermark alignment the source operator may need to buffer excessive amount of data if one split emits data faster than another. For this part from the motivation section, is it accurate? Let's assume one source task consumes from 3 partitions and one of the partition is significantly slower. In this situation, watermark for this source task won't hold back as it is reading recent data from other two Kafka partitions. As a result, it won't hold back the overall watermark. I thought the problem is that we may have late data for this slow partition. I have another question about the restart. Say split alignment is triggered. checkpoint is completed. job failed and restored from the last checkpoint. because alignment decision is not checkpointed, initially alignment won't be enforced until we get a cycle of watermark aggregation and propagation, right? Not saying this corner is a problem. Just want to understand it more. On Thu, Apr 21, 2022 at 8:20 AM Thomas Weise<t...@apache.org> <t...@apache.org> <t...@apache.org> wrote: Thanks for working on this! I wonder if "supporting" split alignment in SourceReaderBase and then doing nothing if the split reader does not implement AlignedSplitReader could be misleading? Perhaps WithSplitsAlignment can instead be added to the specific source reader (i.e. KafkaSourceReader) to make it explicit that the source actually supports it. Thanks, Thomas On Thu, Apr 21, 2022 at 4:57 AM Konstantin Knauf<kna...@apache.org> <kna...@apache.org> <kna...@apache.org> wrote: Hi Sebastian, Hi Dawid, As part of this FLIP, the `AlignedSplitReader` interface (aka the stop & resume behavior) will be implemented for Kafka and Pulsar only, correct? +1 in general. I believe it is valuable to complete the watermark aligned story with this FLIP. Cheers, Konstantin On Thu, Apr 21, 2022 at 12:36 PM Dawid Wysakowicz < dwysakow...@apache.org> wrote: To be explicit, having worked on it, I support it ;) I think we can start a vote thread soonish, as there are no concerns so far. Best, Dawid On 13/04/2022 11:27, Sebastian Mattheis wrote: Dear Flink developers, I would like to open a discussion on FLIP 217 [1] for an extension of Watermark Alignment to perform alignment also in SplitReaders. To do so, SplitReaders must be able to suspend and resume reading from split sources where the SourceOperator coordinates and controlls suspend and resume. To gather information about current watermarks of the SplitReaders, we extend the internal WatermarkOutputMulitplexer and report watermarks to the SourceOperator. There is a PoC for this FLIP [2], prototyped by Arvid Heise and revised and reworked by Dawid Wysakowicz (He did most of the work.) and me. The changes are backwards compatible in a way that if affected components do not support split alignment the behavior is as before. Best, Sebastian [1]https://cwiki.apache.org/confluence/display/FLINK/FLIP-217+Support+watermark+alignment+of+source+splits[2]https://github.com/dawidwys/flink/tree/aligned-splits -- Konstantin Knaufhttps://twitter.com/snntrablehttps://github.com/knaufk
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