@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 affect throughput
if 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 already
implemented in 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, I suggest 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 the basic and required properties of the Source, the person implementing a source must bother with and need to figure out what all of the extra features are about 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 would highly likely 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 affect throughput if 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? 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`. 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> 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 default no-op implementation. I am a little worried about the proliferation of decorative interfaces. I think the most important thing about interfaces is that they are 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 which interface they can decorate and which combinations of multiple decorative interfaces are 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 is why not 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 an interface of AlignedSourceReader? In the future, if there is a new feature added (e.g. sorted or pre-partitioned data aware), are we going to create another interface of SplitReader such as SortedSplitReader or PrePartitionedAware? 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, it is 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 implementation of no-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 give the same user experience, but simpler. c) A related design principle that may be worth discussing is how do we let the Source implementations tell Flink what capability is supported and what is not. Personally speaking I feel the most intuitive place to me is in the Source itself, because that is the entrance of the entire Source connector logic. 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 both SourceReader and 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 that have implemented 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 add more such 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> 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 whichconsumeconcurrently from multiple splits and thus alignment applies. @Thomas: I wonder if "supporting" split alignment in SourceReaderBase and thendoingnothing if the split reader does not implement AlignedSplitReader couldbemisleading? 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 implementWithSplitsAssignment,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 assumeonesource 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 slowpartition.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 shouldwejust 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 exclusivelyina *Public* SourceReader interface. By the way notice SourceReaderBase does extend from WithSplitsAlignment, so effectively all implementationsdohandle 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 thatitis 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 shouldwejust 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 thatitis 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> 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 excessiveamountof data if one split emits data faster than another. For this part from the motivation section, is it accurate? Let's assumeonesource 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 slowpartition.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> 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>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 Knauf https://twitter.com/snntrable https://github.com/knaufk
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