Filed FLIP-167: Watermarks for Sink API: https://cwiki.apache.org/confluence/display/FLINK/FLIP-167%3A+Watermarks+for+Sink+API
I'd like to call a vote next week, is that reasonable? On Wed, May 19, 2021 at 6:28 PM Zhou, Brian <b.z...@dell.com> wrote: > Hi Arvid and Eron, > > Thanks for the discussion and I read through Eron's pull request and I > think this can benefit Pravega Flink connector as well. > > Here is some background. Pravega had the watermark concept through the > event stream since two years ago, and here is a blog introduction[1] for > Pravega watermark. > Pravega Flink connector also had this watermark integration last year that > we wanted to propagate the Flink watermark to Pravega in the SinkFunction, > and at that time we just used the existing Flink API that we keep the last > watermark in memory and check if watermark changes for each event[2] which > is not efficient. With such new interface, we can also manage the watermark > propagation much more easily. > > [1] https://pravega.io/blog/2019/11/08/pravega-watermarking-support/ > [2] > https://github.com/pravega/flink-connectors/blob/master/src/main/java/io/pravega/connectors/flink/FlinkPravegaWriter.java#L465 > > -----Original Message----- > From: Arvid Heise <ar...@apache.org> > Sent: Wednesday, May 19, 2021 16:06 > To: dev > Subject: Re: [DISCUSS] Watermark propagation with Sink API > > > [EXTERNAL EMAIL] > > Hi Eron, > > Thanks for pushing that topic. I can now see that the benefit is even > bigger than I initially thought. So it's worthwhile anyways to include that. > > I also briefly thought about exposing watermarks to all UDFs, but here I > really have an issue to see specific use cases. Could you maybe take a few > minutes to think about it as well? I could only see someone misusing Async > IO as a sink where a real sink would be more appropriate. In general, if > there is not a clear use case, we shouldn't add the functionality as it's > just increased maintenance for no value. > > If we stick to the plan, I think your PR is already in a good shape. We > need to create a FLIP for it though, since it changes Public interfaces > [1]. I was initially not convinced that we should also change the old > SinkFunction interface, but seeing how little the change is, I wouldn't > mind at all to increase consistency. Only when we wrote the FLIP and > approved it (which should be minimal and fast), we should actually look at > the PR ;). > > The only thing which I would improve is the name of the function. > processWatermark sounds as if the sink implementer really needs to > implement it (as you would need to do it on a custom operator). I would > make them symmetric to the record writing/invoking method (e.g. > writeWatermark and invokeWatermark). > > As a follow-up PR, we should then migrate KafkaShuffle to the new API. But > that's something I can do. > > [1] > > https://urldefense.com/v3/__https://cwiki.apache.org/confluence/display/FLINK/Flink*Improvement*Proposals__;Kys!!LpKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dMnp6nc7o$ > [cwiki[.]apache[.]org] > > On Wed, May 19, 2021 at 3:34 AM Eron Wright <ewri...@streamnative.io > .invalid> > wrote: > > > Update: opened an issue and a PR. > > > > https://urldefense.com/v3/__https://issues.apache.org/jira/browse/FLIN > > K-22700__;!!LpKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dM > > plbgRO4$ [issues[.]apache[.]org] > > https://urldefense.com/v3/__https://github.com/apache/flink/pull/15950 > > __;!!LpKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dMtScmG7a > > $ [github[.]com] > > > > > > On Tue, May 18, 2021 at 10:03 AM Eron Wright <ewri...@streamnative.io> > > wrote: > > > > > Thanks Arvid and David for sharing your ideas on this subject. I'm > > > glad to hear that you're seeing use cases for watermark propagation > > > via an enhanced sink interface. > > > > > > As you've guessed, my interest is in Pulsar and am exploring some > > > options for brokering watermarks across stream processing pipelines. > > > I think > > Arvid > > > is speaking to a high-fidelity solution where the difference between > > intra- > > > and inter-pipeline flow is eliminated. My goal is more limited; I > > > want > > to > > > write the watermark that arrives at the sink to Pulsar. Simply > > > imagine that Pulsar has native support for watermarking in its > > > producer/consumer API, and we'll leave the details to another forum. > > > > > > David, I like your invariant. I see lateness as stemming from the > > problem > > > domain and from system dynamics (e.g. scheduling, batching, lag). > > > When > > one > > > depends on order-of-observation to generate watermarks, the app may > > become > > > unduly sensitive to dynamics which bear on order-of-observation. My > > > goal is to factor out the system dynamics from lateness determination. > > > > > > Arvid, to be most valuable (at least for my purposes) the > > > enhancement is needed on SinkFunction. This will allow us to easily > > > evolve the existing Pulsar connector. > > > > > > Next step, I will open a PR to advance the conversation. > > > > > > Eron > > > > > > On Tue, May 18, 2021 at 5:06 AM David Morávek > > > <david.mora...@gmail.com> > > > wrote: > > > > > >> Hi Eron, > > >> > > >> Thanks for starting this discussion. I've been thinking about this > > >> recently as we've run into "watermark related" issues, when > > >> chaining multiple pipelines together. My to cents to the > > >> discussion: > > >> > > >> How I like to think about the problem, is that there should an > > >> invariant that holds for any stream processing pipeline: "NON_LATE > > >> element > > entering > > >> the system, should never become LATE" > > >> > > >> Unfortunately this is exactly what happens in downstream pipelines, > > >> because the upstream one can: > > >> - break ordering (especially with higher parallelism) > > >> - emit elements that are ahead of output watermark > > >> > > >> There is not enough information to re-construct upstream watermark > > >> in latter stages (it's always just an estimate based on previous > > >> pipeline's output). > > >> > > >> It would be great, if we could have a general abstraction, that is > > >> reusable for various sources / sinks (not just Kafka / Pulsar, > > >> thought this would probably cover most of the use-cases) and > > >> systems. > > >> > > >> Is there any other use-case then sharing watermark between > > >> pipelines, > > that > > >> you're trying to solve? > > >> > > >> Arvid: > > >> > > >> 1. Watermarks are closely coupled to the used system (=Flink). I > > >> have a > > >> > hard time imagining that it's useful to use a different stream > > processor > > >> > downstream. So for now, I'm assuming that both upstream and > > >> > downstream > > >> are > > >> > Flink applications. In that case, we probably define both parts > > >> > of the pipeline in the same Flink job similar to KafkaStream's > #through. > > >> > > > >> > > >> I'd slightly disagree here. For example we're "materializing" > > change-logs > > >> produced by Flink pipeline into serving layer (random access db / > > >> in memory view / ..) and we need to know, whether responses we > > >> serve meet the "freshness" requirements (eg. you may want to > > >> respond differently, when watermark is lagging way too much behind > > >> processing time). Also not > > every > > >> stream processor in the pipeline needs to be Flink. It can as well > > >> be a simple element-wise transformation that reads from Kafka and > > >> writes back into separate topic (that's what we do for example with > > >> ML models, that have special hardware requirements). > > >> > > >> Best, > > >> D. > > >> > > >> > > >> On Tue, May 18, 2021 at 8:30 AM Arvid Heise <ar...@apache.org> wrote: > > >> > > >> > Hi Eron, > > >> > > > >> > I think this is a useful addition for storage systems that act as > > >> > pass-through for Flink to reduce recovery time. It is only useful > > >> > if > > you > > >> > combine it with regional fail-over as only a small part of the > > pipeline > > >> is > > >> > restarted. > > >> > > > >> > A couple of thoughts on the implications: > > >> > 1. Watermarks are closely coupled to the used system (=Flink). I > > >> > have > > a > > >> > hard time imagining that it's useful to use a different stream > > processor > > >> > downstream. So for now, I'm assuming that both upstream and > > >> > downstream > > >> are > > >> > Flink applications. In that case, we probably define both parts > > >> > of the pipeline in the same Flink job similar to KafkaStream's > #through. > > >> > 2. The schema of the respective intermediate stream/topic would > > >> > need > > to > > >> be > > >> > managed by Flink to encode both records and watermarks. This > > >> > reduces > > the > > >> > usability quite a bit and needs to be carefully crafted. > > >> > 3. It's not clear to me if constructs like SchemaRegistry can be > > >> properly > > >> > supported (and also if they should be supported) in terms of > > >> > schema evolution. > > >> > 4. Potentially, StreamStatus and LatencyMarker would also need to > > >> > be encoded. > > >> > 5. It's important to have some way to transport backpressure from > > >> > the downstream to the upstream. Or else you would have the same > > >> > issue as KafkaStreams where two separate pipelines can drift so > > >> > far away that > > you > > >> > experience data loss if the data retention period is smaller than > > >> > the drift. > > >> > 6. It's clear that you trade a huge chunk of throughput for lower > > >> overall > > >> > latency in case of failure. So it's an interesting feature for > > >> > use > > cases > > >> > with SLAs. > > >> > > > >> > Since we are phasing out SinkFunction, I'd prefer to only support > > >> > SinkWriter. Having a no-op default sounds good to me. > > >> > > > >> > We have some experimental feature for Kafka [1], which pretty > > >> > much > > >> reflects > > >> > your idea. Here we have an ugly workaround to be able to process > > >> > the watermark by using a custom StreamSink task. We could also > > >> > try to > > >> create a > > >> > FLIP that abstracts the actual system away and then we could use > > >> > the approach for both Pulsar and Kafka. > > >> > > > >> > [1] > > >> > > > >> > > > >> > > https://urldefense.com/v3/__https://github.com/apache/flink/blob/maste > > r/flink-connectors/flink-connector-kafka/src/main/java/org/apache/flin > > k/streaming/connectors/kafka/shuffle/FlinkKafkaShuffle.java*L103__;Iw! > > !LpKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dMvmemHrt$ > > [github[.]com] > > >> > > > >> > > > >> > On Mon, May 17, 2021 at 10:44 PM Eron Wright > > >> > <ewri...@streamnative.io.invalid> wrote: > > >> > > > >> > > I would like to propose an enhancement to the Sink API, the > > >> > > ability > > to > > >> > > receive upstream watermarks. I'm aware that the sink context > > >> provides > > >> > the > > >> > > current watermark for a given record. I'd like to be able to > > >> > > write > > a > > >> > sink > > >> > > function that is invoked whenever the watermark changes. Out > > >> > > of > > scope > > >> > > would be event-time timers (since sinks aren't keyed). > > >> > > > > >> > > For context, imagine that a stream storage system had the > > >> > > ability to persist watermarks in addition to ordinary elements, > > >> > > e.g. to serve > > as > > >> > > source watermarks in a downstream processor. Ideally one could > > >> compose a > > >> > > multi-stage, event-driven application, with watermarks flowing > > >> end-to-end > > >> > > without need for a heuristics-based watermark at each stage. > > >> > > > > >> > > The specific proposal would be a new method on `SinkFunction` > > >> > > and/or > > >> on > > >> > > `SinkWriter`, called 'processWatermark' or 'writeWatermark', > > >> > > with a > > >> > default > > >> > > implementation that does nothing. > > >> > > > > >> > > Thoughts? > > >> > > > > >> > > Thanks! > > >> > > Eron Wright > > >> > > StreamNative > > >> > > > > >> > > > >> > > > > > > > > > -- > > > > > > Eron Wright Cloud Engineering Lead > > > > > > p: +1 425 922 8617 <18163542939> > > > > > > streamnative.io | Meet with me > > > <https://urldefense.com/v3/__https://calendly.com/eronwright/regular > > > -1-hour__;!!LpKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5 > > > dMtQrD25c$ [calendly[.]com]> > > > > > > <https://urldefense.com/v3/__https://github.com/streamnative__;!!LpK > > > I!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dMnQskrSQ$ > > > [github[.]com]> > > > <https://urldefense.com/v3/__https://www.linkedin.com/company/stream > > > native/__;!!LpKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5 > > > dMqO4UZJa$ [linkedin[.]com]> > > > <https://urldefense.com/v3/__https://twitter.com/streamnativeio/__;! > > > !LpKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dMpbyC_rP$ > > > [twitter[.]com]> > > > > > > > > > -- > > > > Eron Wright Cloud Engineering Lead > > > > p: +1 425 922 8617 <18163542939> > > > > streamnative.io | Meet with me > > <https://urldefense.com/v3/__https://calendly.com/eronwright/regular-1 > > -hour__;!!LpKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dMtQ > > rD25c$ [calendly[.]com]> > > > > <https://urldefense.com/v3/__https://github.com/streamnative__;!!LpKI! > > 2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dMnQskrSQ$ > > [github[.]com]> > > <https://urldefense.com/v3/__https://www.linkedin.com/company/streamna > > tive/__;!!LpKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dMqO > > 4UZJa$ [linkedin[.]com]> > > <https://urldefense.com/v3/__https://twitter.com/streamnativeio/__;!!L > > pKI!2IQYKfnjRuBgkNRxnPbJeFvTdhWjpwN0urN3m0yz_6W11H74kY5dMpbyC_rP$ > > [twitter[.]com]> > > > -- Eron Wright Cloud Engineering Lead p: +1 425 922 8617 <18163542939> streamnative.io | Meet with me <https://calendly.com/eronwright/regular-1-hour> <https://github.com/streamnative> <https://www.linkedin.com/company/streamnative/> <https://twitter.com/streamnativeio/>
