Hi All, We had a lot of off-line discussions. As a result I would suggest dropping the idea of introducing an end-to-end-latency concept, until we can properly implement it, which will require more designing and experimenting. I would suggest starting with a more manual solution, where the user needs to configure concrete parameters, like `execution.checkpointing.max-interval` or `execution.flush-interval`.
FLIP-309 looks good to me, I would just rename `execution.checkpointing.interval-during-backlog` to `execution.checkpointing.max-interval`. I would also reference future work, that a solution that would allow set `isProcessingBacklog` for sources like Kafka will be introduced via FLIP-328 [1]. Best, Piotrek [1] https://cwiki.apache.org/confluence/display/FLINK/FLIP-328%3A+Allow+source+operators+to+determine+isProcessingBacklog+based+on+watermark+lag śr., 12 lip 2023 o 03:49 Dong Lin <lindon...@gmail.com> napisał(a): > Hi Piotr, > > I think I understand your motivation for suggeseting > execution.slow-end-to-end-latency now. Please see my followup comments > (after the previous email) inline. > > On Wed, Jul 12, 2023 at 12:32 AM Piotr Nowojski <pnowoj...@apache.org> > wrote: > > > Hi Dong, > > > > Thanks for the updates, a couple of comments: > > > > > If a record is generated by a source when the source's > > isProcessingBacklog is true, or some of the records used to > > > derive this record (by an operator) has isBacklog = true, then this > > record should have isBacklog = true. Otherwise, > > > this record should have isBacklog = false. > > > > nit: > > I think this conflicts with "Rule of thumb for non-source operators to > set > > isBacklog = true for the records it emits:" > > section later on, when it comes to a case if an operator has mixed > > isBacklog = false and isBacklog = true inputs. > > > > > execution.checkpointing.interval-during-backlog > > > > Do we need to define this as an interval config parameter? Won't that add > > an option that will be almost instantly deprecated > > because what we actually would like to have is: > > execution.slow-end-to-end-latency and execution.end-to-end-latency > > > > I guess you are suggesting that we should allow users to specify a higher > end-to-end latency budget for those records that are emitted by two-phase > commit sink, than those records that are emitted by none-two-phase commit > sink. > > My concern with this approach is that it will increase the complexity of > the definition of "processing latency requirement", as well as the > complexity of the Flink runtime code that handles it. Currently, the > FLIP-325 defines end-to-end latency as an attribute of the records that is > statically assigned when the record is generated at the source, regardless > of how it will be emitted later in the topology. If we make the changes > proposed above, we would need to define the latency requirement w.r.t. the > attribute of the operators that it travels through before its result is > emitted, which is less intuitive and more complex. > > For now, it is not clear whether it is necessary to have two categories of > latency requirement for the same job. Maybe it is reasonable to assume that > if a job has two-phase commit sink and the user is OK to emit some results > at 1 minute interval, then more likely than not the user is also OK to emit > all results at 1 minute interval, include those that go through > none-two-phase commit sink? > > If we do want to support different end-to-end latency depending on whether > the operator is emitted by two-phase commit sink, I would prefer to still > use execution.checkpointing.interval-during-backlog instead of > execution.slow-end-to-end-latency. This allows us to keep the concept of > end-to-end latency simple. Also, by explicitly including "checkpointing > interval" in the name of the config that directly affects checkpointing > interval, we can make it easier and more intuitive for users to understand > the impact and set proper value for such configs. > > What do you think? > > Best, > Dong > > > > Maybe we can introduce only `execution.slow-end-to-end-latency` (% maybe > a > > better name), and for the time being > > use it as the checkpoint interval value during backlog? > > > > Or do you envision that in the future users will be configuring only: > > - execution.end-to-end-latency > > and only optionally: > > - execution.checkpointing.interval-during-backlog > > ? > > > > Best Piotrek > > > > PS, I will read the summary that you have just published later, but I > think > > we don't need to block this FLIP on the > > existence of that high level summary. > > > > wt., 11 lip 2023 o 17:49 Dong Lin <lindon...@gmail.com> napisał(a): > > > > > Hi Piotr and everyone, > > > > > > I have documented the vision with a summary of the existing work in > this > > > doc. Please feel free to review/comment/edit this doc. Looking forward > to > > > working with you together in this line of work. > > > > > > > > > > > > https://docs.google.com/document/d/1CgxXvPdAbv60R9yrrQAwaRgK3aMAgAL7RPPr799tOsQ/edit?usp=sharing > > > > > > Best, > > > Dong > > > > > > On Tue, Jul 11, 2023 at 1:07 AM Piotr Nowojski < > piotr.nowoj...@gmail.com > > > > > > wrote: > > > > > > > Hi All, > > > > > > > > Me and Dong chatted offline about the above mentioned issues (thanks > > for > > > > that offline chat > > > > I think it helped both of us a lot). The summary is below. > > > > > > > > > Previously, I thought you meant to add a generic logic in > > > > SourceReaderBase > > > > > to read existing metrics (e.g. backpressure) and emit the > > > > > IsProcessingBacklogEvent to SourceCoordinator. I am sorry if I have > > > > > misunderstood your suggetions. > > > > > > > > > > After double-checking your previous suggestion, I am wondering if > you > > > are > > > > > OK with the following approach: > > > > > > > > > > - Add a job-level config > > > execution.checkpointing.interval-during-backlog > > > > > - Add an API SourceReaderContext#setProcessingBacklog(boolean > > > > > isProcessingBacklog). > > > > > - When this API is invoked, it internally sends an > > > > > internal SourceReaderBacklogEvent to SourceCoordinator. > > > > > - SourceCoordinator should keep track of the latest > > isProcessingBacklog > > > > > status from all its subtasks. And for now, we will hardcode the > logic > > > > such > > > > > that if any source reader says it is under backlog, then > > > > > execution.checkpointing.interval-during-backlog is used. > > > > > > > > > > This approach looks good to me as it can achieve the same > performance > > > > with > > > > > the same number of public APIs for the target use-case. And I > suppose > > > in > > > > > the future we might be able to re-use this API for source reader to > > set > > > > its > > > > > backlog status based on its backpressure metrics, which could be an > > > extra > > > > > advantage over the current approach. > > > > > > > > > > Do you think we can agree to adopt the approach described above? > > > > > > > > Yes, I think that's a viable approach. I would be perfectly fine to > not > > > > introduce > > > > `SourceReaderContext#setProcessingBacklog(boolean > > isProcessingBacklog).` > > > > and sending the `SourceReaderBacklogEvent` from SourceReader to JM > > > > in this FLIP. It could be implemented once we would decide to add > some > > > more > > > > generic > > > > ways of detecting backlog/backpressure on the SourceReader level. > > > > > > > > I think we could also just keep the current proposal of adding > > > > `SplitEnumeratorContext#setIsProcessingBacklog`, and use it in the > > > sources > > > > that > > > > can set it on the `SplitEnumerator` level. Later we could merge this > > with > > > > another > > > > mechanisms of detecting "isProcessingBacklog", like based on > watermark > > > lag, > > > > backpressure, etc, via some component running on the JM. > > > > > > > > At the same time I'm fine with having the "isProcessingBacklog" > concept > > > to > > > > switch > > > > runtime back and forth between high and low latency modes instead of > > > > "backpressure". In FLIP-325 I have asked: > > > > > > > > > I think there is one thing that hasn't been discussed neither here > > nor > > > in > > > > FLIP-309. Given that we have > > > > > three dimensions: > > > > > - e2e latency/checkpointing interval > > > > > - enabling some kind of batching/buffering on the operator level > > > > > - how much resources we want to allocate to the job > > > > > > > > > > How do we want Flink to adjust itself between those three? For > > example: > > > > > a) Should we assume that given Job has a fixed amount of assigned > > > > resources and make it paramount that > > > > > Flink doesn't exceed those available resources? So in case of > > > > backpressure, we > > > > > should extend checkpointing intervals, emit records less > frequently > > > and > > > > in batches. > > > > > b) Or should we assume that the amount of resources is flexible (up > > to > > > a > > > > point?), and the desired e2e latency > > > > > is the paramount aspect? So in case of backpressure, we should > > still > > > > adhere to the configured e2e latency, > > > > > and wait for the user or autoscaler to scale up the job? > > > > > > > > > > In case of a), I think the concept of "isProcessingBacklog" is not > > > > needed, we could steer the behaviour only > > > > > using the backpressure information. > > > > > > > > > > On the other hand, in case of b), "isProcessingBacklog" information > > > might > > > > be helpful, to let Flink know that > > > > > we can safely decrease the e2e latency/checkpoint interval even if > > > there > > > > is no backpressure, to use fewer > > > > > resources (and let the autoscaler scale down the job). > > > > > > > > > > Do we want to have both, or only one of those? Do a) and b) > > complement > > > > one another? If job is backpressured, > > > > > we should follow a) and expose to autoscaler/users information > "Hey! > > > I'm > > > > barely keeping up! I need more resources!". > > > > > While, when there is no backpressure and latency doesn't matter > > > > (isProcessingBacklog=true), we can limit the resource > > > > > usage > > > > > > > > After thinking this over: > > > > - the case that we don't have "isProcessingBacklog" information, but > > the > > > > source operator is > > > > back pressured, must be intermittent. EIther back pressure will go > > > away, > > > > or shortly we should > > > > reach the "isProcessingBacklog" state anyway > > > > - and even if we implement some back pressure detecting algorithm to > > > switch > > > > the runtime into the > > > > "high latency mode", we can always report that as > > "isProcessingBacklog" > > > > anyway, as runtime should > > > > react the same way in both cases (backpressure and > > > "isProcessingBacklog > > > > states). > > > > > > > > =============== > > > > > > > > With a common understanding of the final solution that we want to > have > > in > > > > the future, I'm pretty much fine with the current > > > > FLIP-309 proposal, with a couple of remarks: > > > > 1. Could you include in the FLIP-309 the long term solution as we > have > > > > discussed. > > > > a) Would be nice to have some diagram showing how the > > > > "isProcessingBacklog" information would be travelling, > > > > being aggregated and what will be done with that > > > information. > > > > (from SourceReader/SplitEnumerator to some > > > > "component" aggregating it, and then ... ?) > > > > 2. For me "processing backlog" doesn't necessarily equate to > > > "backpressure" > > > > (HybridSource can be > > > > both NOT backpressured and processing backlog at the same time). > If > > > you > > > > think the same way, can you include that > > > > definition of "processing backlog" in the FLIP including its > > relation > > > > to the backpressure state? If not, we need to align > > > > on that definition first :) > > > > > > > > Also I'm missing a big picture description, that would show what are > > you > > > > trying to achieve and what's the overarching vision > > > > behind all of the current and future FLIPs that you are planning in > > this > > > > area (FLIP-309, FLIP-325, FLIP-327, FLIP-331, ...?). > > > > Or was it described somewhere and I've missed it? > > > > > > > > Best, > > > > Piotrek > > > > > > > > > > > > > > > > czw., 6 lip 2023 o 06:25 Dong Lin <lindon...@gmail.com> napisał(a): > > > > > > > > > Hi Piotr, > > > > > > > > > > I am sorry if you feel unhappy or upset with us for not > > > following/fixing > > > > > your proposal. It is not my intention to give you this feeling. > After > > > > all, > > > > > we are all trying to make Flink better, to support more use-case > with > > > the > > > > > most maintainable code. I hope you understand that just like you, I > > > have > > > > > also been doing my best to think through various design options and > > > > taking > > > > > time to evalute the pros/cons. Eventually, we probably still need > to > > > > reach > > > > > consensus by clearly listing and comparing the objective pros/cons > of > > > > > different proposals and identifying the best choice. > > > > > > > > > > Regarding your concern (or frustration) that we are always finding > > > issues > > > > > in your proposal, I would say it is normal (and probably necessary) > > for > > > > > developers to find pros/cons in each other's solutions, so that we > > can > > > > > eventually pick the right one. I will appreciate anyone who can > > > correctly > > > > > pinpoint the concrete issue in my proposal so that I can improve it > > or > > > > > choose an alternative solution. > > > > > > > > > > Regarding your concern that we are not spending enough effort to > find > > > > > solutions and that the problem in your solution can be solved in a > > > > minute, > > > > > I would like to say that is not true. For each of your previous > > > > proposals, > > > > > I typically spent 1+ hours thinking through your proposal to > > understand > > > > > whether it works and why it does not work, and another 1+ hour to > > write > > > > > down the details and explain why it does not work. And I have had a > > > > variety > > > > > of offline discussions with my colleagues discussing various > > proposals > > > > > (including yours) with 6+ hours in total. Maybe I am not capable > > enough > > > > to > > > > > fix those issues in one minute or so so. If you think your proposal > > can > > > > be > > > > > easily fixed in one minute or so, I would really appreciate it if > you > > > can > > > > > think through your proposal and fix it in the first place :) > > > > > > > > > > For your information, I have had several long discussions with my > > > > > colleagues at Alibaba and also Becket on this FLIP. We have > seriously > > > > > considered your proposals and discussed in detail what are the > > > pros/cons > > > > > and whether we can improve these solutions. The initial version of > > this > > > > > FLIP (which allows the source operator to specify checkpoint > > intervals) > > > > > does not get enough support due to concerns of not being generic > > (i.e. > > > > > users need to specify checkpoint intervals on a per-source basis). > It > > > is > > > > > only after I updated the FLIP to use the job-level > > > > > execution.checkpointing.interval-during-backlog, then they agree to > > > give > > > > +1 > > > > > to the FLIP. What I want to tell you is that your suggestions have > > been > > > > > taken seriously, and the quality of the FLIP has been taken > seriously > > > > > by all those who have voted. As a result of taking your suggestion > > > > > seriously and trying to find improvements, we updated the FLIP to > use > > > > > isProcessingBacklog. > > > > > > > > > > I am wondering, do you think it will be useful to discuss > > face-to-face > > > > via > > > > > video conference call? It is not just between you and me. We can > > invite > > > > the > > > > > developers who are interested to join and help with the discussion. > > > That > > > > > might improve communication efficiency and help us understand each > > > other > > > > > better :) > > > > > > > > > > I am writing this long email to hopefully get your understanding. I > > > care > > > > > much more about the quality of the eventual solution rather than > who > > > > > proposed the solution. Please bear with me and see my comments > > inline, > > > > with > > > > > an explanation of the pros/cons of these proposals. > > > > > > > > > > > > > > > On Wed, Jul 5, 2023 at 11:06 PM Piotr Nowojski < > > > piotr.nowoj...@gmail.com > > > > > > > > > > wrote: > > > > > > > > > > > Hi Guys, > > > > > > > > > > > > I would like to ask you again, to spend a bit more effort on > trying > > > to > > > > > find > > > > > > solutions, not just pointing out problems. For 1.5 months, > > > > > > the discussion doesn't go in circle, but I'm suggesting a > solution, > > > you > > > > > are > > > > > > trying to undermine it with some arguments, I'm coming > > > > > > back with a fix, often an extremely easy one, only for you to try > > to > > > > find > > > > > > yet another "issue". It doesn't bode well, if you are finding > > > > > > a "problem" that can be solved with a minute or so of thinking or > > > even > > > > > has > > > > > > already been solved. > > > > > > > > > > > > I have provided you so far with at least three distinct solutions > > > that > > > > > > could address your exact target use-case. Two [1][2] generic > > > > > > enough to be probably good enough for the foreseeable future, one > > > > > > intermediate and not generic [3] but which wouldn't > > > > > > require @Public API changes or some custom hidden interfaces. > > > > > > > > > > > > > > > > All in all: > > > > > > - [1] with added metric hints like "isProcessingBacklog" solves > > your > > > > > target > > > > > > use case pretty well. Downside is having to improve > > > > > > how JM is collecting/aggregating metrics > > > > > > > > > > > > > > > > Here is my analysis of this proposal compared to the current > approach > > > in > > > > > the FLIP-309. > > > > > > > > > > pros: > > > > > - No need to add the public API > > > > > SplitEnumeratorContext#setIsProcessingBacklog. > > > > > cons: > > > > > - Need to add a public API that subclasses of SourceReader can use > to > > > > > specify its IsProcessingBacklog metric value. > > > > > - Source Coordinator needs to periodically pull the > > isProcessingBacklog > > > > > metrics from all TMs throughout the job execution. > > > > > > > > > > Here is why I think the cons outweigh the pros: > > > > > 1) JM needs to collect/aggregate metrics with extra runtime > overhead, > > > > which > > > > > is not necessary for the target use-case with the push-based > approach > > > in > > > > > FLIP-309. > > > > > 2) For the target use-case, it is simpler and more intuitive for > > source > > > > > operators (e.g. HybridSource, MySQL CDC source) to be able to set > its > > > > > isProcessingBacklog status in the SplitEnumerator. This is because > > the > > > > > switch between bounded/unbounded stages happens in their > > > SplitEnumerator. > > > > > > > > > > > > > > > > > > > > > - [2] is basically an equivalent of [1], replacing metrics with > > > events. > > > > > It > > > > > > also is a superset of your proposal > > > > > > > > > > > > > > > > Previously, I thought you meant to add a generic logic in > > > > SourceReaderBase > > > > > to read existing metrics (e.g. backpressure) and emit the > > > > > IsProcessingBacklogEvent to SourceCoordinator. I am sorry if I have > > > > > misunderstood your suggetions. > > > > > > > > > > After double-checking your previous suggestion, I am wondering if > you > > > are > > > > > OK with the following approach: > > > > > > > > > > - Add a job-level config > > > execution.checkpointing.interval-during-backlog > > > > > - Add an API SourceReaderContext#setProcessingBacklog(boolean > > > > > isProcessingBacklog). > > > > > - When this API is invoked, it internally sends an > > > > > internal SourceReaderBacklogEvent to SourceCoordinator. > > > > > - SourceCoordinator should keep track of the latest > > isProcessingBacklog > > > > > status from all its subtasks. And for now, we will hardcode the > logic > > > > such > > > > > that if any source reader says it is under backlog, then > > > > > execution.checkpointing.interval-during-backlog is used. > > > > > > > > > > This approach looks good to me as it can achieve the same > performance > > > > with > > > > > the same number of public APIs for the target use-case. And I > suppose > > > in > > > > > the future we might be able to re-use this API for source reader to > > set > > > > its > > > > > backlog status based on its backpressure metrics, which could be an > > > extra > > > > > advantage over the current approach. > > > > > > > > > > Do you think we can agree to adopt the approach described above? > > > > > > > > > > > > > > > - [3] yes, it's hacky, but it's a solution that could be thrown > away > > > once > > > > > > we implement [1] or [2] . The only real theoretical > > > > > > downside is that it cannot control the long checkpoint exactly > > > (short > > > > > > checkpoint interval has to be a divisor of the long checkpoint > > > > > > interval, but I simply can not imagine a practical use where > that > > > > would > > > > > > be a blocker for a user. Please..., someone wanting to set > > > > > > short checkpoint interval to 3min and long to 7 minutes, and > that > > > > > someone > > > > > > can not accept the long interval to be 9 minutes? > > > > > > And that's even ignoring the fact that if someone has an issue > > with > > > > > the 3 > > > > > > minutes checkpoint interval, I can hardly think that merely > > > > > > doubling the interval to 7 minutes would significantly solve > any > > > > > problem > > > > > > for that user. > > > > > > > > > > > > > > > > Yes, this is a fabricated example that shows > > > > > execution.checkpointing.interval-during-backlog might not be > > accurately > > > > > enforced with this option. I think you are probably right that it > > might > > > > not > > > > > matter that much. I just think we should try our best to make Flink > > > > public > > > > > API's semantics (including configuration) clear, simple, and > > > enforceable. > > > > > If we can make the user-facing configuration enforceable at the > cost > > of > > > > an > > > > > extra developer facing API (i.e. setProcessingBacklog(...)), I > would > > > > prefer > > > > > to do this. > > > > > > > > > > It seems that we both agree that option [2] is better than [3]. I > > will > > > > skip > > > > > the further comments for this option and we can probably focus on > > > > > option [2] :) > > > > > > > > > > > > > > > > Dong a long time ago you wrote: > > > > > > > Sure. Then let's decide the final solution first. > > > > > > > > > > > > Have you thought about that? Maybe I'm wrong but I don't remember > > you > > > > > > describing in any of your proposals how they could be > > > > > > extended in the future, to cover more generic cases. Regardless > if > > > you > > > > > > either don't believe in the generic solution or struggle to > > > > > > > > > > > > > > > > Yes, I have thought about the plan to extend the current FLIP to > > > support > > > > > metrics (e.g. backpressure) based solution you described earlier. > > > > Actually, > > > > > I mentioned multiple times in the earlier email that your > suggestion > > of > > > > > using metrics is valuable and I will do this in a follow-up FLIP. > > > > > > > > > > Here are my comments from the previous email: > > > > > - See "I will add follow-up FLIPs to make use of the event-time > > metrics > > > > and > > > > > backpressure metrics" from Jul 3, 2023, 6:39 PM > > > > > - See "I agree it is valuable" from Jul 1, 2023, 11:00 PM > > > > > - See "we will create a followup FLIP (probably in FLIP-328)" from > > Jun > > > > 29, > > > > > 2023, 11:01 AM > > > > > > > > > > Frankly speaking, I think the idea around using the backpressure > > > metrics > > > > > still needs a bit more thinking before we can propose a FLIP. But I > > am > > > > > pretty sure we can make use of the watermark/event-time to > determine > > > the > > > > > backlog status. > > > > > > > > > > grasp it, if you can come back with something that can be easily > > > extended > > > > > > in the future, up to a point where one could implement > > > > > > something similar to this backpressure detecting algorithm that I > > > > > mentioned > > > > > > many times before, I would be happy to discuss and > > > > > > support it. > > > > > > > > > > > > > > > > Here is my idea of extending the source reader to support > > > > event-time-based > > > > > backlog detecting algorithms: > > > > > > > > > > - Add a job-level config such as > watermark-lag-threshold-for-backlog. > > > If > > > > > any source reader determines that the event-timestamp is available > > and > > > > the > > > > > system-time - watermark exceeds this threshold, then the source > > reader > > > > > considers its isProcessingBacklog=true. > > > > > - The source reader can send an event to the source coordinator. > Note > > > > that > > > > > this might be doable in the SourceReaderBase without adding any > > public > > > > API > > > > > which the concrete SourceReader subclass needs to explicitly > invoke. > > > > > - And in the future if FLIP-325 is accepted, insteading of sending > > the > > > > > event to SourceCoordinator and let SourceCoordinator inform the > > > > checkpoint > > > > > coordinator, the source reader might just emit the information as > > part > > > of > > > > > the RecordAttributes and let the two-phase commit sink inform the > > > > > checkpoint coordinator. > > > > > > > > > > Note that this is a sketch of the idea and it might need further > > > > > improvement. I just hope you understand that we have thought about > > this > > > > > idea and did quite a lot of thinking for these design options. If > it > > is > > > > OK > > > > > with you, I hope we can make incremental progress and discuss the > > > > > metrics-based solution separately in a follow-up FLIP. > > > > > > > > > > Last but not least, thanks for taking so much time to leave > comments > > > and > > > > > help us improve the FLIP. Please kindly bear with us in this > > > discussion. > > > > I > > > > > am looking forward to collaborating with you to find the best > design > > > for > > > > > the target use-cases. > > > > > > > > > > Best, > > > > > Dong > > > > > > > > > > > > > > > > Hang, about your points 1. and 2., do you think those problems > are > > > > > > insurmountable and blockers for that counter proposal? > > > > > > > > > > > > > 1. It is hard to find the error checkpoint. > > > > > > > > > > > > No it's not, please take a look at what I exactly proposed and > > maybe > > > at > > > > > the > > > > > > code. > > > > > > > > > > > > > 2. (...) The failed checkpoint may make them think the job is > > > > > unhealthy. > > > > > > > > > > > > Please read again what I wrote in [3]. I'm mentioning there a > > > solution > > > > > for > > > > > > this exact "problem". > > > > > > > > > > > > About the necessity of the config value, I'm still not convinced > > > that's > > > > > > needed from the start, but yes we can add some config option > > > > > > if you think otherwise. This option, if named properly, could be > > > > re-used > > > > > in > > > > > > the future for different solutions, so that's fine by me. > > > > > > > > > > > > Best, > > > > > > Piotrek > > > > > > > > > > > > [1] Introduced in my very first e-mail from 23 maj 2023, 16:26, > and > > > > > refined > > > > > > later with point "2." in my e-mail from 16 June 2023, 17:58 > > > > > > [2] Section "2. ===============" in my e-mail from 30 June 2023, > > > 16:34 > > > > > > [3] Section "3. ===============" in my e-mail from 30 June 2023, > > > 16:34 > > > > > > > > > > > > All times in CEST. > > > > > > > > > > > > > > > > > > > > > > > > > > >
