Hi Dong, > Adding regarding the effort to add back the per-component election capability: given that the implementation already follows per-process election, and given that there will likely be a lot of extra design/implementation/test effort needed to achieve the use-cases described above, maybe the change proposed in this thread won't affect the overall effort much?
This might be a misunderstanding; what Chesnay is proposing is _not removing the existing interfaces_, that allow us to split components out to separate processes eventually. Maybe let's be more explicit about what the current state is: 1) _HighAvailabilityServices_ interface contains methods to create _LeaderElectionService_ and _LeaderRetrievalService_ for each component separately 2) In 1.15, we've introduced an alternative implementation -> _MultipleComponentLeaderElectionService_ that can multicast the leader election to multiple components. 3) In 1.16, we've removed the old HA services because they didn't provide any extra capability beyond what _MultipleComponentLeaderElectionService_ offers. It indeed did per-component leader election, but it was still effectively tied with a single JM process, so adding this back would only help a little with the component split efforts. The biggest motivation for re-factoring from my side would be that it was tough to fit the _MultipleComponentLeaderElectionService_ into the existing interfaces, so the implementation is unnecessarily complex. I think what we should do instead is re-thinking these interfaces, so they can still provide the flexibility of letting the user split out some components into a separate process. There is also a pending discussion (FLIP-257 [1]) that hints that some people are already thinking in this direction, and it might be required for their use case. I've also recently started to incline that splitting out the ResourceManager might be crucial for building a large-scale managed service. There are a lot of companies emerging in this area right now, so I don't feel like we should be closing these doors just yet. [1] https://cwiki.apache.org/confluence/display/FLINK/FLIP-257%3A+Flink+JobManager+Process+Split Best, D. On Sat, Dec 10, 2022 at 7:01 AM Dong Lin <lindon...@gmail.com> wrote: > Hi Matthias, > > Thanks for the explanation. I was trying to understand the concrete > user-facing benefits of preserving the flexibility of per-component leader > election. Now I get that maybe they want to scale those components > independently, and maybe run the UI in an environment that is more > accessible > than the other processes. > > I replied to Chesnay's email regarding whether it is worthwhile to keep the > existing interface for those potential but not-yet-realized benefits. > > Thanks, > Dong > > On Fri, Dec 9, 2022 at 5:47 PM Matthias Pohl <matthias.p...@aiven.io > .invalid> > wrote: > > > Hi Dong, > > see my answers below. > > > > Regarding "Interface change might affect other projects that customize HA > > > services", are you referring to those projects which hack into Flink's > > > source code (as opposed to using Flink's public API) to customize HA > > > services? > > > > > > Yes, the proposed change might affect projects that need to have their > own > > HA implementation for whatever reason (interface change) or if a project > > accesses the HA backend to retrieve metadata from the ZK node/k8s > ConfigMap > > (change about how the data is stored in the HA backend). The latter one > was > > actually already the case with the change introduced by FLINK-24038 [1]. > > > > By the way, since Flink already supports zookeeper and kubernetes as the > > > high availability services, are you aware of many projects that still > > need > > > to hack into Flink's code to customize high availability services? > > > > > > I am aware of projects that use customized HA. But based on our > experience > > in FLINK-24038 [1] no one complained. So, making people aware through the > > mailing list might be good enough. > > > > And regarding "We lose some flexibility in terms of per-component > > > LeaderElection", could you explain what flexibility we need so that we > > can > > > gauge the associated downside of losing the flexibility? > > > > > > Just to recap: The current interface allows having per-component > > LeaderElection (e.g. the ResourceManager leader can run on a different > > JobManager than the Dispatcher). This implementation was replaced by > > FLINK-24038 [1] and removed in FLINK-25806 [2]. The new implementation > does > > LeaderElection per process (e.g. ResourceManager and Dispatcher always > run > > on the same JobManager). The changed interface would require us to touch > > the interface again if (for whatever reason) we want to reintroduce > > per-component leader election in some form. > > The interface change is, strictly speaking, not necessary to provide the > > new functionality. But I like the idea of certain requirements > (currently, > > we need per-process leader election to fix what was reported in > FLINK-24038 > > [1]) being reflected in the interface. This makes sure that we don't > > introduce a per-component leader election again accidentally in the > future > > because we thought it's a good idea but forgot about FLINK-24038. > > > > Matthias > > > > [1] https://issues.apache.org/jira/browse/FLINK-24038 > > [2] https://issues.apache.org/jira/browse/FLINK-25806 > > > > On Fri, Dec 9, 2022 at 2:09 AM Dong Lin <lindon...@gmail.com> wrote: > > > > > Hi Matthias, > > > > > > Thanks for the proposal! Overall I am in favor of making this interface > > > change to make Flink's codebase more maintainable. > > > > > > Regarding "Interface change might affect other projects that customize > HA > > > services", are you referring to those projects which hack into Flink's > > > source code (as opposed to using Flink's public API) to customize HA > > > services? If yes, it seems OK to break those projects since we don't > have > > > any backward compatibility guarantee for those projects. > > > > > > By the way, since Flink already supports zookeeper and kubernetes as > the > > > high availability services, are you aware of many projects that still > > need > > > to hack into Flink's code to customize high availability services? > > > > > > And regarding "We lose some flexibility in terms of per-component > > > LeaderElection", could you explain what flexibility we need so that we > > can > > > gauge the associated downside of losing the flexibility? > > > > > > Thanks! > > > Dong > > > > > > > > > > > > On Wed, Dec 7, 2022 at 4:28 PM Matthias Pohl <matthias.p...@aiven.io > > > .invalid> > > > wrote: > > > > > > > Hi everyone, > > > > > > > > The Flink community introduced a new way how leader election works in > > > Flink > > > > 1.15 with FLINK-24038 [1]. Instead of a per-component leader > election, > > > all > > > > components (i.e. ResourceManager, Dispatcher, REST server, JobMaster) > > > use a > > > > single (per-JM-process) leader election instance. It was meant to fix > > > some > > > > issues with deregistering Flink applications in multi-JM setups [1] > and > > > > reduce load on the HA backend. Users were able to opt-out and switch > > back > > > > to the old implementation [2]. > > > > > > > > The new approach was kind of complicated to implement while still > > > > maintaining support for the old implementation through the existing > > > > interfaces. With FLINK-25806 [3], the old implementation was removed > in > > > > Flink 1.16. This enables us to clean things up in the > > > > HighAvailabilityServices. > > > > > > > > The proposed change would mean touching the HighAvailabilityServices > > > > interface. Currently, the interface provides factory methods for > > > > LeaderElectionServices of the aforementioned components. All of these > > > > LeaderElectionServices are internally based on the same > LeaderElection > > > > instance handled in DefaultMultipleComponentLeaderElectionService. > > > > Therefore, we can replace all these factory methods by a single one > > which > > > > returns a LeaderElectionService instance that’s going to be used by > all > > > > components. Of course, we could also stick to the old > > > > HighAvailabilityServices and return the same LeaderElectionService > > > instance > > > > through each of the four factory methods (similar to what’s done now > > with > > > > the MultipleComponentLeaderElectionService). > > > > > > > > A similar question appears for the corresponding > > LeaderRetrievalService: > > > We > > > > could create a single listener instead of having individual > > per-component > > > > listeners to reflect the current requirement of having a > per-JM-process > > > > leader election and align it with the LeaderElectionService approach > > (if > > > we > > > > decide on modifying the HA interface). > > > > > > > > I didn’t come up with a dedicated FLIP: HighAvailabilityServices are > > not > > > > considered a public interface. Still, I am aware it might affect > users > > > > (e.g. if they implemented their own HA services or if the project > > > monitors > > > > HA information in the HA backend outside of Flink). That’s why I > wanted > > > to > > > > start a discussion here. I’m happy to create a FLIP, if someone > thinks > > > it’s > > > > worth it. The work is going to be covered by FLINK-26522 [4] > > > > > > > > Pro’s (for changing the interface methods): > > > > > > > > - > > > > > > > > It reflects the requirements stated in FLINK-24038 [1] about > having > > a > > > > per-JM-process LeaderElection > > > > - > > > > > > > > It helps reducing the complexity of the JobManager > > > > > > > > Con’s: > > > > > > > > - > > > > > > > > We lose some flexibility in terms of per-component LeaderElection > > > > - > > > > > > > > Interface change might affect other projects that customize HA > > > services > > > > > > > > > > > > I’m in favor of reducing the amount of factory methods in > > > > HighAvailabilityServices considering that it’s not a public > interface. > > > I’m > > > > looking forward to your opinions. > > > > > > > > Matthias > > > > > > > > [1] https://issues.apache.org/jira/browse/FLINK-24038 > > > > > > > > [2] > > > > > > > > > > > > > > https://nightlies.apache.org/flink/flink-docs-release-1.15/docs/deployment/config/#high-availability-use-old-ha-services > > > > > > > > [3] https://issues.apache.org/jira/browse/FLINK-25806 > > > > > > > > [4] https://issues.apache.org/jira/browse/FLINK-26522 > > > > > > > > > > > > -- > > > > > > > > [image: Aiven] > > > > > > > > Matthias Pohl > > > > > > > > Software Engineer, Aiven > > > > > > > > matthias.p...@aiven.io <i...@aiven.io> > > > > > > > > aiven.io <https://www.aiven.io> | > > > > <https://www.facebook.com/aivencloud> < > > > > https://www.facebook.com/aivencloud/> > > > > <https://www.linkedin.com/company/aiven/> > > > > <https://www.linkedin.com/company/aiven> < > > > https://twitter.com/aiven_io> > > > > <https://twitter.com/aiven_io> > > > > > > > > Aiven Deutschland GmbH > > > > > > > > Immanuelkirchstraße 26, 10405 Berlin > > > > > > > > Geschäftsführer: Oskari Saarenmaa & Hannu Valtonen > > > > > > > > Amtsgericht Charlottenburg, HRB 209739 B > > > > > > > > > >
