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
