Hi Yu, I haven't thought too much about the compatibility design before. By the nature of the problem, it's impossible to make V3 compatible with V2, what we can do is to somewhat better inform users when switching the hasher, but I don't have any good idea so far. Do you have any suggestions on this?
Best, Zhanghao Chen ________________________________ From: Yu Chen <[email protected]> Sent: Thursday, January 11, 2024 13:52 To: [email protected] <[email protected]> Cc: Piotr Nowojski <[email protected]>; [email protected] <[email protected]> Subject: Re: [DISCUSS] FLIP 411: Chaining-agnostic Operator ID generation for improved state compatibility on parallelism change Hi Zhanghao, Thanks for driving this, that’s really painful for us when we need to switch config `pipeline.operator-chaining`. But I have a Concern, according to FLIP description, modifying `isChainable` related code in `StreamGraphHasherV2` will cause the generated operator id to be changed, which will result in the user unable to recover from the old state (old and new Operator IDs can't be mapped). Therefore switching Hasher strategy (V2->V3 or V3->V2) will lead to an incompatibility, is there any relevant compatibility design considered? Best, Yu Chen 2024年1月10日 10:25，Zhanghao Chen <[email protected]> 写道： Hi David, Thanks for the comments. AFAIK, unaligned checkpoints are disabled for pointwise connections according to [1], let's wait Piotr for confirmation. The issue itself is not directly related to this proposal as well. If a user manually specifies UIDs for each of the chained operators and has unaligned checkpoints enabled, we will encounter the same issue if they decide to break the chain on a later restart and try to recover from a retained cp. [1] https://nightlies.apache.org/flink/flink-docs-release-1.18/docs/ops/state/checkpointing_under_backpressure/ Best, Zhanghao Chen ________________________________ From: David Morávek <[email protected]> Sent: Wednesday, January 10, 2024 6:26 To: [email protected] <[email protected]>; Piotr Nowojski <[email protected]> Subject: Re: [DISCUSS] FLIP 411: Chaining-agnostic Operator ID generation for improved state compatibility on parallelism change Hi Zhanghao, Thanks for the FLIP. What you're proposing makes a lot of sense +1 Have you thought about how this works with unaligned checkpoints in case you go from unchained to chained? I think it should be fine because this scenario should only apply to forward/rebalance scenarios where we, as far as I recall, force alignment anyway, so there should be no exchanges to snapshot. It might just work, but something to double-check. Maybe @Piotr Nowojski <[email protected]> could confirm it. Best, D. On Wed, Jan 3, 2024 at 7:10 AM Zhanghao Chen <[email protected]> wrote: Dear Flink devs, I'd like to start a discussion on FLIP 411: Chaining-agnostic Operator ID generation for improved state compatibility on parallelism change [1]. Currently, when user does not explicitly set operator UIDs, the chaining behavior will still affect state compatibility, as the generation of the Operator ID is dependent on its chained output nodes. For example, a simple source->sink DAG with source and sink chained together is state incompatible with an otherwise identical DAG with source and sink unchained (either because the parallelisms of the two ops are changed to be unequal or chaining is disabled). This greatly limits the flexibility to perform chain-breaking/building for performance tuning. The dependency on chained output nodes for Operator ID generation can be traced back to Flink 1.2. It is unclear at this point on why chained output nodes are involved in the algorithm, but the following history background might be related: prior to Flink 1.3, Flink runtime takes the snapshots by the operator ID of the first vertex in a chain, so it somewhat makes sense to include chained output nodes into the algorithm as chain-breaking/building is expected to break state-compatibility anyway. Given that operator-level state recovery within a chain has long been supported since Flink 1.3, I propose to introduce StreamGraphHasherV3 that is agnostic of the chaining behavior of operators, so that users are free to tune the parallelism of individual operators without worrying about state incompatibility. We can make the V3 hasher an optional choice in Flink 1.19, and make it the default hasher in 2.0 for backwards compatibility. Looking forward to your suggestions on it, thanks~ [1] https://cwiki.apache.org/confluence/display/FLINK/FLIP-411%3A+Chaining-agnostic+Operator+ID+generation+for+improved+state+compatibility+on+parallelism+change Best, Zhanghao Chen
