Thanks for the clarification Yuan and Gen, I agree that the checkpointing of the sources needs to support the rescaling case, otherwise it does not work. Is there currently a source implementation where this wouldn't work? For Kafka it should work because we store the offset per assigned partition. For Kinesis it is probably the same. For the Filesource we store the set of unread input splits in the source coordinator and the state of the assigned splits in the sources. This should probably also work since new splits are only handed out to running tasks.
Cheers, Till On Mon, Feb 7, 2022 at 10:29 AM Yuan Mei <yuanmei.w...@gmail.com> wrote: > Hey Till, > > > Why rescaling is a problem for pipelined regions/independent execution > subgraphs: > > Take a simplified example : > job graph : source (2 instances) -> sink (2 instances) > execution graph: > source (1/2) -> sink (1/2) [pieplined region 1] > source (2/2) -> sink (2/2) [pieplined region 2] > > Let's assume checkpoints are still triggered globally, meaning different > pipelined regions share the global checkpoint id (PR1 CP1 matches with PR2 > CP1). > > Now let's assume PR1 completes CP10 and PR2 completes CP8. > > Let's say we want to rescale to parallelism 3 due to increased input. > > - Notice that we can not simply rescale based on the latest completed > checkpoint (CP8), because PR1 has already had data (CP8 -> CP10) output > externally. > - Can we take CP10 from PR1 and CP8 from PR2? I think it depends on how the > source's offset redistribution is implemented. > The answer is yes if we treat each input partition as independent from > each other, *but I am not sure whether we can make that assumption*. > > If not, the rescaling cannot happen until PR1 and PR2 are aligned with CPs. > > Best > -Yuan > > > > > > > > On Mon, Feb 7, 2022 at 4:17 PM Till Rohrmann <trohrm...@apache.org> wrote: > > > Hi everyone, > > > > Yuan and Gen could you elaborate why rescaling is a problem if we say > that > > separate pipelined regions can take checkpoints independently? > > Conceptually, I somehow think that a pipelined region that is failed and > > cannot create a new checkpoint is more or less the same as a pipelined > > region that didn't get new input or a very very slow pipelined region > which > > couldn't read new records since the last checkpoint (assuming that the > > checkpoint coordinator can create a global checkpoint by combining > > individual checkpoints (e.g. taking the last completed checkpoint from > each > > pipelined region)). If this comparison is correct, then this would mean > > that we have rescaling problems under the latter two cases. > > > > Cheers, > > Till > > > > On Mon, Feb 7, 2022 at 8:55 AM Gen Luo <luogen...@gmail.com> wrote: > > > > > Hi Gyula, > > > > > > Thanks for sharing the idea. As Yuan mentioned, I think we can discuss > > this > > > within two scopes. One is the job subgraph, the other is the execution > > > subgraph, which I suppose is the same as PipelineRegion. > > > > > > An idea is to individually checkpoint the PipelineRegions, for the > > > recovering in a single run. > > > > > > Flink has now supported PipelineRegion based failover, with a subset > of a > > > global checkpoint snapshot. The checkpoint barriers are spread within a > > > PipelineRegion, so the checkpointing of individual PipelineRegions is > > > actually independent. Since in a single run of a job, the > PipelineRegions > > > are fixed, we can individually checkpoint separated PipelineRegions, > > > despite what status the other PipelineRegions are, and use a snapshot > of > > a > > > failing region to recover, instead of the subset of a global snapshot. > > This > > > can support separated job subgraphs as well, since they will also be > > > separated into different PipelineRegions. I think this can fulfill your > > > needs. > > > > > > In fact the individual snapshots of all PipelineRegions can form a > global > > > snapshot, and the alignment of snapshots of individual regions is not > > > necessary. But rescaling this global snapshot can be potentially > > complex. I > > > think it's better to use the individual snapshots in a single run, and > > take > > > a global checkpoint/savepoint before restarting the job, rescaling it > or > > > not. > > > > > > A major issue of this plan is that it breaks the checkpoint mechanism > of > > > Flink. As far as I know, even in the approximate recovery, the snapshot > > > used to recover a single task is still a part of a global snapshot. To > > > implement the individual checkpointing of PipelineRegions, there may > need > > > to be a checkpoint coordinator for each PipelineRegion, and a new > global > > > checkpoint coordinator. When the scale goes up, there can be many > > > individual regions, which can be a big burden to the job manager. The > > > meaning of the checkpoint id will also be changed, which can affect > many > > > aspects. There can be lots of work and risks, and the risks still exist > > if > > > we only individually checkpoint separated job subgraphs, since the > > > mechanism is still broken. If that is what you need, maybe separating > > them > > > into different jobs is an easier and better choice, as Caizhi and Yuan > > > mentioned. > > > > > > On Mon, Feb 7, 2022 at 11:39 AM Yuan Mei <yuanmei.w...@gmail.com> > wrote: > > > > > > > Hey Gyula, > > > > > > > > That's a very interesting idea. The discussion about the `Individual` > > vs > > > > `Global` checkpoint was raised before, but the main concern was from > > two > > > > aspects: > > > > > > > > - Non-deterministic replaying may lead to an inconsistent view of > > > > checkpoint > > > > - It is not easy to form a clear cut of past and future and hence no > > > clear > > > > cut of where the start point of the source should begin to replay > from. > > > > > > > > Starting from independent subgraphs as you proposed may be a good > > > starting > > > > point. However, when we talk about subgraph, do we mention it as a > job > > > > subgraph (each vertex is one or more operators) or execution subgraph > > > (each > > > > vertex is a task instance)? > > > > > > > > If it is a job subgraph, then indeed, why not separate it into > multiple > > > > jobs as Caizhi mentioned. > > > > If it is an execution subgraph, then it is difficult to handle > > rescaling > > > > due to inconsistent views of checkpoints between tasks of the same > > > > operator. > > > > > > > > `Individual/Subgraph Checkpointing` is definitely an interesting > > > direction > > > > to think of, and I'd love to hear more from you! > > > > > > > > Best, > > > > > > > > Yuan > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Feb 7, 2022 at 10:16 AM Caizhi Weng <tsreape...@gmail.com> > > > wrote: > > > > > > > > > Hi Gyula! > > > > > > > > > > Thanks for raising this discussion. I agree that this will be an > > > > > interesting feature but I actually have some doubts about the > > > motivation > > > > > and use case. If there are multiple individual subgraphs in the > same > > > job, > > > > > why not just distribute them to multiple jobs so that each job > > contains > > > > > only one individual graph and can now fail without disturbing the > > > others? > > > > > > > > > > > > > > > Gyula Fóra <gyf...@apache.org> 于2022年2月7日周一 05:22写道： > > > > > > > > > > > Hi all! > > > > > > > > > > > > At the moment checkpointing only works for healthy jobs with all > > > > running > > > > > > (or some finished) tasks. This sounds reasonable in most cases > but > > > > there > > > > > > are a few applications where it would make sense to checkpoint > > > failing > > > > > jobs > > > > > > as well. > > > > > > > > > > > > Due to how the checkpointing mechanism works, subgraphs that > have a > > > > > failing > > > > > > task cannot be checkpointed without violating the exactly-once > > > > semantics. > > > > > > However if the job has multiple independent subgraphs (that are > not > > > > > > connected to each other), even if one subgraph is failing, the > > other > > > > > > completely running one could be checkpointed. In these cases the > > > tasks > > > > of > > > > > > the failing subgraph could simply inherit the last successful > > > > checkpoint > > > > > > metadata (before they started failing). This logic would produce > a > > > > > > consistent checkpoint. > > > > > > > > > > > > The job as a whole could now make stateful progress even if some > > > > > subgraphs > > > > > > are constantly failing. This can be very valuable if for some > > reason > > > > the > > > > > > job has a larger number of independent subgraphs that are > expected > > to > > > > > fail > > > > > > every once in a while, or if some subgraphs can have longer > > downtimes > > > > > that > > > > > > would now cause the whole job to stall. > > > > > > > > > > > > What do you think? > > > > > > > > > > > > Cheers, > > > > > > Gyula > > > > > > > > > > > > > > > > > > > > >
