Hi, I'm not entirely sure if I fully understand the raised concerns here. So let me maybe step back in the discussion a bit and address the original points from Roman.
> 2. Instead of forcing re-upload, can we "inverse control" in no-claim mode? I second the concerns from Dawid. This is effectively what we have right now, but with an extra (Async?) API call. It's not conceptually simple, it's hard to explain to the users, it might take actually forever to release the artefacts. Furthermore I don't think the implementation would be trivial. On the other hand the current proposal of having (a) `--claim` and (b) `--no-claim` mode are conceptually very simple. (a) being perfectly efficient, without any overheads. If you have concerns that (b) will cause some overheads, slower first checkpoint etc, keep in mind that the user can always pick option (a). Starting a new job from an existing savepoint/externalised checkpoint in general shouldn't be time critical, so users can always even manually copy the files and still use option (a), or just be fine accepting the price of a slower first checkpoint. For other use cases - restarting the same job after a downtime - (b) sounds to me to be an acceptable option. I would also like to point out that the "force full snapshot"/"do not use previous artefacts" option we will need either way for the incremental intermediate savepoints (subject of a next FLIP). From this perspective, we are getting the "--no-claim" option basically for free. > 3. Alternatively, re-upload not necessarily on 1st checkpoint, but after a configured number of checkpoints? I don't see a reason why we couldn't provide an option like that at some point in the future. However as it's more complicated to reason about, more complicated to implement and I'm not entirely sure how much actually needed given the (a) `--claim` mode, I think we can wait for feedback from the users before actually implementing it. > 6. Enforcing re-upload by a single task and skew > If we use some greedy logic like subtask 0 always re-uploads then it > might be overloaded. > So we'll have to obtain a full list of subtasks first (then probably > choose randomly or round-robin). > However, that requires rebuilding Task snapshot, which is doable but > not trivial (which I think supports "reverse API option"). What do you mean by "rebuilding Task snapshot"? During some early discussions about this point, I've hoped that a state backend like changelog could embed into the state handle information which operator should actually be responsible for duplicating such shared states. However now that I'm thinking about it, indeed there might be an issue if we combine the fact that state handles can be shared across multiple different operators and with a job modification, like dropping an operator. In that case it looks like we would need some extra logic during recovery, that would have an overview of the whole job to make a decision which particular parallel instance of an operator should be responsible for duplicating the underlying file? Best, Piotrek wt., 23 lis 2021 o 12:28 Dawid Wysakowicz <dwysakow...@apache.org> napisał(a): > Some user has usage to restore different jobs based on a same externalized > checkpoint. I think this usage would break after introducing this FLIP, and > we must tell users explicitly if choose to make Flink manage the > checkpoints by default. > > Could you elaborate what do you mean? The proposal is to use the > "no-claim" mode by default which should let users start as many jobs as > they wish from the same externalized checkpoints and it should not cause > them any harm. Each job effectively will create effectively it's own > private "copy" of the initial checkpoint along with the 1st taken > checkpoint. > > If the 1st full checkpoint did not complete in the end, the next > checkpoints have to try to reupload all artifacts again. I think this > problem could be mitigated if task knows some files have been uploaded > before. > > I don't know how we could achieve that easily. Besides, we have the same > situation for all, even regular checkpoints don't we? Do we check if e.g. > diff files has been successfully uploaded in a previous aborted checkpoint? > I am not saying it's a wrong suggestion, just that I feel it is orthogonal > and I can't see a straightforward way to implement it. > > Best, > > Dawid > On 23/11/2021 07:52, Yun Tang wrote: > > Hi, > > For the likelihood of never deleting some SST files by RocksDB. > Unfortunately, it could happen as current level compaction strategy in > RocksDB is triggered by upper input level size reached to the threshold and > the compaction priority cannot guarantee all files would be choosed during > several round compactions. > > Actually, I am a bit in favor of this FLIP to manage checkpoints within Flink > as we have heared from many users that they cannot delete older checkpoints > after several rounds of re-launching Flink jobs. Current Flink would not > delete older checkpoints automatically when restoring from older retained > checkpoint, which makes the base checkpoint directory becomes larger and > larger. However, if they decide to delete the older checkpoint directory of > other job-ids, they might not be able to recover from the last completed > checkpoint as it might depend on some artifacts in older checkpoint directory. > > And I think reuploading would indeed increase the 1st checkpoint duration > after restoring. For aliyun oss, the developer said that copping files > (larger than 32MB) from one location to another within same bucket on DFS > could cause hundreds millseconds. However, from my experiences, copying on > HDFS might not be so quick. Maybe some numbers here could be better. > > I have two questions here: > 1. If the 1st full checkpoint did not complete in the end, the next > checkpoints have to try to reupload all artifacts again. I think this problem > could be mitigated if task knows some files have been uploaded before. > 2. Some user has usage to restore different jobs based on a same externalized > checkpoint. I think this usage would break after introducing this FLIP, and > we must tell users explicitly if choose to make Flink manage the checkpoints > by default. > > Best > Yun Tang > > > On 2021/11/22 19:49:11 Dawid Wysakowicz wrote: > > There is one more fundamental issue with either of your two > proposals that've just came to my mind. > What happens if you have externalized checkpoints and the job fails > before the initial checkpoint can be safely removed? > > You start the job from the latest created checkpoint and wait for it > to be allowed for deletion. Then you can delete it, and all previous > checkpoints (or am I missing something?) > > > Let me clarify it with an example. You start with chk-42, Flink takes > e.g. three checkpoints chk-43, chk-44, chk-45 all still reference chk-42 > files. After that it fails. We have externalized checkpoints enabled, > therefore we have retained all checkpoints. Users starts a new program > from let's say chk-45. At this point your proposal does not give the > user any help in regards when chk-42 can be safely removed. (This is > also how Flink works right now). > > To make it even harder you can arbitrarily complicate it, 1) start a job > from chk-44, 2) start a job from a chk-47 which depends on chk-45, 3) > never start a job from chk-44, it is not claimed by any job, thus it is > never deleted, users must remember themselves that chk-44 originated > from chk-42 etc.) User would be forced to build a lineage system for > checkpoints to track which checkpoints depend on each other. > > I mean that the design described by FLIP implies the following (PCIIW): > 1. treat SST files from the initial checkpoint specially: re-upload or > send placeholder - depending on those attributes in state handle > 2. (SST files from newer checkpoints are re-uploaded depending on > confirmation currently; so yes there is tracking, but it's different) > 3. SharedStateRegistry must allow replacing state under the existing > key; otherwise, if a new key is used then other parallel subtasks > should learn somehow this key and use it; However, allowing > replacement must be limited to this scenario, otherwise it can lead to > previous checkpoint corruption in normal cases > > I might not understand your points, but I don't think FLIP implies any > of this. The FLIP suggests to send along with the CheckpointBarrier a > flag "force full checkpoint". Then the state backend should respect it > and should not use any of the previous shared handles. Now let me > explain how that would work for RocksDB incremental checkpoints. > > 1. Simplest approach: upload all local RocksDB files. This works > exactly the same as the first incremental checkpoint for a fresh start. > 2. Improvement on 1) we already do know which files were uploaded for > the initial checkpoint. Therefore instead of uploading the local > files that are same with files uploaded for the initial checkpoint > we call duplicate for those files and upload just the diff. > > It does not require any changes to the SharedStateRegistry nor to state > handles, at least for RocksDB. > > Best, > > Dawid > > > On 22/11/2021 19:33, Roman Khachatryan wrote: > > If you assume the 1st checkpoint needs to be "full" you know you are not > allowed to use any shared files. > It's true you should know about the shared files of the previous checkpoint, > but e.g. RocksDB already tracks that. > > I mean that the design described by FLIP implies the following (PCIIW): > 1. treat SST files from the initial checkpoint specially: re-upload or > send placeholder - depending on those attributes in state handle > 2. (SST files from newer checkpoints are re-uploaded depending on > confirmation currently; so yes there is tracking, but it's different) > 3. SharedStateRegistry must allow replacing state under the existing > key; otherwise, if a new key is used then other parallel subtasks > should learn somehow this key and use it; However, allowing > replacement must be limited to this scenario, otherwise it can lead to > previous checkpoint corruption in normal cases > > Forcing a full checkpoint after completing N checkpoints instead of > immediately would only require enabling (1) after N checkpoints. > And with the "poll API until checkpoint released" approach, those > changes aren't necessary. > > > There is one more fundamental issue with either of your two proposals that've > just came to my mind. > What happens if you have externalized checkpoints and the job fails before > the initial checkpoint can be safely removed? > > You start the job from the latest created checkpoint and wait for it > to be allowed for deletion. Then you can delete it, and all previous > checkpoints (or am I missing something?) > > > With tracking the shared files on JM you can not say if you can clear the > files after couple of checkpoints or 10s, 100s or 1000s, > which translates into minutes/hours/days/weeks of processing. > > This doesn't necessarily translate into higher cost (because of saved > RPC etc., as I mentioned above). > However, I do agree that an infinite or arbitrary high delay is unacceptable. > > The added complexity above doesn't seem negligible to me (especially > in SharedStateHandle); and should therefore be weighted against those > operational disadvantages (given that the number of checkpoints to > wait is bounded in practice). > > Regards, > Roman > > > > > On Mon, Nov 22, 2021 at 5:05 PM Dawid Wysakowicz <dwysakow...@apache.org> > <dwysakow...@apache.org> wrote: > > There is one more fundamental issue with either of your two proposals > that've just came to my mind. What happens if you have externalized > checkpoints and the job fails before the initial checkpoint can be > safely removed? You have a situation where you have a retained > checkpoint that was built on top of the original one. Basically ending > in a situation we have right now that you never know when it is safe to > delete a retained checkpoint. > > BTW, the intention for the "claim" mode was to support cases when users > are concerned with the performance of the first checkpoint. In those > cases they can claim the checkpoint on don't pay the additional cost of > the first checkpoint. > > Best, > > Dawid > > On 22/11/2021 14:09, Roman Khachatryan wrote: > > Thanks Dawid, > > Regarding clarity, > I think that all proposals require waiting for some event: re-upload / > checkpoint completion / api response. > But with the current one, there is an assumption: "initial checkpoint > can be deleted once a new one completes" (instead of just "initial > checkpoint can be deleted once the API says it can be deleted"). > So I think it's actually more clear to offer this explicit API and rely on it. > > Regarding delaying the deletion, > I agree that it can delay deletion, but how important is it? > Checkpoints are usually stored on relatively cheap storage like S3, so > some delay shouldn't be an issue (especially taking rounding into > account); it can even be cheaper or comparable to paying for > re-upload/duplicate calls. > > Infinite delay can be an issue though, I agree. > Maybe @Yun can clarify the likelihood of never deleting some SST files > by RocksDB? > For the changelog backend, old files won't be used once > materialization succeeds. > > Yes, my concern is checkpointing time, but also added complexity: > > It would be a bit invasive though, as we would have to somehow keep track > which files should not be reused on TMs. > > I think we need this anyway if we choose to re-upload files once the > job is running. > The new checkpoint must be formed by re-uploaded old artifacts AND > uploaded new artifacts. > > > Regards, > Roman > > > On Mon, Nov 22, 2021 at 12:42 PM Dawid Wysakowicz<dwysakow...@apache.org> > <dwysakow...@apache.org> wrote: > > @Yun > > I think it is a good comment with I agree in principal. However, we use > --fromSavepoint (cli), savepointPath (REST API), SavepointRestoreSettings for > both restoring from a savepoint and an externalized checkpoint already. I > wanted to voice that concern. Nevertheless I am fine with changing it to > execution.restore-mode, if there are no other comments on that matter, I will > change it. > > @Roman: > > Re 1. Correct, stop-with-savepoint should commit side-effects. Will add that > to the doc. > > Re.2 What I don't like about this counter proposal is that it still has no > clearly defined point in time when it is safe to delete the original > checkpoint. Users would have a hard time reasoning about it and debugging. > Even worse, I think worst case it might never happen that all the original > files are no longer in use (I am not too familiar with RocksDB compaction, > but what happens if there are key ranges that are never accessed again?) I > agree it is unlikely, but possible, isn't it? Definitely it can take a > significant time and many checkpoints to do so. > > Re. 3 I believe where you are coming from is that you'd like to keep the > checkpointing time minimal and reuploading files may increase it. The > proposal so far builds on the assumption we could in most cases use a cheap > duplicate API instead of re-upload. I could see this as a follow-up if it > becomes a bottleneck. It would be a bit invasive though, as we would have to > somehow keep track which files should not be reused on TMs. > > Re. 2 & 3 Neither of the counter proposals work well for taking incremental > savepoints. We were thinking of building incremental savepoints on the same > concept. I think delaying the completion of an independent savepoint to a > closer undefined future is not a nice property of savepoints. > > Re 4. Good point. We should make sure the first completed checkpoint has the > independent/full checkpoint property rather than just the first triggered. > > Re. 5 & 6 I need a bit more time to look into it. > > Best, > > Dawid > > On 22/11/2021 11:40, Roman Khachatryan wrote: > > Hi, > > Thanks for the proposal Dawid, I have some questions and remarks: > > 1. How will stop-with-savepoint be handled? > Shouldn't side effects be enforced in this case? (i.e. send > notifyCheckpointComplete) > > 2. Instead of forcing re-upload, can we "inverse control" in no-claim mode? > Anyways, any external tool will have to poll Flink API waiting for the > next (full) checkpoint, before deleting the retained checkpoint, > right? > Instead, we can provide an API which tells whether the 1st checkpoint > is still in use (and not force re-upload it). > > Under the hood, it can work like this: > - for the checkpoint Flink recovers from, remember all shared state > handles it is adding > - when unregistering shared state handles, remove them from the set above > - when the set becomes empty the 1st checkpoint can be deleted externally > > Besides not requiring re-upload, it seems much simpler and less invasive. > On the downside, state deletion can be delayed; but I think this is a > reasonable trade-off. > > 3. Alternatively, re-upload not necessarily on 1st checkpoint, but > after a configured number of checkpoints? > There is a high chance that after some more checkpoints, initial state > will not be used (because of compaction), > so backends won't have to re-upload anything (or small part). > > 4. Re-uploaded artifacts must not be deleted on checkpoin abortion > This should be addressed in https://issues.apache.org/jira/browse/FLINK-24611. > If not, I think the FLIP should consider this case. > > 5. Enforcing re-upload by a single task and Changelog state backend > With Changelog state backend, a file can be shared by multiple operators. > Therefore, getIntersection() is irrelevant here, because operators > might not be sharing any key groups. > (so we'll have to analyze "raw" file usage I think). > > 6. Enforcing re-upload by a single task and skew > If we use some greedy logic like subtask 0 always re-uploads then it > might be overloaded. > So we'll have to obtain a full list of subtasks first (then probably > choose randomly or round-robin). > However, that requires rebuilding Task snapshot, which is doable but > not trivial (which I think supports "reverse API option"). > > 7. I think it would be helpful to list file systems / object stores > that support "fast" copy (ideally with latency numbers). > > Regards, > Roman > > On Mon, Nov 22, 2021 at 9:24 AM Yun Gao <yungao...@aliyun.com.invalid> > <yungao...@aliyun.com.invalid> wrote: > > Hi, > > Very thanks Dawid for proposing the FLIP to clarify the ownership for the > states. +1 for the overall changes since it makes the behavior clear and > provide users a determined method to finally cleanup savepoints / retained > checkpoints. > > Regarding the changes to the public interface, it seems currently the changes > are all bound > to the savepoint, but from the FLIP it seems perhaps we might also need to > support the claim declaration > for retained checkpoints like in the cli side[1] ? If so, then might it be > better to change the option name > from `execution.savepoint.restore-mode` to something like > `execution.restore-mode`? > > Best, > Yun > > > [1] > https://nightlies.apache.org/flink/flink-docs-master/docs/ops/state/checkpoints/#resuming-from-a-retained-checkpoint > > > ------------------------------------------------------------------ > From:Konstantin Knauf <kna...@apache.org> <kna...@apache.org> > Send Time:2021 Nov. 19 (Fri.) 16:00 > To:dev <dev@flink.apache.org> <dev@flink.apache.org> > Subject:Re: [DISCUSS] FLIP-193: Snapshots ownership > > Hi Dawid, > > Thanks for working on this FLIP. Clarifying the differences and > guarantees around savepoints and checkpoints will make it easier and safer > for users and downstream projects and platforms to work with them. > > +1 to the changing the current (undefined) behavior when recovering from > retained checkpoints. Users can now choose between claiming and not > claiming, which I think will make the current mixed behavior obsolete. > > Cheers, > > Konstantin > > On Fri, Nov 19, 2021 at 8:19 AM Dawid Wysakowicz <dwysakow...@apache.org> > <dwysakow...@apache.org> > wrote: > > Hi devs, > > I'd like to bring up for a discussion a proposal to clean up ownership > of snapshots, both checkpoints and savepoints. > > The goal here is to make it clear who is responsible for deleting > checkpoints/savepoints files and when can that be done in a safe manner. > > Looking forward for your feedback! > > Best, > > Dawid > > [1] https://cwiki.apache.org/confluence/x/bIyqCw > > > > -- > > Konstantin Knauf > https://twitter.com/snntrable > https://github.com/knaufk > >
