Hi all,
Very thanks for the discussion and the valuable opinions! Currently there
are several ongoing issues and we would like to show what we are thinking
in the next few mails.
It seems that the biggest issue now is about the topology of the sinks.
Before deciding what the sink API would look like, I would like to first
summarize the different topologies we have mentioned so that we could sync
on the same page and gain more insights about this issue. There are four
types of topology I could see. Please correct me if I misunderstand what
you mean:
1.
Commit individual files. (StreamingFileSink)
1.
FileWriter -> FileCommitter
2.
Commit a directory (HiveSink)
1.
FileWriter -> FileCommitter -> GlobalCommitter
3.
Commit a bundle of files (Iceberg)
1.
DataFileWriter -> GlobalCommitter
4.
Commit a directory with merged files(Some user want to merge the files
in a directory before committing the directory to Hive meta store)
1.
FileWriter -> SingleFileCommit -> FileMergeWriter -> GlobalCommitter
It can be seen from the above that the topologies are different according
to different requirements. Not only that there may be other options for the
second and third categories. E.g
A alternative topology option for the IcebergSink might be : DataFileWriter
-> Agg -> GlobalCommitter. One pro of this method is that we can let Agg
take care of the cleanup instead of coupling the cleanup logic to the
committer.
In the long run I think we might provide the sink developer the ability to
build arbitrary topologies. Maybe Flink could only provide a basic commit
transformation and let the user build other parts of the topology. In the
1.12 we might first provide different patterns for these different
scenarios at first and I think these components could be reused in the
future.
Best,
Guowei
On Mon, Sep 14, 2020 at 11:19 PM Dawid Wysakowicz <dwysakow...@apache.org>
wrote:
> Hi all,
>
> > I would think that we only need flush() and the semantics are that it
> > prepares for a commit, so on a physical level it would be called from
> > "prepareSnapshotPreBarrier". Now that I'm thinking about it more I
> > think flush() should be renamed to something like "prepareCommit()".
>
> Generally speaking it is a good point that emitting the committables
> should happen before emitting the checkpoint barrier downstream.
> However, if I remember offline discussions well, the idea behind
> Writer#flush and Writer#snapshotState was to differentiate commit on
> checkpoint vs final checkpoint at the end of the job. Both of these
> methods could emit committables, but the flush should not leave any in
> progress state (e.g. in case of file sink in STREAM mode, in
> snapshotState it could leave some open files that would be committed in
> a subsequent cycle, however flush should close all files). The
> snapshotState as it is now can not be called in
> prepareSnapshotPreBarrier as it can store some state, which should
> happen in Operator#snapshotState as otherwise it would always be
> synchronous. Therefore I think we would need sth like:
>
> void prepareCommit(boolean flush, WriterOutput<CommT> output);
>
> ver 1:
>
> List<StateT> snapshotState();
>
> ver 2:
>
> void snapshotState(); // not sure if we need that method at all in option 2
>
> > The Committer is as described in the FLIP, it's basically a function
> > "void commit(Committable)". The GobalCommitter would be a function "void
> > commit(List<Committable>)". The former would be used by an S3 sink where
> > we can individually commit files to S3, a committable would be the list
> > of part uploads that will form the final file and the commit operation
> > creates the metadata in S3. The latter would be used by something like
> > Iceberg where the Committer needs a global view of all the commits to be
> > efficient and not overwhelm the system.
> >
> > I don't know yet if sinks would only implement on type of commit
> > function or potentially both at the same time, and maybe Commit can
> > return some CommitResult that gets shipped to the GlobalCommit function.
> I must admit it I did not get the need for Local/Normal + Global
> committer at first. The Iceberg example helped a lot. I think it makes a
> lot of sense.
>
> > For Iceberg, writers don't need any state. But the GlobalCommitter
> > needs to
> > checkpoint StateT. For the committer, CommT is "DataFile". Since a single
> > committer can collect thousands (or more) data files in one checkpoint
> > cycle, as an optimization we checkpoint a single "ManifestFile" (for the
> > collected thousands data files) as StateT. This allows us to absorb
> > extended commit outages without losing written/uploaded data files, as
> > operator state size is as small as one manifest file per checkpoint cycle
> > [2].
> > ------------------
> > StateT snapshotState(SnapshotContext context) throws Exception;
> >
> > That means we also need the restoreCommitter API in the Sink interface
> > ---------------
> > Committer<CommT, StateT> restoreCommitter(InitContext context, StateT
> > state);
> I think this might be a valid case. Not sure though if I would go with a
> "state" there. Having a state in a committer would imply we need a
> collect method as well. So far we needed a single method commit(...) and
> the bookkeeping of the committables could be handled by the framework. I
> think something like an optional combiner in the GlobalCommitter would
> be enough. What do you think?
>
> GlobalCommitter<CommT, GlobalCommT> {
>
> void commit(GlobalCommT globalCommittables);
>
> GlobalCommT combine(List<CommT> committables);
>
> }
>
> A different problem that I see here is how do we handle commit failures.
> Should the committables (both normal and global be included in the next
> cycle, shall we retry it, ...) I think it would be worth laying it out
> in the FLIP.
>
> @Aljoscha I think you can find the code Steven was referring in here:
>
> https://github.com/Netflix-Skunkworks/nfflink-connector-iceberg/blob/master/nfflink-connector-iceberg/src/main/java/com/netflix/spaas/nfflink/connector/iceberg/sink/IcebergCommitter.java
>
> Best,
>
> Dawid
>
> On 14/09/2020 15:19, Aljoscha Krettek wrote:
> > On 14.09.20 01:23, Steven Wu wrote:
> >> ## Writer interface
> >>
> >> For the Writer interface, should we add "*prepareSnapshot"* before the
> >> checkpoint barrier emitted downstream? IcebergWriter would need it. Or
> >> would the framework call "*flush*" before the barrier emitted
> >> downstream?
> >> that guarantee would achieve the same goal.
> >
> > I would think that we only need flush() and the semantics are that it
> > prepares for a commit, so on a physical level it would be called from
> > "prepareSnapshotPreBarrier". Now that I'm thinking about it more I
> > think flush() should be renamed to something like "prepareCommit()".
> >
> > @Guowei, what do you think about this?
> >
> >> In [1], we discussed the reason for Writer to emit (checkpointId, CommT)
> >> tuple to the committer. The committer needs checkpointId to separate out
> >> data files for different checkpoints if concurrent checkpoints are
> >> enabled.
> >
> > When can this happen? Even with concurrent checkpoints the snapshot
> > barriers would still cleanly segregate the input stream of an operator
> > into tranches that should manifest in only one checkpoint. With
> > concurrent checkpoints, all that can happen is that we start a
> > checkpoint before a last one is confirmed completed.
> >
> > Unless there is some weirdness in the sources and some sources start
> > chk1 first and some other ones start chk2 first?
> >
> > @Piotrek, do you think this is a problem?
> >
> >> For the Committer interface, I am wondering if we should split the
> >> single
> >> commit method into separate "*collect"* and "*commit"* methods? This
> >> way,
> >> it can handle both single and multiple CommT objects.
> >
> > I think we can't do this. If the sink only needs a regular Commiter,
> > we can perform the commits in parallel, possibly on different
> > machines. Only when the sink needs a GlobalCommitter would we need to
> > ship all commits to a single process and perform the commit there. If
> > both methods were unified in one interface we couldn't make the
> > decision of were to commit in the framework code.
> >
> >> For Iceberg, writers don't need any state. But the GlobalCommitter
> >> needs to
> >> checkpoint StateT. For the committer, CommT is "DataFile". Since a
> >> single
> >> committer can collect thousands (or more) data files in one checkpoint
> >> cycle, as an optimization we checkpoint a single "ManifestFile" (for the
> >> collected thousands data files) as StateT. This allows us to absorb
> >> extended commit outages without losing written/uploaded data files, as
> >> operator state size is as small as one manifest file per checkpoint
> >> cycle
> >
> > You could have a point here. Is the code for this available in
> > open-source? I was checking out
> >
> https://github.com/apache/iceberg/blob/master/flink/src/main/java/org/apache/iceberg/flink/sink/IcebergFilesCommitter.java
> > and didn't find the ManifestFile optimization there.
> >
> > Best,
> > Aljoscha
> >
>
>
