Thanks this clarifies a lot. For writer, I think it's great if you can utilize existing FileIO.Sink implementations even if you have to reimplement some of the logic (for example compression, temp file handling) that is already implemented in Beam FileIO/WriteFiles transforms in your SMB sink transform.
For reader, you are right that there's no FileIO.Read. What we have are various implementations of FileBasedSource/FileBasedReader classes that are currently intentionally hidden since Beam IO transforms are expected to be the intended public interface for users. If you can expose and re-use these classes with slight modifications (keeping backwards compatibility) I'm OK with it. Otherwise you'll have to write your own reader implementations. In general, seems like SMB has very strong requirements related to sharding/hot-key management that are not easily achievable by implementing SMB source/sink as a composite transform that utilizes existing source/sink transforms. This forces you to implement this logic in your own DoFns and existing Beam primitives are not easily re-usable in this context. Thanks, Cham On Tue, Jul 16, 2019 at 8:26 AM Neville Li <neville....@gmail.com> wrote: > A little clarification of the IO requirement and my understanding of the > current state of IO. > > tl;dr: not sure if there're reusable bits for the reader. It's possible to > reuse some for the writer but with heavy refactoring. > > *Reader* > > - For each bucket (containing the same key partition, sorted) across > multiple input data sets, we stream records from bucket files and merge > sort. > - We open the files in a DoFn, and emit KV<K, CoGbkResult> where the > CGBKR encapsulates Iterable<V> from each input. > - Basically we need a simple API like ResourceId -> Iterator<T>, i.e. > sequential read, no block/offset/split requirement. > - FileBasedSource.FileBasedReader seems the closest fit but they're > nested & decoupled. > - There's no FileIO.Read, only a ReadMatches[1], which can be used > with ReadAllViaFileBasedSource<T>. But that's not the granularity we need, > since we lose ordering of the input records, and can't merge 2+ sources. > > *Writer* > > - We get a `PCollection<BucketShardId, Iterable<T>>` after bucket and > and sort, where Iterable<T> is the records sorted by key and BucketShardId > is used to produce filename, e.g. bucket-00001-shard-00002.avro. > - We write each Iterable<T> to a temp file and move to final > destination when done. Both should ideally reuse existing code. > - Looks like FileIO.Sink (and impls in AvroIO, TextIO, TFRecordIO) > supports record writing into a WritableByteChannel, but some logic like > compression is handled in FileIO through ViaFileBasedSink which extends > FileBasedSink. > - FileIO uses WriteFiles[3] to shard and write of PCollection<T>. > Again we lose ordering of the output records or custom file naming scheme. > However, WriteShardsIntoTempFilesFn[4] and FinalizeTempFileBundles[5] in > WriteFiles seem closest to our need but would have to be split out and > generalized. > > *Note on reader block/offset/split requirement* > > - Because of the merge sort, we can't split or offset seek a bucket > file. Because without persisting the offset index of a key group somewhere, > we can't efficiently skip to a key group without exhausting the previous > ones. Furthermore we need to merge sort and align keys from multiple > sources, which may not have the same key distribution. It might be possible > to binary search for matching keys but that's extra complication. IMO the > reader work distribution is better solved by better bucket/shard strategy > in upstream writer. > > *References* > > 1. ReadMatches extends PTransform<PCollection<MatchResult.Metadata>, > PCollection<ReadableFile>> > 2. ReadAllViaFileBasedSource<T> extends > PTransform<PCollection<ReadableFile>, PCollection<T>> > 3. WriteFiles<UserT, DestinationT, OutputT> extends > PTransform<PCollection<UserT>, WriteFilesResult<DestinationT>> > 4. WriteShardsIntoTempFilesFn extends DoFn<KV<ShardedKey<Integer>, > Iterable<UserT>>, FileResult<DestinationT>> > 5. FinalizeTempFileBundles extends PTransform< > PCollection<List<FileResult<DestinationT>>>, > WriteFilesResult<DestinationT>> > > > On Tue, Jul 16, 2019 at 5:15 AM Robert Bradshaw <rober...@google.com> > wrote: > >> On Mon, Jul 15, 2019 at 7:03 PM Eugene Kirpichov <kirpic...@google.com> >> wrote: >> > >> > Quick note: I didn't look through the document, but please do not build >> on either FileBasedSink or FileBasedReader. They are both remnants of the >> old, non-composable IO world; and in fact much of the composable IO work >> emerged from frustration with their limitations and recognizing that many >> other IOs were suffering from the same limitations. >> > Instead of FileBasedSink, build on FileIO.write; instead of >> FileBasedReader, build on FileIO.read. >> >> +1 >> >> I think the sink could be written atop FileIO.write, possibly using >> dynamic destinations. At the very least the FileSink interface, which >> handles the details of writing a single shard, would be an ideal way >> to parameterize an SMB sink. It seems that none of our existing IOs >> (publically?) expose FileSink implementations. >> >> FileIO.read is not flexible enough to do the merging. Eugene, is there >> a composable analogue to FileSink, for sources, i.e. something that >> can turn a file handle (possibly with offsets) into a set of records >> other than FileBasedReader? >> >> > On Mon, Jul 15, 2019 at 9:01 AM Gleb Kanterov <g...@spotify.com> wrote: >> >> >> >> I share the same concern with Robert regarding re-implementing parts >> of IO. At the same time, in the past, I worked on internal libraries that >> try to re-use code from existing IO, and it's hardly possible because it >> feels like it wasn't designed for re-use. There are a lot of classes that >> are nested (non-static) or non-public. I can understand why they were made >> non-public, it's a hard abstraction to design well and keep compatibility. >> As Neville mentioned, decoupling readers and writers would not only benefit >> for this proposal but for any other use-case that has to deal with >> low-level API such as FileSystem API, that is hardly possible today without >> copy-pasting, >> >> >> >> >> >> >> >> >> >> >> >> On Mon, Jul 15, 2019 at 5:05 PM Neville Li <neville....@gmail.com> >> wrote: >> >>> >> >>> Re: avoiding mirroring IO functionality, what about: >> >>> >> >>> - Decouple the nested FileBasedSink.Writer and >> FileBasedSource.FileBasedReader, make them top level and remove references >> to parent classes. >> >>> - Simplify the interfaces, while maintaining support for block/offset >> read & sequential write. >> >>> - As a bonus, the refactored IO classes can be used standalone in >> case when the user wants to perform custom IO in a DoFn, i.e. a >> PTransform<PCollection<URI>, PCollection<KV<URI, GenericRecord>>>. Today >> this requires a lot of copy-pasted Avro boilerplate. >> >>> - For compatibility, we can delegate to the new classes from the old >> ones and remove them in the next breaking release. >> >>> >> >>> Re: WriteFiles logic, I'm not sure about generalizing it, but what >> about splitting the part handling writing temp files into a new >> PTransform<PCollection<KV<ResourceId, Iterable<UserT>>>, >> PCollection<WriteFilesResult<DestinationT>>>? That splits the bucket-shard >> logic from actual file IO. >> >>> >> >>> On Mon, Jul 15, 2019 at 10:27 AM Robert Bradshaw <rober...@google.com> >> wrote: >> >>>> >> >>>> I agree that generalizing the existing FileIO may not be the right >> >>>> path forward, and I'd only make their innards public with great care. >> >>>> (Would this be used like like >> >>>> SmbSink(MyFileIO.sink(parameters).getWriter[Factory]())?) SMB is a >> bit >> >>>> unique that the source and sink are much more coupled than other >> >>>> sources and sinks (which happen to be completely independent, if >> >>>> complementary implementations, whereas SMB attempts to be a kind of >> >>>> pipe where one half is instanciated in each pipeline). >> >>>> >> >>>> In short, an SMB source/sink that is parameterized by an arbitrary, >> >>>> existing IO would be ideal (but possibly not feasible (per existing >> >>>> prioritizations)), or an SMB source/sink that works as a pair. What >> >>>> I'd like to avoid is a set of parallel SMB IO classes that >> (partially, >> >>>> and incompletely) mirror the existing IO ones (from an API >> >>>> perspective--how much implementation it makes sense to share is an >> >>>> orthogonal issue that I'm sure can be worked out.) >> >>>> >> >>>> On Mon, Jul 15, 2019 at 4:18 PM Neville Li <neville....@gmail.com> >> wrote: >> >>>> > >> >>>> > Hi Robert, >> >>>> > >> >>>> > I agree, it'd be nice to reuse FileIO logic of different file >> types. But given the current code structure of FileIO & scope of the >> change, I feel it's better left for future refactor PRs. >> >>>> > >> >>>> > Some thoughts: >> >>>> > - SMB file operation is simple single file sequential >> reads/writes, which already exists as Writer & FileBasedReader but are >> private inner classes, and have references to the parent Sink/Source >> instance. >> >>>> > - The readers also have extra offset/split logic but that can be >> worked around. >> >>>> > - It'll be nice to not duplicate temp->destination file logic but >> again WriteFiles is assuming a single integer shard key, so it'll take some >> refactoring to reuse it. >> >>>> > >> >>>> > All of these can be done in backwards compatible way. OTOH >> generalizing the existing components too much (esp. WriteFiles, which is >> already complex) might lead to two logic paths, one specialized for the SMB >> case. It might be easier to decouple some of them for better reuse. But >> again I feel it's a separate discussion. >> >>>> > >> >>>> > On Mon, Jul 15, 2019 at 9:45 AM Claire McGinty < >> claire.d.mcgi...@gmail.com> wrote: >> >>>> >> >> >>>> >> Thanks Robert! >> >>>> >> >> >>>> >> We'd definitely like to be able to re-use existing I/O >> components--for example the Writer<DestinationT, >> OutputT>/FileBasedReader<T> (since they operate on a >> WritableByteChannel/ReadableByteChannel, which is the level of granularity >> we need) but the Writers, at least, seem to be mostly private-access. Do >> you foresee them being made public at any point? >> >>>> >> >> >>>> >> - Claire >> >>>> >> >> >>>> >> On Mon, Jul 15, 2019 at 9:31 AM Robert Bradshaw < >> rober...@google.com> wrote: >> >>>> >>> >> >>>> >>> I left some comments on the doc. >> >>>> >>> >> >>>> >>> I think the general idea is sound, but one thing that worries me >> is >> >>>> >>> the introduction of a parallel set of IOs that mirrors the >> (existing) >> >>>> >>> FileIOs. I would suggest either (1) incorporate this >> functionality >> >>>> >>> into the generic FileIO infrastructure, or let it be >> parameterized by >> >>>> >>> arbitrary IO (which I'm not sure is possible, especially for the >> Read >> >>>> >>> side (and better would be the capability of supporting arbitrary >> >>>> >>> sources, aka an optional "as-sharded-source" operation that >> returns a >> >>>> >>> PTransform<..., KV<shard-id, Iterable<KV<K, V>>> where the >> iterable is >> >>>> >>> promised to be in key order)) or support a single SMB aka >> >>>> >>> "PreGrouping" source/sink pair that's aways used together (and >> whose >> >>>> >>> underlying format is not necessarily public). >> >>>> >>> >> >>>> >>> On Sat, Jul 13, 2019 at 3:19 PM Neville Li < >> neville....@gmail.com> wrote: >> >>>> >>> > >> >>>> >>> > 4 people have commented but mostly clarifying details and not >> much on the overall design. >> >>>> >>> > >> >>>> >>> > It'd be great to have thumbs up/down on the design, >> specifically metadata, bucket & shard strategy, etc., since that affects >> backwards compatibility of output files. >> >>>> >>> > Some breaking changes, e.g. dynamic # of shards, are out of >> scope for V1 unless someone feels strongly about it. The current scope >> should cover all our use cases and leave room for optimization. >> >>>> >>> > >> >>>> >>> > Once green lighted we can start adopting internally, ironing >> out rough edges while iterating on the PRs in parallel. >> >>>> >>> > >> >>>> >>> > Most of the implementation is self-contained in the >> extensions:smb module, except making a few core classes/methods public for >> reuse. So despite the amount of work it's still fairly low risk to the code >> base. There're some proposed optimization & refactoring involving core (see >> appendix) but IMO they're better left for followup PRs. >> >>>> >>> > >> >>>> >>> > On Fri, Jul 12, 2019 at 11:34 PM Kenneth Knowles < >> k...@apache.org> wrote: >> >>>> >>> >> >> >>>> >>> >> I've seen some discussion on the doc. I cannot tell whether >> the questions are resolved or what the status of review is. Would you mind >> looping this thread with a quick summary? This is such a major piece of >> work I don't want it to sit with everyone thinking they are waiting on >> someone else, or any such thing. (not saying this is happening, just >> pinging to be sure) >> >>>> >>> >> >> >>>> >>> >> Kenn >> >>>> >>> >> >> >>>> >>> >> On Mon, Jul 1, 2019 at 1:09 PM Neville Li < >> neville....@gmail.com> wrote: >> >>>> >>> >>> >> >>>> >>> >>> Updated the doc a bit with more future work (appendix). IMO >> most of them are non-breaking and better done in separate PRs later since >> some involve pretty big refactoring and are outside the scope of MVP. >> >>>> >>> >>> >> >>>> >>> >>> For now we'd really like to get feedback on some fundamental >> design decisions and find a way to move forward. >> >>>> >>> >>> >> >>>> >>> >>> On Thu, Jun 27, 2019 at 4:39 PM Neville Li < >> neville....@gmail.com> wrote: >> >>>> >>> >>>> >> >>>> >>> >>>> Thanks. I responded to comments in the doc. More inline. >> >>>> >>> >>>> >> >>>> >>> >>>> On Thu, Jun 27, 2019 at 2:44 PM Chamikara Jayalath < >> chamik...@google.com> wrote: >> >>>> >>> >>>>> >> >>>> >>> >>>>> Thanks added few comments. >> >>>> >>> >>>>> >> >>>> >>> >>>>> If I understood correctly, you basically assign elements >> with keys to different buckets which are written to unique files and merge >> files for the same key while reading ? >> >>>> >>> >>>>> >> >>>> >>> >>>>> Some of my concerns are. >> >>>> >>> >>>>> >> >>>> >>> >>>>> (1) Seems like you rely on an in-memory sorting of >> buckets. Will this end up limiting the size of a PCollection you can >> process ? >> >>>> >>> >>>> >> >>>> >>> >>>> The sorter transform we're using supports spilling and >> external sort. We can break up large key groups further by sharding, >> similar to fan out in some GBK transforms. >> >>>> >>> >>>> >> >>>> >>> >>>>> (2) Seems like you rely on Reshuffle.viaRandomKey() which >> is actually implemented using a shuffle (which you try to replace with this >> proposal). >> >>>> >>> >>>> >> >>>> >>> >>>> That's for distributing task metadata, so that each DoFn >> thread picks up a random bucket and sort merge key-values. It's not >> shuffling actual data. >> >>>> >>> >>>> >> >>>> >>> >>>>> >> >>>> >>> >>>>> (3) I think (at least some of the) shuffle implementations >> are implemented in ways similar to this (writing to files and merging). So >> I'm wondering if the performance benefits you see are for a very specific >> case and may limit the functionality in other ways. >> >>>> >>> >>>> >> >>>> >>> >>>> This is for the common pattern of few core data producer >> pipelines and many downstream consumer pipelines. It's not intended to >> replace shuffle/join within a single pipeline. On the producer side, by >> pre-grouping/sorting data and writing to bucket/shard output files, the >> consumer can sort/merge matching ones without a CoGBK. Essentially we're >> paying the shuffle cost upfront to avoid them repeatedly in each consumer >> pipeline that wants to join data. >> >>>> >>> >>>> >> >>>> >>> >>>>> >> >>>> >>> >>>>> Thanks, >> >>>> >>> >>>>> Cham >> >>>> >>> >>>>> >> >>>> >>> >>>>> >> >>>> >>> >>>>> On Thu, Jun 27, 2019 at 8:12 AM Neville Li < >> neville....@gmail.com> wrote: >> >>>> >>> >>>>>> >> >>>> >>> >>>>>> Ping again. Any chance someone takes a look to get this >> thing going? It's just a design doc and basic metadata/IO impl. We're not >> talking about actual source/sink code yet (already done but saved for >> future PRs). >> >>>> >>> >>>>>> >> >>>> >>> >>>>>> On Fri, Jun 21, 2019 at 1:38 PM Ahmet Altay < >> al...@google.com> wrote: >> >>>> >>> >>>>>>> >> >>>> >>> >>>>>>> Thank you Claire, this looks promising. Explicitly >> adding a few folks that might have feedback: +Ismaël Mejía +Robert Bradshaw >> +Lukasz Cwik +Chamikara Jayalath >> >>>> >>> >>>>>>> >> >>>> >>> >>>>>>> On Mon, Jun 17, 2019 at 2:12 PM Claire McGinty < >> claire.d.mcgi...@gmail.com> wrote: >> >>>> >>> >>>>>>>> >> >>>> >>> >>>>>>>> Hey dev@! >> >>>> >>> >>>>>>>> >> >>>> >>> >>>>>>>> Myself and a few other Spotify data engineers have put >> together a design doc for SMB Join support in Beam, and have a working Java >> implementation we've started to put up for PR ([0], [1], [2]). There's more >> detailed information in the document, but the tl;dr is that SMB is a >> strategy to optimize joins for file-based sources by modifying the initial >> write operation to write records in sorted buckets based on the desired >> join key. This means that subsequent joins of datasets written in this way >> are only sequential file reads, no shuffling involved. We've seen some >> pretty substantial performance speedups with our implementation and would >> love to get it checked in to Beam's Java SDK. >> >>>> >>> >>>>>>>> >> >>>> >>> >>>>>>>> We'd appreciate any suggestions or feedback on our >> proposal--the design doc should be public to comment on. >> >>>> >>> >>>>>>>> >> >>>> >>> >>>>>>>> Thanks! >> >>>> >>> >>>>>>>> Claire / Neville >> >> >> >> >> >> >> >> -- >> >> Cheers, >> >> Gleb >> >
