So I spent one afternoon trying some ideas for reusing the last few
transforms WriteFiles.
WriteShardsIntoTempFilesFn extends DoFn<KV<*ShardedKey<Integer>*,
Iterable<UserT>>, *FileResult*<DestinationT>>
=> GatherResults<ResultT> extends PTransform<PCollection<ResultT>,
PCollection<List<ResultT>>>
=> FinalizeTempFileBundles extends PTransform<PCollection<List<
*FileResult<DestinationT>*>>, WriteFilesResult<DestinationT>>
I replaced FileResult<DestinationT> with KV<DestinationT, ResourceId> so I
can use pre-compute SMB destination file names for the transforms.
I'm also thinking of parameterizing ShardedKey<Integer> for SMB's
bucket/shard to reuse WriteShardsIntoTempFilesFn. These transforms are
private and easy to change/pull out.
OTOH they are somewhat coupled with the package private
{Avro,Text,TFRecord}Sink and their WriteOperation impl (where the bulk of
temp file handing logic lives). Might be hard to decouple either modifying
existing code or creating new transforms, unless if we re-write most of
FileBasedSink from scratch.
Let me know if I'm on the wrong track.
WIP Branch https://github.com/spotify/beam/tree/neville/write-files
On Tue, Jul 23, 2019 at 4:22 PM Chamikara Jayalath <[email protected]>
wrote:
>
>
> On Mon, Jul 22, 2019 at 1:41 PM Robert Bradshaw <[email protected]>
> wrote:
>
>> On Mon, Jul 22, 2019 at 7:39 PM Eugene Kirpichov <[email protected]>
>> wrote:
>> >
>> > On Mon, Jul 22, 2019 at 7:49 AM Robert Bradshaw <[email protected]>
>> wrote:
>> >>
>> >> On Mon, Jul 22, 2019 at 4:04 PM Neville Li <[email protected]>
>> wrote:
>> >> >
>> >> > Thanks Robert. Agree with the FileIO point. I'll look into it and
>> see what needs to be done.
>> >> >
>> >> > Eugene pointed out that we shouldn't build on
>> FileBased{Source,Sink}. So for writes I'll probably build on top of
>> WriteFiles.
>> >>
>> >> Meaning it could be parameterized by FileIO.Sink, right?
>> >>
>> >>
>> https://github.com/apache/beam/blob/release-2.13.0/sdks/java/core/src/main/java/org/apache/beam/sdk/io/FileIO.java#L779
>> >
>> > Yeah if possible, parameterize FileIO.Sink.
>> > I would recommend against building on top of WriteFiles either. FileIO
>> being implemented on top of WriteFiles was supposed to be a temporary
>> measure - the longer-term plan was to rewrite it from scratch (albeit with
>> a similar structure) and throw away WriteFiles.
>> > If possible, I would recommend to pursue this path: if there are parts
>> of WriteFiles you want to reuse, I would recommend to implement them as new
>> transforms, not at all tied to FileBasedSink (but ok if tied to
>> FileIO.Sink), with the goal in mind that FileIO could be rewritten on top
>> of these new transforms, or maybe parts of WriteFiles could be swapped out
>> for them incrementally.
>>
>> Thanks for the feedback. There's a lot that was done, but looking at
>> the code it feels like there's a lot that was not yet done either, and
>> the longer-term plan wasn't clear (though perhaps I'm just not finding
>> the right docs).
>>
>
> I'm also a bit unfamiliar with original plans for WriteFiles and for
> updating source interfaces, but I prefer not significantly modifying
> existing IO transforms to suite the SMB use-case. If there are existing
> pieces of code that can be easily re-used that is fine, but existing
> sources/sinks are designed to perform a PCollection -> file transformation
> and vice versa with (usually) runner determined sharding. Things specific
> to SMB such as sharding restrictions, writing metadata to a separate file,
> reading multiple files from the same abstraction, does not sound like
> features that should be included in our usual file read/write transforms.
>
>
>> >> > Read might be a bigger change w.r.t. collocating ordered elements
>> across files within a bucket and TBH I'm not even sure where to start.
>> >>
>> >> Yeah, here we need an interface that gives us ReadableFile ->
>> >> Iterable<T>. There are existing PTransform<PCollection<ReadableFile>,
>> >> PCollection<T>> but such an interface is insufficient to extract
>> >> ordered records per shard. It seems the only concrete implementations
>> >> are based on FileBasedSource, which we'd like to avoid, but there's no
>> >> alternative. An SDF, if exposed, would likely be overkill and
>> >> cumbersome to call (given the reflection machinery involved in
>> >> invoking DoFns).
>> >
>> > Seems easiest to just define a new regular Java interface for this.
>> > Could be either, indeed, ReadableFile -> Iterable<T>, or something
>> analogous, e.g. (ReadableFile, OutputReceiver<T>) -> void. Depends on how
>> much control over iteration you need.
>>
>> For this application, one wants to iterate over several files in
>> parallel. The downside of a new interface is that it shares almost
>> nothing with the "normal" sources (e.g. when features (or
>> optimizations) get added to one, they won't get added to the other).
>
>
>
>>
>> > And yes, DoFn's including SDF's are not designed to be used as Java
>> interfaces per se. If you need DoFn machinery in this interface (e.g. side
>> inputs), use Contextful - s.apache.org/context-fn.
>>
>> Yeah, one of the primary downsides to the NewDoFns is how hard it is
>> to build new DoFns out of others (or, really, use them in any context
>> other than as an argument to ParDo).
>>
>> >> > I'll file separate PRs for core changes needed for discussion. WDYT?
>> >>
>> >> Sounds good.
>>
>
> +1
>
>
>> >>
>> >> > On Mon, Jul 22, 2019 at 4:20 AM Robert Bradshaw <[email protected]>
>> wrote:
>> >> >>
>> >> >> On Fri, Jul 19, 2019 at 5:16 PM Neville Li <[email protected]>
>> wrote:
>> >> >> >
>> >> >> > Forking this thread to discuss action items regarding the change.
>> We can keep technical discussion in the original thread.
>> >> >> >
>> >> >> > Background: our SMB POC showed promising performance & cost
>> saving improvements and we'd like to adopt it for production soon (by EOY).
>> We want to contribute it to Beam so it's better generalized and maintained.
>> We also want to avoid divergence between our internal version and the PR
>> while it's in progress, specifically any breaking change in the produced
>> SMB data.
>> >> >>
>> >> >> All good goals.
>> >> >>
>> >> >> > To achieve that I'd like to propose a few action items.
>> >> >> >
>> >> >> > 1. Reach a consensus about bucket and shard strategy, key
>> handling, bucket file and metadata format, etc., anything that affect
>> produced SMB data.
>> >> >> > 2. Revise the existing PR according to #1
>> >> >> > 3. Reduce duplicate file IO logic by reusing FileIO.Sink,
>> Compression, etc., but keep the existing file level abstraction
>> >> >> > 4. (Optional) Merge code into extensions::smb but mark clearly as
>> @experimental
>> >> >> > 5. Incorporate ideas from the discussion, e.g. ShardingFn,
>> GroupByKeyAndSortValues, FileIO generalization, key URN, etc.
>> >> >> >
>> >> >> > #1-4 gives us something usable in the short term, while #1
>> guarantees that production data produced today are usable when #5 lands on
>> master. #4 also gives early adopters a chance to give feedback.
>> >> >> > Due to the scope of #5, it might take much longer and a couple of
>> big PRs to achieve, which we can keep iterating on.
>> >> >> >
>> >> >> > What are your thoughts on this?
>> >> >>
>> >> >> I would like to see some resolution on the FileIO abstractions
>> before
>> >> >> merging into experimental. (We have a FileBasedSink that would
>> mostly
>> >> >> already work, so it's a matter of coming up with an analogous Source
>> >> >> interface.) Specifically I would not want to merge a set of per file
>> >> >> type smb IOs without a path forward to this or the determination
>> that
>> >> >> it's not possible/desirable.
>>
>
