I see, thank you. For such a scenario, implementations would need to define a "UserDefinedCodec" interface to enable codecs to be registered from third party code, similar to what is done for extension types [1]
I'll update this thread when I get my experimental C++ patch up to see what I'm thinking at least for the built-in codecs we have like ZSTD. https://github.com/apache/arrow/blob/apache-arrow-0.16.0/docs/source/format/Columnar.rst#extension-types On Thu, Mar 5, 2020 at 7:56 AM Fan Liya <liya.fa...@gmail.com> wrote: > > Hi Wes, > > Thanks a lot for your further clarification. > > Some of my prelimiary thoughts: > > 1. We assign a unique GUID to each pair of compression/decompression > strategies. The GUID is stored as part of the Message.custom_metadata. When > receiving the GUID, the receiver knows which decompression strategy to use. > > 2. We serialize the decompression strategy, and store it into the > Message.custom_metadata. The receiver can decompress data after > deserializing the strategy. > > Method 1 is generally used in static strategy scenarios while method 2 is > generally used in dynamic strategy scenarios. > > Best, > Liya Fan > > On Wed, Mar 4, 2020 at 11:39 PM Wes McKinney <wesmck...@gmail.com> wrote: > > > Okay, I guess my question is how the receiver is going to be able to > > determine how to "rehydrate" the record batch buffers: > > > > What I've proposed amounts to the following: > > > > * UNCOMPRESSED: the current behavior > > * ZSTD/LZ4/...: each buffer is compressed and written with an int64 > > length prefix > > > > (I'm close to putting up a PR implementing an experimental version of > > this that uses Message.custom_metadata to transmit the codec, so this > > will make the implementation details more concrete) > > > > So in the USER_DEFINED case, how will the library know how to obtain > > the uncompressed buffer? Is some additional metadata structure > > required to provide instructions? > > > > On Wed, Mar 4, 2020 at 8:05 AM Fan Liya <liya.fa...@gmail.com> wrote: > > > > > > Hi Wes, > > > > > > I am thinking of adding an option named "USER_DEFINED" (or something > > > similar) to enum CompressionType in your proposal. > > > IMO, this option should be used primarily in Flight. > > > > > > Best, > > > Liya Fan > > > > > > On Wed, Mar 4, 2020 at 11:12 AM Wes McKinney <wesmck...@gmail.com> > > wrote: > > > > > > > On Tue, Mar 3, 2020, 8:11 PM Fan Liya <liya.fa...@gmail.com> wrote: > > > > > > > > > Sure. I agree with you that we should not overdo this. > > > > > I am wondering if we should provide an option to allow users to > > plugin > > > > > their customized compression strategies. > > > > > > > > > > > > > Can you provide a patch showing changes to Message.fbs (or Schema.fbs) > > that > > > > make this idea more concrete? > > > > > > > > > > > > > Best, > > > > > Liya Fan > > > > > > > > > > On Tue, Mar 3, 2020 at 9:47 PM Wes McKinney <wesmck...@gmail.com> > > wrote: > > > > > > > > > > > On Tue, Mar 3, 2020, 7:36 AM Fan Liya <liya.fa...@gmail.com> > > wrote: > > > > > > > > > > > > > I am so glad to see this discussion, and I am willing to provide > > help > > > > > > from > > > > > > > the Java side. > > > > > > > > > > > > > > In the proposal, I see the support for basic compression > > strategies > > > > > > > (e.g.gzip, snappy). > > > > > > > IMO, applying a single basic strategy is not likely to achieve > > > > > > performance > > > > > > > improvement for most scenarios. > > > > > > > The optimal compression strategy is often obtained by composing > > basic > > > > > > > strategies and tuning parameters. > > > > > > > > > > > > > > I hope we can support such highly customized compression > > strategies. > > > > > > > > > > > > > > > > > > > I think very much beyond trivial one-shot buffer level compression > > is > > > > > > probably out of the question for addition to the current > > "RecordBatch" > > > > > > Flatbuffers type, because the additional metadata would add > > undesirable > > > > > > bloat (which I would be against). If people have other ideas it > > would > > > > be > > > > > > great to see exactly what you are thinking as far as changes to the > > > > > > protocol files. > > > > > > > > > > > > I'll try to assemble some examples to show the before/after > > results of > > > > > > applying the simple strategy. > > > > > > > > > > > > > > > > > > > > > > > > > > Best, > > > > > > > Liya Fan > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Tue, Mar 3, 2020 at 8:15 PM Antoine Pitrou < > > anto...@python.org> > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > If we want to use a HTTP header, it would be more of a > > > > > Accept-Encoding > > > > > > > > header, no? > > > > > > > > > > > > > > > > In any case, we would have to put non-standard values there > > (e.g. > > > > > lz4), > > > > > > > > so I'm not sure how desirable it is to repurpose HTTP headers > > for > > > > > that, > > > > > > > > rather than add some dedicated field to the Flight messages. > > > > > > > > > > > > > > > > Regards > > > > > > > > > > > > > > > > Antoine. > > > > > > > > > > > > > > > > > > > > > > > > Le 03/03/2020 à 12:52, David Li a écrit : > > > > > > > > > gRPC supports headers so for Flight, we could send > > essentially an > > > > > > > Accept > > > > > > > > > header and perhaps a Content-Type header. > > > > > > > > > > > > > > > > > > David > > > > > > > > > > > > > > > > > > On Mon, Mar 2, 2020, 23:15 Micah Kornfield < > > > > emkornfi...@gmail.com> > > > > > > > > wrote: > > > > > > > > > > > > > > > > > >> Hi Wes, > > > > > > > > >> A few thoughts on this. In general, I think it is a good > > idea. > > > > > But > > > > > > > > before > > > > > > > > >> proceeding, I think the following points are worth > > discussing: > > > > > > > > >> 1. Does this actually improve throughput/latency for > > Flight? (I > > > > > > think > > > > > > > > you > > > > > > > > >> mentioned you would follow-up with benchmarks). > > > > > > > > >> 2. I think we should limit the number of supported > > compression > > > > > > > schemes > > > > > > > > to > > > > > > > > >> only 1 or 2. I think the criteria for selection speed and > > > > native > > > > > > > > >> implementations available across the widest possible > > languages. > > > > > As > > > > > > > far > > > > > > > > as > > > > > > > > >> i can tell zstd only have bindings in java via JNI, but my > > > > > > > > understanding is > > > > > > > > >> it is probably the type of compression for our use-cases. > > So I > > > > > > think > > > > > > > > >> zstd + potentially 1 more. > > > > > > > > >> 3. Commitment from someone on the Java side to implement > > this. > > > > > > > > >> 4. This doesn't need to be coupled with this change per-se > > but > > > > > for > > > > > > > > >> something like flight it would be good to have a standard > > > > > mechanism > > > > > > > for > > > > > > > > >> negotiating server/client capabilities (e.g. client doesn't > > > > > support > > > > > > > > >> compression or only supports a subset). > > > > > > > > >> > > > > > > > > >> > > > > > > > > >> Thanks, > > > > > > > > >> Micah > > > > > > > > >> > > > > > > > > >> On Sun, Mar 1, 2020 at 1:24 PM Wes McKinney < > > > > wesmck...@gmail.com> > > > > > > > > wrote: > > > > > > > > >> > > > > > > > > >>> On Sun, Mar 1, 2020 at 3:14 PM Antoine Pitrou < > > > > > anto...@python.org> > > > > > > > > >> wrote: > > > > > > > > >>>> > > > > > > > > >>>> > > > > > > > > >>>> Le 01/03/2020 à 22:01, Wes McKinney a écrit : > > > > > > > > >>>>> In the context of a "next version of the Feather format" > > > > > > ARROW-5510 > > > > > > > > >>>>> (which is consumed only by Python and R at the moment), I > > > > have > > > > > > been > > > > > > > > >>>>> looking at compressing buffers using fast compressors > > like > > > > ZSTD > > > > > > > when > > > > > > > > >>>>> writing the RecordBatch bodies. This could be handled > > > > privately > > > > > > as > > > > > > > an > > > > > > > > >>>>> implementation detail of the Feather file, but since ZSTD > > > > > > > compression > > > > > > > > >>>>> could improve throughput in Flight, for example, I > > thought I > > > > > > would > > > > > > > > >>>>> bring it up for discussion. > > > > > > > > >>>>> > > > > > > > > >>>>> I can see two simple compression strategies: > > > > > > > > >>>>> > > > > > > > > >>>>> * Compress the entire message body in one-shot, writing > > the > > > > > > result > > > > > > > > >> out > > > > > > > > >>>>> with an 8-byte int64 prefix indicating the uncompressed > > size > > > > > > > > >>>>> * Compress each non-zero-length constituent Buffer prior > > to > > > > > > writing > > > > > > > > >> to > > > > > > > > >>>>> the body (and using the same uncompressed-length-prefix > > when > > > > > > > writing > > > > > > > > >>>>> the compressed buffer) > > > > > > > > >>>>> > > > > > > > > >>>>> The latter strategy is preferable for scenarios where we > > may > > > > > > > project > > > > > > > > >>>>> out only a few fields from a larger record batch (such as > > > > > reading > > > > > > > > >> from > > > > > > > > >>>>> a memory-mapped file). > > > > > > > > >>>> > > > > > > > > >>>> Agreed. It may also allow using different compression > > > > > strategies > > > > > > > for > > > > > > > > >>>> different kinds of buffers (for example a bytestream > > splitting > > > > > > > > strategy > > > > > > > > >>>> for floats and doubles, or a delta encoding strategy for > > > > > > integers). > > > > > > > > >>> > > > > > > > > >>> If we wanted to allow for different compression to apply to > > > > > > different > > > > > > > > >>> buffers, I think we will need a new Message type because > > this > > > > > would > > > > > > > > >>> inflate metadata sizes in a way that is not likely to be > > > > > acceptable > > > > > > > > >>> for the current uncompressed use case. > > > > > > > > >>> > > > > > > > > >>> Here is my strawman proposal > > > > > > > > >>> > > > > > > > > >>> > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://github.com/apache/arrow/compare/master...wesm:compression-strawman > > > > > > > > >>> > > > > > > > > >>>>> Implementation could be accomplished by one of the > > following > > > > > > > methods: > > > > > > > > >>>>> > > > > > > > > >>>>> * Setting a field in Message.custom_metadata > > > > > > > > >>>>> * Adding a new field to Message > > > > > > > > >>>> > > > > > > > > >>>> I think it has to be a new field in Message. Making it an > > > > > > ignorable > > > > > > > > >>>> metadata field means non-supporting receivers will decode > > and > > > > > > > > interpret > > > > > > > > >>>> the data wrongly. > > > > > > > > >>>> > > > > > > > > >>>> Regards > > > > > > > > >>>> > > > > > > > > >>>> Antoine. > > > > > > > > >>> > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
