I'd be open to looking at a proposal for a human-readable text representation, but I'm definitely wary about making any kind of cross-version compatibility guarantees (beyond "we will do our best"). If we were to make the same kinds of forward/backward compatibility guarantees as with Flatbuffers it could create a lot of work for maintainers.
On Thu, Dec 12, 2019 at 12:43 AM Micah Kornfield <emkornfi...@gmail.com> wrote: > > > > > With these two together, it would seem not too difficult to create a text > > representation for Arrow schemas that (at some point) has some > > compatibility guarantees, but maybe I'm missing something? > > > I think the main risk is if somehow flatbuffers JSON parsing doesn't handle > backward compatible changes to the arrow schema message. Given the way the > documentation is describing the JSON functionality I think this would be > considered a bug. > > The one downside to calling the "schema" canonical is the flatbuffers JSON > functionality only appears to be available in C++ and Java via JNI, so it > wouldn't have cross language support. I think this issue is more one of > semantics though (i.e. does the JSON description become part of the "Arrow > spec" or does it live as a C++/Python only feature). > > -Micah > > > On Tue, Dec 10, 2019 at 10:51 AM Christian Hudon <chr...@elementai.com> > wrote: > > > Micah: I didn't know that Flatbuffers supported serialization to/from JSON, > > thanks. That seems like a very good start, at least. I'll aim to create a > > draft pull request that at least wires everything up in Arrow so we can > > load/save a Schema.fbs instance from/to JSON. At least it'll make it easier > > for me to see how Arrow schemas would look in JSON with that. > > > > Otherwise, I'm still gathering requirements internally here. For example, > > one thing that would be nice would be to be able to output a JSON Schema > > from at least a subset of the Arrow schema. (That way our users could start > > by passing around JSON with a given schema, and transition pieces of a > > workflow to Arrow as they're ready.) But that part can also be done outside > > of the Arrow code, if deemed not relevant to have in the Arrow codebase > > itself. > > > > One core requirement for us, however, would be eventual compatibility > > between Arrow versions for a given text representation of a schema. > > Meaning, if you have a text description of a given Arrow schema, you can > > load it into different versions of Arrow and it creates a valid Schema > > Flatbuffer description, that Arrow can use. Wes, were you thinking of that, > > or of something else, when you wrote "only makes sense if it is offered > > without any backward/forward compatibility guarantees"? > > > > For the now, or me, assuming the JSON serialization done by the Flatbuffer > > libraries is usable, it seems we have all the pieces to make this happen: > > 1) The binary Schema.fbs data structures has to be compatible between > > different versions of Arrow, otherwise two processes with different Arrow > > versions won't be able to interoperate, no? > > 2) The Flatbuffer <-> JSON serialization supplied by the Flatbuffers > > library also has to be compatible between different versions of the > > Flatbuffers library, since the main use case seems to be storing > > Flatbuffers assets into version control. Breaking changes there will also > > be painful to their users. > > > > With these two together, it would seem not too difficult to create a text > > representation for Arrow schemas that (at some point) has some > > compatibility guarantees, but maybe I'm missing something? > > > > Thanks, > > > > Christian > > > > Le lun. 9 déc. 2019, à 07 h 00, Wes McKinney <wesmck...@gmail.com> a > > écrit : > > > > > The only "canonical" representation of schemas at the moment is the > > > Flatbuffers data structure [1] > > > > > > Having a human-readable/parseable text representation I think only > > > makes sense if it is offered without any backward/forward > > > compatibility guarantees. > > > > > > Note I had previously opened > > > https://issues.apache.org/jira/browse/ARROW-3730 where I noted that > > > there's no way (aside from generating the Flatbuffers messages) to > > > generate a schema representation that can be used later to reconstruct > > > a schema in a program. If such a representation were human > > > readable/editable that seems beneficial. > > > > > > > > > > > > [1]: https://github.com/apache/arrow/blob/master/format/Schema.fbs > > > > > > On Sat, Dec 7, 2019 at 11:56 AM Maarten Ballintijn <maart...@xs4all.nl> > > > wrote: > > > > > > > > > > > > Is there a syntax specified for schemas? > > > > > > > > Cheers, > > > > Maarten. > > > > > > > > > > > > > On Dec 6, 2019, at 5:01 PM, Micah Kornfield <emkornfi...@gmail.com> > > > wrote: > > > > > > > > > > Hi Christian, > > > > > As far as I know no-one is working on a canonical text representation > > > for > > > > > schemas. A JSON serializer exists for integration test purposes, but > > > > > IMO it shouldn't be relied upon as canonical. > > > > > > > > > > It looks like Flatbuffers supports serialization to/from JSON [1 > > > > > <https://google.github.io/flatbuffers/flatbuffers_guide_use_cpp.html > > > >], > > > > > using that functionality might be a promising avenue to pursue for a > > > human > > > > > readable schema. I could see adding a helper method someplace under > > > IPC for > > > > > this. Would that meet your needs? I think if there are other > > > > > requirements, then a proposal would be welcome. Ideally, a solution > > > would > > > > > not require additional build/runtime dependencies. > > > > > > > > > > > > > > > Thanks, > > > > > Micah > > > > > > > > > > [1] See Text & schema parsing > > > > > https://google.github.io/flatbuffers/flatbuffers_guide_use_cpp.html > > > > > > > > > > On Fri, Dec 6, 2019 at 1:26 PM Christian Hudon <chr...@elementai.com > > > > > > wrote: > > > > > > > > > >> Hi, > > > > >> > > > > >> For the uses I would like to make of Arrow, I would need a > > > human-readable > > > > >> and -writable version of an Arrow Schema, that could be converted to > > > and > > > > >> from the Arrow Schema C++ object. Going through the doc for 0.15.1, > > I > > > don't > > > > >> see anything to that effect, with the closest being the ToString() > > > method > > > > >> on DataType instances, but which is meant for debugging only. (I > > need > > > an > > > > >> expression of an Arrow Schema that people can read, and that can > > live > > > > >> outside of the code for a particular operation.) > > > > >> > > > > >> Is a text representation of an Arrow Schema something that is being > > > worked > > > > >> on now? If not, would you folks be interested in me putting up an > > > initial > > > > >> proposal for discussion? Any design constraints I should pay > > > attention to, > > > > >> then? > > > > >> > > > > >> Thanks, > > > > >> > > > > >> Christian > > > > >> -- > > > > >> > > > > >> > > > > >> │ Christian Hudon > > > > >> > > > > >> │ Applied Research Scientist > > > > >> > > > > >> Element AI, 6650 Saint-Urbain #500 > > > > >> > > > > >> Montréal, QC, H2S 3G9, Canada > > > > >> Elementai.com > > > > >> > > > > > > > > > > > > > -- > > > > > > │ Christian Hudon > > > > │ Applied Research Scientist > > > > Element AI, 6650 Saint-Urbain #500 > > > > Montréal, QC, H2S 3G9, Canada > > Elementai.com > >
