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From Jacques Nadeau <jacq...@apache.org>
Subject Re: [DISCUSS] C-level in-process array protocol
Date Tue, 08 Oct 2019 18:54:26 GMT
I removing all my objections to this work.

I wish there was more feedback from additional community members. I
continue to be concerned about fragmentation. I don't agree with the
arguments here that we need to add a new api to make it easy for people to
*not* use Arrow codebase. It seems like a punt on building useful libraries
within the project that will ultimately hurt the interoperability story.

As a side note, it seems like much of this is about people's distaste for
flatbuffers. I know I regret using it. If we had a chance to do it over
again, I would have chosen to use protobuf for everything except the data
header, where I would hand write the encoding (since it is so simple
anyway). If it is such a problem that people are contorting to work around
it, maybe we should address that? Just a thought.

Thanks for the discourse and patience.

On Wed, Oct 2, 2019 at 10:12 PM Micah Kornfield <emkornfield@gmail.com>
wrote:

> Hi Wes,
> I agree for third-parties "A" (Field data structures) is the most useful.
>
> At least in my mind the discussion was for both first and third-parties.  I
> was trying to point out that "A" is less necessary as a first step for
> first-party integrations and could potentially require more effort if we
> already have the code that does "B" (field reassembly).
>
> Thanks,
> Micah
>
> On Wed, Oct 2, 2019 at 10:28 PM Wes McKinney <wesmckinn@gmail.com> wrote:
>
> > On Wed, Oct 2, 2019 at 11:05 PM Micah Kornfield <emkornfield@gmail.com>
> > wrote:
> > >
> > > I've tried to summarize my understanding of the debate so far and give
> > some
> > > initial thoughts. I think there are two potentially different sets of
> > users
> > > that we are targeting with a stable C API/ABI ourselves and external
> > > parties.
> > >
> > > 1.  Different language implementations within the Arrow project that
> want
> > > to call into each other's code.  We still don't have a great story
> around
> > > this in terms of reusable libraries and questions like [1] are a
> > motivating
> > > examples of making something better in this context.
> > > 2.  third-parties wishing to support/integrate with Arrow.  Some
> > > conjectures about these users:
> > >   - Users in this group are NOT necessarily familiar with existing
> > > technologies Arrow uses (i.e. flatbuffers)
> > >   - The stability of the API is the primary concern (consumers don't
> want
> > > to change when a new version of the library ships)
> > >   - An important secondary concern is additional libraries that need to
> > be
> > > integrated in addition to the API
> > >
> > > The main debate points seems to be:
> > >
> > > 1.  Vector/Array oriented API vs existing Record Batch.  Will an
> > additional
> > > column oriented API become too much of a maintenance headache/cause
> > > fragmentation?
> > >
> > >  - In my mind the question here is which set of users we are
> > prioritizing.
> > > IMO the combination of flatbuffers and translation to/from RecordBatch
> > > format offers too much friction to make it easy for a third-party
> > > implementer to use. If we are prioritizing for our own internal
> > use-cases I
> > > think we should try out a RecordBatch+Flatbuffers based C-API. We
> already
> > > have all the necessary building blocks.
> > >
> >
> > If a C function passes you a string containing a RecordBatch
> > Flatbuffers message, what happens next? This message has to be
> > reassembled into a recursive data structure before you can "do"
> > anything with it. Are we expecting every third party project to
> > implement:
> >
> > A. Data structures appropriate to represent a logical "field" in a
> > record batch (which have to be recursive to account for nested types'
> > children)
> > B. The logic to convert from the flattened Flatbuffers representation
> > to some implementation of A
> >
> > I'm arguing that we should provide both to third parties. To build B,
> > you need A. Some consumers will only use A. This discussion is
> > essentially about developing an ultraminimalist "drop-in" C
> > implementation of A.
> >
> > > 2.  How onerous is the dependency on flat-buffers both from a learning
> > > curve perspective and as dependency for third-party integrators?
> > > - Flatbuffers aren't entirely straight-forward and I think if we do
> move
> > > forward with an API based on Column/Array we should consider
> alternatives
> > > as long as the necessary parsing code can be done in a small amount of
> > code
> > > (I'm personally against JSON for this, but can see the arguments for
> it).
> > >
> > > 3.  Do all existing library implementations need to support both
> > > Column/Array a ABI?  How will compliance be checked for the new
> API/ABI?
> > >
> > > - I'm still thinking this through.
> > >
> > > [1]
> > >
> >
> https://lists.apache.org/thread.html/18244b294d0b9bd568b5cfd1b1ac2b6a25088383a08202cc7a8a3563@%3Cuser.arrow.apache.org%3E
> > >
> > > On Wed, Oct 2, 2019 at 6:46 PM Jacques Nadeau <jacques@apache.org>
> > wrote:
> > >
> > > > I'd like to hear more opinions from others on this topic. This
> > conversation
> > > > seems mostly dominated by comments from myself, Wes and Antoine.
> > > >
> > > > I think it is reasonable to argue that keeping any ABI (or
> > header/struct
> > > > pattern) as narrow as possible would allow us to minimize overlap
> with
> > the
> > > > existing in-memory specification. In Arrow's case, this could be as
> > simple
> > > > as a single memory pointer for schema (backed by flatbuffers) and a
> > single
> > > > memory location for data (that references the record batch header,
> > which in
> > > > turn provides pointers into the actual arrow data). Extensions would
> > need
> > > > to be added for reference management as done here but I continue to
> > think
> > > > we should defer discussion of that until the base data structures are
> > > > resolved. I see the comments here as arguing for a much broader ABI,
> in
> > > > part to support having people build "Arrow" components that
> > interconnect
> > > > using this new interface. I understand the desire to expand the ABI
> to
> > be
> > > > driven by needs to reduce dependencies and ease usability.
> > > >
> > > > The representation within the related patch is being presented as a
> > way for
> > > > applications to share Arrow data but is not easily accessible to all
> > > > languages. I want to avoid a situation where someone says "I produced
> > an
> > > > Arrow API" when what they've really done is created a C interface
> which
> > > > only a small subset of languages can actually leverage. For example,
> > every
> > > > language now knows how to parse the existing schema definition as
> > rendered
> > > > in flatbuf. In order to interact with something that implements this
> > new
> > > > pattern one would also be required to implement completely new schema
> > > > consumption code. In the proposal itself it suggests this (for
> example
> > > > enhancing the C++ library to consume structures produced this way).
> > > >
> > > > As I said, I really want to hear more opinions. Running this past
> > various
> > > > developers I know, many have echoed my concerns but that really
> doesn't
> > > > matter (and who knows how much of that is colored by my presentation
> > of the
> > > > issue). What do people here think? If someone builds an "Arrow"
> library
> > > > that implements this set of structures, how does one use it in Node?
> In
> > > > Java? Does it drive creation of a secondary set of interfaces in each
> > of
> > > > those languages to work with this kind of pattern? (For example, in a
> > JVM
> > > > view of the world, working with a plain struct in java rather than a
> > set of
> > > > memory pointers against our existing IPC formats would be quite
> > painful and
> > > > we'd definitely need to create some glue code for users. I worry the
> > same
> > > > pattern would occur in many other languages.)
> > > >
> > > > To respond directly to some of Wes's most recent comments from the
> > email
> > > > below. I struggle to map your description of the situation to the
> rest
> > of
> > > > the thread and the proposed patch.  For example, you say that a
> > non-goal is
> > > > "creating a new canonical way to serialize metadata" bute the patch
> > > > proposes a concrete string based encoding system to describe data
> > types.
> > > > Aren't those things in conflict?
> > > >
> > > > I'll also think more on this and challenge my own perspective. This
> > isn't
> > > > where my focus is so my comments aren't as developed/thoughtful as
> I'd
> > > > like.
> > > >
> > > >
> > > > On Tue, Oct 1, 2019 at 7:33 PM Wes McKinney <wesmckinn@gmail.com>
> > wrote:
> > > >
> > > > > hi Jacques,
> > > > >
> > > > > I think we've veered off course a bit and maybe we could reframe
> the
> > > > > discussion.
> > > > >
> > > > > Goals
> > > > > * A "drop-in" header-only C file that projects can use as a
> > > > > programming interface either internally only or to expose in-memory
> > > > > data structures between C functions at call sites. Ideally little
> to
> > > > > no disassembly/reassembly should be required on either "side" of
> the
> > > > > call site.
> > > > > * Simplifying adoption of Arrow for C programmers, or languages
> based
> > > > > around C FFI
> > > > >
> > > > > Non-goals
> > > > > * Expanding the columnar format or creating an alternative
> canonical
> > > > > in-memory representation
> > > > > * Creating a new canonical way to serialize metadata
> > > > >
> > > > > Note that this use case has been on my mind for more than 2 years:
> > > > > https://issues.apache.org/jira/browse/ARROW-1058
> > > > >
> > > > > I think there are a couple of potentially misleading things at play
> > here
> > > > >
> > > > > 1. The use of the word "protocol". In C, a struct has a
> well-defined
> > > > > binary layout, so a C API is also an ABI. Using C structs to
> > > > > communicate data can be considered to be a protocol, but it means
> > > > > something different in the context of the "Arrow protocol". I think
> > we
> > > > > need to call this a "C API"
> > > > >
> > > > > 2. The documentation for this in Antoine's PR is in the format/
> > > > > directory. It would probably be better to have a "C API" section
in
> > > > > the documentation.
> > > > >
> > > > > The header file under discussion and the documentation about it is
> > > > > best considered as a "library".
> > > > >
> > > > > It might be useful at some point to create a C99 implementation of
> > the
> > > > > IPC protocol as well using FlatCC with the goal of having a
> complete
> > > > > implementation of the columnar format in C with minimal binary
> > > > > footprint. This is analogous to the NanoPB project which is an
> > > > > implementation of Protocol Buffers with small code size
> > > > >
> > > > > https://github.com/nanopb/nanopb
> > > > >
> > > > > Let me know if this makes more sense.
> > > > >
> > > > > I think it's important to communicate clearly about this primarily
> > for
> > > > > the benefit of the outside world which can confuse easily as we
> have
> > > > > observed over the last few years =)
> > > > >
> > > > > Wes
> > > > >
> > > > > On Tue, Oct 1, 2019 at 2:55 PM Jacques Nadeau <jacques@apache.org>
> > > > wrote:
> > > > > >
> > > > > > I disagree with this statement:
> > > > > >
> > > > > > - the IPC format is meant for serialization while the C data
> > protocol
> > > > is
> > > > > > meants for in-memory communication, so different concerns apply
> > > > > >
> > > > > > If that is how the a particular implementation presents it,
that
> > is a
> > > > > > weaknesses of the implementation, not the format. The primary
use
> > case
> > > > I
> > > > > > was focused on when working on the initial format was
> communication
> > > > > within
> > > > > > the same process. It seems like this is being used as a basis
for
> > the
> > > > > > introduction of new things when the premise is inconsistent
with
> > the
> > > > > > intention of the creation. The specific reason we used
> flatbuffers
> > in
> > > > the
> > > > > > project was to collapse the separation of in-process and
> > out-of-process
> > > > > > communication. It means the same thing it does with the Arrow
> data
> > > > > itself:
> > > > > > that a consumer doesn't have to use a particular library to
> > interact
> > > > with
> > > > > > and use the data.
> > > > > >
> > > > > > It seems like there are two ideas here:
> > > > > >
> > > > > > 1) How do we make it easier for people to use Arrow?
> > > > > > 2) Should we implement a new in memory representation of Arrow
> > that is
> > > > > > language specific.
> > > > > >
> > > > > > I'm entirely in support of number one. If for a particular type
> of
> > > > > domain,
> > > > > > people want an easier way to interact with Arrow, let's make
a
> new
> > > > > library
> > > > > > that helps with that. In easy of our current libraries, we do
> many
> > > > things
> > > > > > to make it easier to work with Arrow. None of those require
a
> > change to
> > > > > the
> > > > > > core format or are formalized as a new in-memory standard. The
> > > > in-memory
> > > > > > representation of rust or javascript or java objects are
> > implementation
> > > > > > details.
> > > > > >
> > > > > > I'm against number two as it creates a fragmentation problem.
> > Arrow is
> > > > > > about having a single canonical format for memory for both
> > metadata and
> > > > > > data. Having multiple in-memory formats (especially when some
are
> > not
> > > > > > language independent) is counter to the goals of the project.
> > > > >
> > > > > I don't think anyone is proposing anything that would cause
> > > > fragmentation.
> > > > >
> > > > > A central question is whether it is useful to define a reusable C
> ABI
> > > > > for the Arrow columnar format, and if there is sufficient
> interest, a
> > > > > tiny C implementation of the IPC protocol (which uses the
> Flatbuffers
> > > > > message) that assembles and disassembles the data structures
> defined
> > > > > in the C ABI.
> > > > >
> > > > > We could separately create a tiny implementation of the Arrow IPC
> > > > > protocol using FlatCC that could be dropped into applications
> > > > > requiring only a C compiler and nothing else.
> > > > >
> > > > >
> > > > > >
> > > > > > Two other, separate comments:
> > > > > > 1) I don't understand the idea that we need to change the way
> Arrow
> > > > > > fundamentally works so that people can avoid using a dependency.
> > If the
> > > > > > dependency is small, open source and easy to build, people can
> > fork it
> > > > > and
> > > > > > include directly if they want to. Let's not violate project
> > principles
> > > > > > because DuckDB has a religious perspective on dependencies.
If
> the
> > > > > problem
> > > > > > is people have to swallow too large of a pill to do basic things
> > with
> > > > > Arrow
> > > > > > in C, let's focus on fixing that (to our definition of ease,
not
> > > > someone
> > > > > > else's). If FlatCC solves some those things, great. If we need
to
> > > > build a
> > > > > > baby integration library that is more C centric, great. Neither
> of
> > > > those
> > > > > > things require implementing something at the format level.
> > > > > >
> > > > > > 2) It seems like we should discuss the data structure problem
> > > > separately
> > > > > > from the reference management concern.
> > > > > >
> > > > > >
> > > > > > On Tue, Oct 1, 2019 at 5:42 AM Wes McKinney <wesmckinn@gmail.com
> >
> > > > wrote:
> > > > > >
> > > > > > > hi Antoine,
> > > > > > >
> > > > > > > On Tue, Oct 1, 2019 at 4:29 AM Antoine Pitrou <
> > antoine@python.org>
> > > > > wrote:
> > > > > > > >
> > > > > > > >
> > > > > > > > Le 01/10/2019 à 00:39, Wes McKinney a écrit :
> > > > > > > > > A couple things:
> > > > > > > > >
> > > > > > > > > * I think a C protocol / FFI for Arrow array/vectors
would
> be
> > > > > better
> > > > > > > > > to have the same "shape" as an assembled array.
Note that
> > the C
> > > > > > > > > structs here have very nearly the same "shape"
as the data
> > > > > structure
> > > > > > > > > representing a C++ Array object [1]. The disassembly
and
> > > > reassembly
> > > > > > > > > here is substantially simpler than the IPC protocol.
A
> > recursive
> > > > > > > > > structure in Flatbuffers would make RecordBatch
messages
> much
> > > > > larger,
> > > > > > > > > so the flattened / disassembled representation
we use for
> > > > > serialized
> > > > > > > > > record batches is the correct one
> > > > > > > >
> > > > > > > > I'm not sure I agree:
> > > > > > > >
> > > > > > > > - indeed, it's not a coincidence that the ArrowArray
struct
> > looks
> > > > > quite
> > > > > > > > closely like the C++ ArrayData object :-)  We have
good
> > experience
> > > > > with
> > > > > > > > that abstraction and it has proven to work quite well
> > > > > > > >
> > > > > > > > - the IPC format is meant for serialization while
the C data
> > > > > protocol is
> > > > > > > > meants for in-memory communication, so different concerns
> apply
> > > > > > > >
> > > > > > > > - the fact that this makes the layout slightly larger
doesn't
> > seem
> > > > > > > > important at all; we're not talking about transferring
data
> > over
> > > > the
> > > > > wire
> > > > > > > >
> > > > > > > > There's also another argument for having a recursive
struct:
> it
> > > > > > > > simplifies how the data type is represented, since
we can
> > encode
> > > > each
> > > > > > > > child type individually instead of encoding it in
the
> parent's
> > > > format
> > > > > > > > string (same applies for metadata and individual flags).
> > > > > > > >
> > > > > > >
> > > > > > > I was saying something different here. I was making an
argument
> > about
> > > > > > > why we use the flattened array-of-structs in the IPC protocol.
> > One
> > > > > > > reason is that it's a more compact representation. That
is not
> > very
> > > > > > > important here because this protocol is only for *in-process*
> > (for
> > > > > > > languages that have a C FFI facility) rather than
> *inter-process*
> > > > > > > communication.
> > > > > > >
> > > > > > > I agree also that the type encoding is simple, here, too,
since
> > we
> > > > > > > aren't having to split the schema and record batch between
> > different
> > > > > > > serialized messages. There is some potential waste with
having
> to
> > > > > > > populate the type fields multiple times when communicating
a
> > sequence
> > > > > > > of "chunks" from the same logical dataset.
> > > > > > >
> > > > > > > > > * The "formal" C protocol having the "assembled"
shape
> means
> > that
> > > > > many
> > > > > > > > > minimal Arrow users won't have to implement any
separate
> data
> > > > > > > > > structures. They can just use the C struct directly
or a
> > slightly
> > > > > > > > > wrapped version thereof with some convenience
functions.
> > > > > > > >
> > > > > > > > Yes, but the same applies to the current proposal.
> > > > > > > >
> > > > > > > > > * I think that requiring building a Flatbuffer
for minimal
> > use
> > > > > cases
> > > > > > > > > (e.g. communicating simple record batches with
primitive
> > types)
> > > > > passes
> > > > > > > > > on implementation burden to minimal users.
> > > > > > > >
> > > > > > > > It certainly does.
> > > > > > > >
> > > > > > > > > I think the mantra of the C protocol should be
the
> following:
> > > > > > > > >
> > > > > > > > > * Users of the protocol have to write little
to no code to
> > use
> > > > it.
> > > > > For
> > > > > > > > > example, populating an INT32 array should require
only a
> few
> > > > lines
> > > > > of
> > > > > > > > > code
> > > > > > > >
> > > > > > > > Agreed.  As a sidenote, the spec should have an example
of
> > doing
> > > > > this in
> > > > > > > > raw C.
> > > > > > > >
> > > > > > > > Regards
> > > > > > > >
> > > > > > > > Antoine.
> > > > > > >
> > > > >
> > > >
> >
>

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