arrow-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Micah Kornfield <emkornfi...@gmail.com>
Subject Re: [DISCUSS] C-level in-process array protocol
Date Thu, 03 Oct 2019 05:12:03 GMT
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.
> > > > > >
> > > >
> > >
>

Mime
  • Unnamed multipart/alternative (inline, None, 0 bytes)
View raw message