On Wed, Oct 2, 2019 at 7:46 PM Jacques Nadeau <jacq...@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).
I think we are creating a C-based in-memory representation of Arrow (significantly simpler than what we have in C++, which involves smart pointers and other C++ concepts) and how people use these structs is up to them. > 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.) > I'm fine to wait for more opinions, but I don't think that creating a strict C programming interface means that all languages have to figure out how to do FFI with it. > 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? > Each language implementation represents in-memory schemas in a different way. In C++ we have the arrow::DataType classes. If the goal is to create a very compact C-based data model or Arrow, why is using a string representation of types instead of a more verbose object model inappropriate? > 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 <wesmck...@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 <jacq...@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 <wesmck...@gmail.com> wrote: > > > > > > > hi Antoine, > > > > > > > > On Tue, Oct 1, 2019 at 4:29 AM Antoine Pitrou <anto...@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. > > > > > >
