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 <wesmck...@gmail.com> wrote: > On Wed, Oct 2, 2019 at 11:05 PM Micah Kornfield <emkornfi...@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 <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). > > > > > > 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 <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. > > > > > > > > > > > > > >
