hi Jacques, On Tue, Oct 8, 2019 at 1:54 PM Jacques Nadeau <jacq...@apache.org> wrote: > > 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. >
I think we'll have to take a "wait and see" approach. I believe the community needs to build accessible libraries that offer value to third party users, and we will continue to do that. I think there are use cases here that fall outside of which library to use, but time will tell. > 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. > I think that using an Protobuf-like with IDL and a compiler presents a problem. Note that Flatbuffers is much better for C/C++ programmers and I still think it was the right choice for the project. Unlike Flatbuffers, C/C++ applications must either link libprotobuf.so or libprotobuf.a. Flatbuffers in C++ is a header-only dependency that is trivial to bundle with a project [1]. The same is true for Thrift, and this came up in the TF discussion [2] [1]: https://github.com/apache/arrow/tree/master/cpp/thirdparty/flatbuffers/include/flatbuffers [2]: https://github.com/tensorflow/community/pull/162#discussion_r332610486 > Thanks for the discourse and patience. > > On Wed, Oct 2, 2019 at 10:12 PM Micah Kornfield <emkornfi...@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 <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. >> > > > > > > >> > > > > >> > > > >> >
