Thank you both for the explanation, it makes sense. Another feedback I have is around flight.proto - some of the message (such as FlightDescriptor and FlightPutInstruction) is not very clear to me - it would be helpful to get some more explanation for those here or on the PR.
Thanks! Li On Sun, Aug 26, 2018 at 6:14 PM Jacques Nadeau <jacq...@apache.org> wrote: > Wes nailed my thinking. There are autobindings for every language for the > envelope if you use protobuf meaning someone can send/receive an arrow > stream without having to know how to read the arrow stream. > > On Sat, Aug 25, 2018 at 6:00 PM Wes McKinney <wesmck...@gmail.com> wrote: > > > Hi Li -- Protobuf is the "native" wire format for GRPC [1]. You can use > > Flatbuffers with it, too [2], but if we are aiming for fairly broad > support > > at the RPC level then using Protobuf is probably a safer bet. > > > > One question might be "Well, Arrow already uses Flatbuffers". That's > true, > > but a system could make Flight RPCs and delegate handling of the messages > > to third party code -- so the RPC handler does not need to know anything > > about Flatbuffers or Arrow columnar format for that matter. > > > > The main thing we need to be concerned about re: zero copy is the > > FlightData. > > > > As an aside: I still believe that Flatbuffers was the right choice for > > Arrow's metadata serialization. We've suffered a bit from weakness in > > implementation for languages like Rust, but to have the option to > > selectively read only a small part of a potentially very large message > is a > > big benefit (vs. having to do an all-or-nothing parse of the proto). It > > would be useful to quantify this benefit at some point by creating some > > benchmarks vs. a protobuf-based version of Arrow's metadata > > > > - Wes > > > > [1]: https://grpc.io/docs/guides/concepts.html#overview > > [2]: https://grpc.io/blog/flatbuffers > > > > On Fri, Aug 24, 2018, 5:05 PM Li Jin <ice.xell...@gmail.com> wrote: > > > > > One question I have is around the choice of using protobufs - It seems > > that > > > flatbuffers has better support for zero-copy and works with grpc as > well. > > > What's the rational behind picking protobuf over flatbuffer? > > > > > > On Thu, Aug 16, 2018 at 7:41 PM Wes McKinney <wesmck...@gmail.com> > > wrote: > > > > > > > hi Julian, > > > > > > > > Thanks for chiming in. > > > > > > > > On Thu, Aug 16, 2018 at 1:16 PM, Julian Hyde <jh...@apache.org> > wrote: > > > > > If your use case is SQL RPC, then you are getting close to > Avatica's > > > > > territory. Avatica[1] is a protocol for implementing > > > > > language-independent JDBC and ODBC stacks. > > > > > > > > I'm not proposing to develop a SQL RPC system inside Apache Arrow. > But > > > > Arrow Flight could be used to build one > > > > > > > > > > > > > > Now, I agree that many ODBC implementations are inefficient. Some > > ODBC > > > > > stacks make more round trips than necessary, and do more copying > than > > > > > necessary. In Avatica we are trying to squeeze out those > > > > > inefficiencies, for example minimizing the number of RPCs. We would > > > > > also love to use Arrow as the data format and reduce copying on the > > > > > server side and client side. > > > > > > > > Indeed -- what I would like to see instead is for Avatica to _use_ > > > > Arrow Flight to provide an alternative platform to offer Arrow-native > > > > connectivity in addition to the slower JDBC and ODBC standards. > > > > > > > > > > > > > > But conversely, people who start with a simple RPC use case - send > > > > > SQL, get the results - may soon find themselves needing a more > > complex > > > > > protocol - authentication, sessions, prepared statements, bind > > > > > variables, getting metadata before executing, cursors, skipping > over > > > > > rows. In other words, find themselves wanting substantial portions > of > > > > > an ODBC or JDBC driver. > > > > > > > > > > You could find yourselves building Avatica all over again. We saw > all > > > > > of this happen in XML-RPC, and it was sad. > > > > > > > > Agreed. I don't think this is in the cards, and what's being proposed > > > > now is orthogonal. > > > > > > > > > > > > > > I suggest to keep flight for the truly simple use case, and for the > > > > > more complex use case, invest effort putting Arrow into Avatica. We > > > > > are always happy to welcome new contributors. > > > > > > > > +1 > > > > > > > > > > > > > > Julian > > > > > > > > > > [1] https://calcite.apache.org/avatica/docs/ > > > > > On Thu, Aug 16, 2018 at 7:56 AM Wes McKinney <wesmck...@gmail.com> > > > > wrote: > > > > >> > > > > >> To give some extra color on my personal motivation for interest in > > > > Arrow Flight: > > > > >> > > > > >> Systems that expose databases on a network frequently send data > very > > > > >> slowly. For example, ODBC is in general extremely slow. What I > would > > > > >> like to see is servers that can expose a "sql" action type. > > > > >> > > > > >> So, in consideration of the protocol as it stands now [1], example > > > > >> session goes like this: > > > > >> > > > > >> * Client issues ListActions -> returns one or more ActionType, > > suppose > > > > >> one is "sql" > > > > >> * Client issues DoAction with type sql and body "select * from > > $TABLE" > > > > >> * Server returns stream URI for query result set and Ticket in the > > > > Result proto > > > > >> * Client issues GetFlightInfo using URI to obtain schema of result > > set > > > > >> * Client issues DoGet with ticket returned by sql DoAction > > > > >> > > > > >> There's some possible refinements to this workflow; for example, > if > > we > > > > >> wanted to enable DoAction to return more structured results (e.g. > to > > > > >> avoid the extra GetFlightInfo RPC to get the schema of the query > > > > >> result set) > > > > >> > > > > >> - Wes > > > > >> > > > > >> [1]: > > > > https://github.com/apache/arrow/blob/c52897274035f8b5192d7647b9711c > > > 68d9c54ccc/java/flight/src/main/protobuf/flight.proto > > > > >> > > > > >> On Thu, Aug 16, 2018 at 10:29 AM, Jacques Nadeau < > > jacq...@apache.org> > > > > wrote: > > > > >> > I'm out of town this week (vacation) and will be reviewing your > > > > feedback > > > > >> > next week. Thanks for the feedback! > > > > >> > > > > > >> > On Thu, Aug 9, 2018, 8:45 PM Wes McKinney <wesmck...@gmail.com> > > > > wrote: > > > > >> > > > > > >> >> hi folks, > > > > >> >> > > > > >> >> I left some feedback on this PR. If others could take a look > > > > >> >> (particularly at the .proto service definition) that would be > > > useful. > > > > >> >> > > > > >> >> We should decide on an approach to getting multiple > > > production-worthy > > > > >> >> Flight/RPC implementations ready to go. It would be a good goal > > to > > > > >> >> deliver (end-to-end send/receive data between Python and Java, > or > > > > >> >> Python and other Python processes) in the next couple releases. > > > > >> >> > > > > >> >> - Wes > > > > >> >> > > > > >> >> On Wed, May 30, 2018 at 12:44 PM, Jacques Nadeau < > > > jacq...@apache.org > > > > > > > > > >> >> wrote: > > > > >> >> > Correct, I'm maintaining standard protobuf encoding so a > > consumer > > > > that > > > > >> >> > doesn't go byte by byte can still consumer/produce the > > messages. > > > > >> >> > > > > > >> >> > More impls: for sure. > > > > >> >> > > > > > >> >> > On Wed, May 30, 2018 at 9:01 AM, Wes McKinney < > > > wesmck...@gmail.com > > > > > > > > > >> >> wrote: > > > > >> >> > > > > > >> >> >> I see; looking more closely I see you've sidestepped the > > > standard > > > > >> >> >> Protobuf serialization to write the stream as tagged > > components: > > > > >> >> >> > > > > >> >> >> > > > > > > https://github.com/apache/arrow/compare/master...jacques-n:flight#diff- > > > > >> >> >> 02cfc9235e22653fce8a7636c9f95507R241 > > > > >> >> >> > > > > >> >> >> and then reading the fields of the message tag by tag > > > > >> >> >> > > > > >> >> >> > > > > > > https://github.com/apache/arrow/compare/master...jacques-n:flight#diff- > > > > >> >> >> 02cfc9235e22653fce8a7636c9f95507R159 > > > > >> >> >> > > > > >> >> >> Would it be correct that if a GRPC implementation doesn't > > > provide > > > > >> >> >> sufficient access to the byte stream (or if it doesn't care > > > enough > > > > >> >> >> about zero copy) that you could allow GRPC to return an > > instance > > > > of > > > > >> >> >> the FlightData structure? > > > > >> >> >> > > > > >> >> >> I expect we'd want to see a few interoperable > implementations > > (I > > > > >> >> >> suggest Java, C++, Go) to harden the fine details. > > > > >> >> >> > > > > >> >> >> - Wes > > > > >> >> >> > > > > >> >> >> On Mon, May 28, 2018 at 3:32 PM, Jacques Nadeau < > > > > jacq...@apache.org> > > > > >> >> >> wrote: > > > > >> >> >> > Cutting through the layers of GRPC will be a per language > > > > approach > > > > >> >> thing. > > > > >> >> >> > Assuming that each GRPC language implementation does a > good > > > job > > > > of > > > > >> >> >> > separating message encapsulation from the base library, > this > > > > should be > > > > >> >> >> > straight-forward-ish. Hope improves around this as I see > > > > creation of > > > > >> >> >> > non-protobuf protocols built on top of the base GRPC [1]. > > How > > > > to do > > > > >> >> this > > > > >> >> >> in > > > > >> >> >> > each language will probably take time looking at the GRPC > > > > internals > > > > >> >> for > > > > >> >> >> > that language but can be a secondary step once you get the > > > > protocol > > > > >> >> >> working > > > > >> >> >> > (you can just pay for extra copies until then). > > > > >> >> >> > > > > > >> >> >> > In my Java approach I believe I do one read copy and zero > > > write > > > > copies > > > > >> >> >> > (needs more testing) which was my target. (Getting to > > > zero-copy > > > > on > > > > >> >> read > > > > >> >> >> > means a lot more complexity because your socket-reading > has > > to > > > > be > > > > >> >> >> protocol > > > > >> >> >> > aware: even our bespoke layer in Dremio doesn't try to do > > > that. > > > > I'd > > > > >> >> guess > > > > >> >> >> > KRPC does the same but haven't reviewed the code to > > confirm.) > > > > >> >> >> > > > > > >> >> >> > Will try to get some more slides/readme and a proper > > proposed > > > > patch up > > > > >> >> >> soon. > > > > >> >> >> > > > > > >> >> >> > [1] https://grpc.io/blog/flatbuffers > > > > >> >> >> > > > > > >> >> >> > > > > > >> >> >> > > > > > >> >> >> > On Mon, May 28, 2018 at 1:05 AM, Wes McKinney < > > > > wesmck...@gmail.com> > > > > >> >> >> wrote: > > > > >> >> >> > > > > > >> >> >> >> hey Jacques, > > > > >> >> >> >> > > > > >> >> >> >> This is great news, I look forward to digging into this. > My > > > > biggest > > > > >> >> >> >> initial question is the Protobuf encapsulation, > > specifically: > > > > >> >> >> >> > > > > >> >> >> >> > > https://github.com/jacques-n/arrow/blob/flight/java/flight/ > > > > >> >> >> >> src/main/protobuf/flight.proto#L99 > > > > >> >> >> >> > > > > >> >> >> >> My understanding of Protocol Buffers is that on read, the > > > > "data_body" > > > > >> >> >> >> memory would be copied out of the serialized protobuf > that > > > came > > > > >> >> across > > > > >> >> >> >> the wire. Your comment in the .proto says this "comes > last > > in > > > > the > > > > >> >> >> >> definition to help with sidecar patterns" -- my read is > > that > > > > it would > > > > >> >> >> >> be up to us to do our own sidecar implementation, similar > > to > > > > how > > > > >> >> >> >> Apache Kudu has zero-copy sidecars in their KRPC system > [1] > > > > (the > > > > >> >> >> >> comment there describes pretty much exactly the problem > we > > > > have). I > > > > >> >> >> >> saw that you also replied on a GRPC thread about this > issue > > > > [2]. > > > > >> >> Could > > > > >> >> >> >> you summarize what (if anything) stands in the way to get > > > > zero-copy > > > > >> >> on > > > > >> >> >> >> write and read? > > > > >> >> >> >> > > > > >> >> >> >> - Wes > > > > >> >> >> >> > > > > >> >> >> >> [1]: https://github.com/apache/ > > > kudu/blob/master/src/kudu/rpc/ > > > > >> >> >> >> rpc_sidecar.h#L34 > > > > >> >> >> >> [2]: > > > > https://github.com/grpc/grpc-java/issues/1054#issuecomment- > > > > >> >> >> 391692087 > > > > >> >> >> >> > > > > >> >> >> >> On Thu, May 24, 2018 at 6:57 AM, Jacques Nadeau < > > > > jacq...@apache.org> > > > > >> >> >> >> wrote: > > > > >> >> >> >> > FYI, if you want to see an example server you can run > > with > > > a > > > > GRPC > > > > >> >> >> >> generated > > > > >> >> >> >> > client, you can run the ExampleFlightServer located at > > [1]. > > > > Very > > > > >> >> basic > > > > >> >> >> >> > 'test' with that class and client is located at [2]. > > > > >> >> >> >> > > > > > >> >> >> >> > [1] > > > > >> >> >> >> > https://github.com/jacques-n/ > > > arrow/tree/flight/java/flight/ > > > > >> >> >> >> src/main/java/org/apache/arrow/flight/example > > > > >> >> >> >> > [2] > > > > >> >> >> >> > https://github.com/jacques-n/ > > > arrow/blob/flight/java/flight/ > > > > >> >> >> >> > > > > src/test/java/org/apache/arrow/flight/example/TestExampleServer.java > > > > >> >> >> >> > > > > > >> >> >> >> > > > > > >> >> >> >> > On Thu, May 24, 2018 at 11:51 AM, Jacques Nadeau < > > > > >> >> jacq...@apache.org> > > > > >> >> >> >> wrote: > > > > >> >> >> >> > > > > > >> >> >> >> >> Hey All, > > > > >> >> >> >> >> > > > > >> >> >> >> >> I used my Strata talk today as a forcing function to > > make > > > > >> >> additional > > > > >> >> >> >> >> progress on a GRPC-based Arrow RPC protocol [1]. I’m > > > > calling it > > > > >> >> >> “Apache > > > > >> >> >> >> >> Arrow Flight”. You can take a look at the work here > [2]. > > > > I’ll > > > > >> >> work to > > > > >> >> >> >> clean > > > > >> >> >> >> >> up my work and explain my thoughts about the protocol > in > > > the > > > > >> >> coming > > > > >> >> >> >> days. > > > > >> >> >> >> >> High-level: use protobuf as a encapsulation format so > > that > > > > any > > > > >> >> client > > > > >> >> >> >> that > > > > >> >> >> >> >> is supported in GRPC will work. However, we can > optimize > > > the > > > > >> >> >> read/write > > > > >> >> >> >> >> path for targeted languages and hand control the > > > > >> >> >> >> >> serialization/deserialization and memory handling. (I > > did > > > > that in > > > > >> >> >> this > > > > >> >> >> >> Java > > > > >> >> >> >> >> patch [3][4][5].) I also looked at starting to use > GRPC > > > > generated > > > > >> >> >> >> bindings > > > > >> >> >> >> >> within Python but it looks like some glue code may be > > > > needed in > > > > >> >> the > > > > >> >> >> C++ > > > > >> >> >> >> >> layer since Python delegates down frequently. I also > am > > > > still > > > > >> >> trying > > > > >> >> >> to > > > > >> >> >> >> >> understand GRPC back-pressure patterns and whether the > > > > protocol > > > > >> >> >> >> >> realistically needs to change to cover real-world high > > > > performance > > > > >> >> >> use > > > > >> >> >> >> >> cases. > > > > >> >> >> >> >> > > > > >> >> >> >> >> I’ll send out some slides about the ideas and update > > > > README, etc. > > > > >> >> >> soon. > > > > >> >> >> >> >> > > > > >> >> >> >> >> Thanks, > > > > >> >> >> >> >> Jacques > > > > >> >> >> >> >> > > > > >> >> >> >> >> [1] > > > > https://github.com/jacques-n/arrow/blob/flight/java/flight/ > > > > >> >> >> >> >> src/main/protobuf/flight.proto > > > > >> >> >> >> >> [2] http://github.com/jacques-n/arrow/ > > > > >> >> >> >> >> [3] https://github.com/jacques-n/arrow/tree/flight/ > > > > >> >> >> >> >> java/flight/src/main/java/org/apache/arrow/flight/grpc > > > > >> >> >> >> >> [4] https://github.com/jacques-n/arrow/blob/flight/ > > > > >> >> >> >> >> java/flight/src/main/java/org/apache/arrow/flight/ > > > > >> >> >> >> ArrowMessage.java#L253 > > > > >> >> >> >> >> < > > > > https://github.com/jacques-n/arrow/blob/flight/java/flight/ > > > > >> >> >> >> src/main/java/org/apache/arrow/flight/ArrowMessage. > > > java#L253> > > > > >> >> >> >> >> [5] https://github.com/jacques-n/arrow/blob/flight/ > > > > >> >> >> >> >> java/flight/src/main/java/org/apache/arrow/flight/ > > > > >> >> >> >> ArrowMessage.java#L185 > > > > >> >> >> >> >> > > > > >> >> >> >> >> > > > > >> >> >> >> > > > > >> >> >> > > > > >> >> > > > > > > > > > >
