I've been poking around the project, and I'm growing concerned that our use of the KeyValue field has already been non-compliant in many cases since we do not validate UTF8-ness. Since we also use KeyValue to handle opaque data serialization for extension types [1], the fact that the specification does not clarify that binary data (such as the output of ExtensionType::Serialize) must be base64-encoded or similar makes this a bit of a minefield at the moment.
It seems that there are no particularly excellent solutions, and maintaining the status quo (having now identified these inconsistencies / vagueries in at least the C++ implementation) is probably not a good idea either. In Parquet, where we store a serialized binary Arrow schema, we have to base64-encode [2] [1]: https://arrow.apache.org/docs/format/Columnar.html#custom-application-metadata [2]: https://github.com/apache/arrow/blob/e990d177b1f1dec962315487682f613d46be573c/cpp/src/parquet/arrow/writer.cc#L423 On Tue, Aug 10, 2021 at 3:27 PM Wes McKinney <wesmck...@gmail.com> wrote: > > Ah, that's definitely a no-go then (I believe we verify messages > unconditionally in C++). That's unfortunate (and I feel responsible > for missing this, but I suppose we had a lot of opportunities to fix > it prior to the 1.0.0 format version) — so to have actual binary > values (which was the intention in the first place for the metadata) > we would need to add a new metadata field. > > On Tue, Aug 10, 2021 at 6:53 AM Micah Kornfield <emkornfi...@gmail.com> wrote: > > > > One issue with changing it to byte is it would effectively break any reader > > that is validating flatbuffer data, because flatbuffers verifies null > > termination [1]. While this might comply with forward compatibility > > guarantees it seems like a pretty large blast radius. > > > > [1] > > https://github.com/google/flatbuffers/blob/master/include/flatbuffers/flatbuffers.h#L2457 > > > > On Mon, Jul 12, 2021 at 12:38 PM Wes McKinney <wesmck...@gmail.com> wrote: > >> > >> pyarrow at least treats the KeyValue values as binary and not UTF-8. > >> > >> On Sun, Jul 11, 2021 at 9:40 PM Micah Kornfield <emkornfi...@gmail.com> > >> wrote: > >> > > >> > I think other languages (e.g. java, python) might make more of > >> > distinction between utf-8 compatible strings and raw bytes. For python > >> > it might be less of a concern if the c++ wrapper already makes the value > >> > field look like a bytes field > >> > > >> > On Sunday, July 11, 2021, Wes McKinney <wesmck...@gmail.com> wrote: > >> >> > >> >> We could certainly "upgrade" KeyValue to have a binary value field > >> >> everywhere KeyValue is used, but there is some risk of code in the > >> >> wild expecting there to be a null terminator after the string data. > >> >> The Flatbuffers-generated accessor APIs do not depend on the existence > >> >> of the null terminator, though. Not ideal, but I would not be thrilled > >> >> about adding an extra [ BinaryKeyValue ] everyplace we currently have > >> >> [ KeyValue ]. > >> >> > >> >> That said, I doubt that we have any endogenous forward compatibility > >> >> problems related to this in Apache Arrow-maintained libraries, the > >> >> risk would come from users who are interacting with the Flatbuffers > >> >> data manually / without using one of our libraries. We could implement > >> >> the changes and run a set of forward compatibility integration tests > >> >> to see if anyone of our released libraries have an issue. > >> >> > >> >> On Fri, Jul 9, 2021 at 11:33 AM Wes McKinney <wesmck...@gmail.com> > >> >> wrote: > >> >> > > >> >> > The cost of an empty vector in Flatbuffers appears to be 4 bytes. > >> >> > > >> >> > On Wed, Jul 7, 2021 at 5:50 PM Micah Kornfield > >> >> > <emkornfi...@gmail.com> wrote: > >> >> > > > >> >> > > Retitling and forking the discussion to talk about key value pairs. > >> >> > > > >> >> > > What is the byte cost of an empty list? Another option would be to > >> >> > > introduce a new BinaryKeyValue table and add binary metadata. > >> >> > > > >> >> > > On Wed, Jul 7, 2021 at 8:32 AM Nate Bauernfeind < > >> >> > > natebauernfe...@deephaven.io> wrote: > >> >> > > > >> >> > > > Deephaven and I are very supportive of "upgrading" the value half > >> >> > > > of the kv > >> >> > > > pair to a byte vector. What is the best way to find out if there > >> >> > > > is > >> >> > > > sufficient interest? > >> >> > > > > >> >> > > > > >> >> > > > I've been stewing on the ideas here around schema evolution, and > >> >> > > > I realize > >> >> > > > the specific feature I am missing is the ability to encode that a > >> >> > > > field > >> >> > > > (i.e. its FieldNode and accompanying Buffers in the RecordBatch) > >> >> > > > is > >> >> > > > empty/has-no-data in O(0) cost (yes; for free). > >> >> > > > > >> >> > > > Might there be interest in adding a "field_id" to the FieldNode > >> >> > > > (which is > >> >> > > > encoded on the RecordBatch flatbuffer)? I see a simple > >> >> > > > forward-compatible > >> >> > > > upgrade (by either keying off of 0, or explicitly set the field > >> >> > > > default to > >> >> > > > -1) which would allow the sender to "skip" fields that have 1) > >> >> > > > FieldNode > >> >> > > > length of zero, and 2) all Buffer's associated at that level (and > >> >> > > > further > >> >> > > > nested) are also equally empty (i.e. Buffer length is zero). > >> >> > > > > >> >> > > > I understand this concept slightly interferes with RecordBatch's > >> >> > > > `length` > >> >> > > > field, and that many implementations use that length to resize the > >> >> > > > root-level FieldNodes. The use-case I have in mind has different > >> >> > > > logical > >> >> > > > lengths per field node; current implementations require sending a > >> >> > > > RecordBatch length of the max length across all root level field > >> >> > > > nodes. I > >> >> > > > believe this requires a copy of data whenever a field node is too > >> >> > > > short; I > >> >> > > > don't know if there is a decent solution to this slight > >> >> > > > inefficiency. I am > >> >> > > > bringing it up because if "skipping a field node when it is > >> >> > > > empty" is a > >> >> > > > feature, then we may not want to allocate space for those nodes > >> >> > > > given that > >> >> > > > the record batch length will likely be greater than zero. > >> >> > > > > >> >> > > > On Wed, Jul 7, 2021 at 8:12 AM Wes McKinney <wesmck...@gmail.com> > >> >> > > > wrote: > >> >> > > > > >> >> > > > > On Wed, Jul 7, 2021 at 2:53 PM David Li <apa...@lidavidm.me> > >> >> > > > > wrote: > >> >> > > > > > > >> >> > > > > > From the Flatbuffers internals doc[1] it appears they are the > >> >> > > > > > same: > >> >> > > > > "Strings are simply a vector of bytes, and are always > >> >> > > > > null-terminated." > >> >> > > > > > >> >> > > > > I see. I took a look at flatbuffers.h, and it appears that > >> >> > > > > changing > >> >> > > > > this field from string to [byte] would be backward-compatible > >> >> > > > > and > >> >> > > > > forward-compatible except with code that expects a null > >> >> > > > > terminator. > >> >> > > > > This is something we could discuss separately if there were > >> >> > > > > enough > >> >> > > > > interest. > >> >> > > > > > >> >> > > > > > [1]: > >> >> > > > > > https://google.github.io/flatbuffers/flatbuffers_internals.html > >> >> > > > > > > >> >> > > > > > -David > >> >> > > > > > > >> >> > > > > > On Wed, Jul 7, 2021, at 05:08, Wes McKinney wrote: > >> >> > > > > > > On Tue, Jul 6, 2021 at 6:33 PM Micah Kornfield < > >> >> > > > emkornfi...@gmail.com> > >> >> > > > > wrote: > >> >> > > > > > > > > >> >> > > > > > > > > > >> >> > > > > > > > > Right, I had wanted to focus the discussion on Flight > >> >> > > > > > > > > as I think > >> >> > > > > schema > >> >> > > > > > > > > evolution or multiplexing streams (more so the latter) > >> >> > > > > > > > > is a > >> >> > > > > property of the > >> >> > > > > > > > > transport and not the stream format itself. If we are > >> >> > > > > > > > > leaning > >> >> > > > > towards just > >> >> > > > > > > > > schema evolution then maybe it makes sense to discuss > >> >> > > > > > > > > it for the > >> >> > > > > IPC stream > >> >> > > > > > > > > format and leverage that in Flight. I'd be interested > >> >> > > > > > > > > in what > >> >> > > > > others think. > >> >> > > > > > > > > >> >> > > > > > > > I tend to agree, I think stream multiplexing is likely a > >> >> > > > > > > > transport > >> >> > > > > level > >> >> > > > > > > > issue. IMO I think schema evolution should be consistent > >> >> > > > > > > > with the > >> >> > > > > IPC > >> >> > > > > > > > stream format and flight. > >> >> > > > > > > > > >> >> > > > > > > > > >> >> > > > > > > > > Nate: it may be worth starting a separate discussion > >> >> > > > > > > > > about more > >> >> > > > > general > >> >> > > > > > > > > metadata in the IPC message. I'm not aware of why > >> >> > > > > > > > > key-value > >> >> > > > > metadata was > >> >> > > > > > > > > chosen/if opaque bytes were considered in the past. > >> >> > > > > > > > > >> >> > > > > > > > > >> >> > > > > > > > I think this was an unfortunate design of the key value > >> >> > > > > > > > metadata > >> >> > > > in > >> >> > > > > > > > Schema.fbs, but I don't think I was around when this > >> >> > > > > > > > decision was > >> >> > > > > made. > >> >> > > > > > > > >> >> > > > > > > I agree that it's unfortunate that we did not use [ byte ] > >> >> > > > > > > instead of > >> >> > > > > > > string for the value in the KeyValue metadata — I think > >> >> > > > > > > this was more > >> >> > > > > > > of an oversight than a deliberate choice (e.g. it was not > >> >> > > > > > > our intent > >> >> > > > > > > to require binary data to be base64-encoded — this is > >> >> > > > > > > something that > >> >> > > > > > > we have to do when encoding binary data in Thrift KeyValue > >> >> > > > > > > metadata > >> >> > > > > > > for Parquet, for example). Is the binary representation of > >> >> > > > > > > [byte] > >> >> > > > > > > different from string? > >> >> > > > > > > > >> >> > > > > > > > >> >> > > > > > > > >> >> > > > > > > > Side Question: Why isn't the IPC stream format a series > >> >> > > > > > > > of the > >> >> > > > flight > >> >> > > > > > > > > protobufs? > >> >> > > > > > > > > >> >> > > > > > > > In addition to what David said, protobufs can't be read > >> >> > > > > > > > directly > >> >> > > > > from a > >> >> > > > > > > > memory-mapped file (they need decoding). This was one of > >> >> > > > > > > > the > >> >> > > > design > >> >> > > > > > > > considerations of using flatbuffers and IPC Stream/File > >> >> > > > > > > > format. > >> >> > > > > > > > > >> >> > > > > > > > I was thinking Micah's comment is more that whatever we > >> >> > > > > > > > do, it > >> >> > > > > should be > >> >> > > > > > > > > clearly specified and edge cases should be considered, > >> >> > > > > > > > > especially > >> >> > > > > if we > >> >> > > > > > > > > might want to 'backport' this into the stream format > >> >> > > > > > > > > later. > >> >> > > > > > > > > >> >> > > > > > > > > >> >> > > > > > > > Yes, for dictionaries we just need to be careful to define > >> >> > > > semantics > >> >> > > > > and > >> >> > > > > > > > ensure implementations are validating them with regards to > >> >> > > > > dictionaries. > >> >> > > > > > > > There likely isn't any need to change current > >> >> > > > > > > > implementations > >> >> > > > though. > >> >> > > > > > > > > >> >> > > > > > > > On Mon, Jun 28, 2021 at 1:25 PM David Li > >> >> > > > > > > > <lidav...@apache.org> > >> >> > > > > wrote: > >> >> > > > > > > > > >> >> > > > > > > > > Right, I had wanted to focus the discussion on Flight > >> >> > > > > > > > > as I think > >> >> > > > > schema > >> >> > > > > > > > > evolution or multiplexing streams (more so the latter) > >> >> > > > > > > > > is a > >> >> > > > > property of the > >> >> > > > > > > > > transport and not the stream format itself. If we are > >> >> > > > > > > > > leaning > >> >> > > > > towards just > >> >> > > > > > > > > schema evolution then maybe it makes sense to discuss > >> >> > > > > > > > > it for the > >> >> > > > > IPC stream > >> >> > > > > > > > > format and leverage that in Flight. I'd be interested > >> >> > > > > > > > > in what > >> >> > > > > others think. > >> >> > > > > > > > > > >> >> > > > > > > > > Especially if we are looking at multiplexing streams - > >> >> > > > > > > > > I would > >> >> > > > > wonder if > >> >> > > > > > > > > that's actually better served by making it easier to > >> >> > > > > > > > > implement > >> >> > > > > using the > >> >> > > > > > > > > Flight implementation as it stands (by managing > >> >> > > > > > > > > concurrent RPC > >> >> > > > > calls and/or > >> >> > > > > > > > > performing the union-of-structs encoding trick for > >> >> > > > > > > > > you), instead > >> >> > > > > of having > >> >> > > > > > > > > to change the protocol. > >> >> > > > > > > > > > >> >> > > > > > > > > Nate: it may be worth starting a separate discussion > >> >> > > > > > > > > about more > >> >> > > > > general > >> >> > > > > > > > > metadata in the IPC message. I'm not aware of why > >> >> > > > > > > > > key-value > >> >> > > > > metadata was > >> >> > > > > > > > > chosen/if opaque bytes were considered in the past. Off > >> >> > > > > > > > > the top > >> >> > > > of > >> >> > > > > my head > >> >> > > > > > > > > if it's for on-disk storage and fully > >> >> > > > > > > > > application-defined it may > >> >> > > > > make sense > >> >> > > > > > > > > to store as a separate file alongside the Arrow file > >> >> > > > > > > > > (indexed by > >> >> > > > > record > >> >> > > > > > > > > batch index) where you can take advantage of whatever > >> >> > > > > > > > > format is > >> >> > > > > most > >> >> > > > > > > > > suitable. > >> >> > > > > > > > > > >> >> > > > > > > > > -David > >> >> > > > > > > > > > >> >> > > > > > > > > On Sun, Jun 27, 2021, at 07:50, Gosh Arzumanyan wrote: > >> >> > > > > > > > > > Hi guys, > >> >> > > > > > > > > > > >> >> > > > > > > > > > 1. Regarding IPC vs Flight: in fact my initial > >> >> > > > > > > > > > suggestion was > >> >> > > > to > >> >> > > > > add this > >> >> > > > > > > > > > feature starting from the IPC(I moved initial write > >> >> > > > > > > > > > up steps to > >> >> > > > > the > >> >> > > > > > > > > bottom > >> >> > > > > > > > > > of the doc). Afterwards David suggested focusing on > >> >> > > > > > > > > > Flight and > >> >> > > > > that's how > >> >> > > > > > > > > > we ended up with the protobufs change in the > >> >> > > > > > > > > > proposal. This > >> >> > > > > being said I > >> >> > > > > > > > > do > >> >> > > > > > > > > > think that the place where this should be impemented > >> >> > > > > > > > > > is a good > >> >> > > > > question > >> >> > > > > > > > > on > >> >> > > > > > > > > > its own. Maybe it makes sense to have this kind of a > >> >> > > > > > > > > > feature in > >> >> > > > > IPC and > >> >> > > > > > > > > > somehow use it in Flight, maybe not. > >> >> > > > > > > > > > 2. The point about dictionaries deserves a dedicated > >> >> > > > > > > > > > section in > >> >> > > > > the > >> >> > > > > > > > > > proposal. Nate and David brought it up and shared some > >> >> > > > insights. > >> >> > > > > I'll try > >> >> > > > > > > > > > to aggregate them and we can continue the discussion > >> >> > > > > > > > > > form > >> >> > > > there. > >> >> > > > > > > > > > > >> >> > > > > > > > > > Cheers, > >> >> > > > > > > > > > Gosh > >> >> > > > > > > > > > > >> >> > > > > > > > > > On Sat., 26 Jun. 2021, 17:26 Nate Bauernfeind, < > >> >> > > > > > > > > natebauernfe...@deephaven.io> > >> >> > > > > > > > > > wrote: > >> >> > > > > > > > > > > >> >> > > > > > > > > > > > > >> >> > > > > > > > > > > > > > makes it more difficult to bring schema > >> >> > > > > > > > > > > > > > evolution back > >> >> > > > > into the > >> >> > > > > > > > > > > > > > IPC Stream format (i.e. it would live only in > >> >> > > > > > > > > > > > > > flight) > >> >> > > > > > > > > > > > > > >> >> > > > > > > > > > > > > Gosh's proposal extends the flatbuffer > >> >> > > > > > > > > > > > > structures not the > >> >> > > > > > > > > protobufs. > >> >> > > > > > > > > > > Can > >> >> > > > > > > > > > > > > you help me understand how difficult it would > >> >> > > > > > > > > > > > > be to bring > >> >> > > > > the > >> >> > > > > > > > > > > `schema_id` > >> >> > > > > > > > > > > > > approach to the IPC stream format? > >> >> > > > > > > > > > > > > >> >> > > > > > > > > > > > I thought we were talking solely about the Flight > >> >> > > > > > > > > > > > Protobuf > >> >> > > > > > > > > definitions - > >> >> > > > > > > > > > > > not the Flatbuffers (and the Google doc at least > >> >> > > > > > > > > > > > only talks > >> >> > > > > about the > >> >> > > > > > > > > > > > Protobufs). > >> >> > > > > > > > > > > > > >> >> > > > > > > > > > > > >> >> > > > > > > > > > > I somehow missed that schema_id is being added to > >> >> > > > > > > > > > > protobuf in > >> >> > > > > the > >> >> > > > > > > > > document. > >> >> > > > > > > > > > > It feels to me that the schema_id is a property > >> >> > > > > > > > > > > that would > >> >> > > > > ideally only > >> >> > > > > > > > > > > apply to the RecordBatch. I better understand > >> >> > > > > > > > > > > Micah's > >> >> > > > > dictionary > >> >> > > > > > > > > concerns, > >> >> > > > > > > > > > > now, too. > >> >> > > > > > > > > > > > >> >> > > > > > > > > > > > Side Question: Why isn't the IPC stream format a > >> >> > > > > > > > > > > > series of > >> >> > > > > the flight > >> >> > > > > > > > > > > > > protobufs? It's a real shame that there is no > >> >> > > > > > > > > > > > > standard > >> >> > > > way > >> >> > > > > to > >> >> > > > > > > > > > > > > capture/replay a stream with app_metadata. > >> >> > > > > > > > > > > > > (Obviously > >> >> > > > > ignoring the > >> >> > > > > > > > > > > > > annoyances around protobuf wrapping > >> >> > > > > > > > > > > > > flatbuffers.) > >> >> > > > > > > > > > > > > >> >> > > > > > > > > > > > The IPC format was defined long before Flight, > >> >> > > > > > > > > > > > and Flight's > >> >> > > > > > > > > app_metadata > >> >> > > > > > > > > > > > was added after Flight's initial definition. Note > >> >> > > > > > > > > > > > an IPC > >> >> > > > > message does > >> >> > > > > > > > > > > have > >> >> > > > > > > > > > > > a provision for key-value metadata, though I > >> >> > > > > > > > > > > > think APIs for > >> >> > > > > that are > >> >> > > > > > > > > not > >> >> > > > > > > > > > > > fully exposed. (See ARROW-6940: > >> >> > > > > > > > > > > > https://issues.apache.org/jira/browse/ARROW-6940 > >> >> > > > > > > > > > > > and > >> >> > > > > despite my > >> >> > > > > > > > > comments > >> >> > > > > > > > > > > > there perhaps we need to unify or at least > >> >> > > > > > > > > > > > consider how > >> >> > > > > Flight's > >> >> > > > > > > > > > > > app_metadata relates to the IPC message > >> >> > > > > > > > > > > > custom_metadata. > >> >> > > > Also > >> >> > > > > > > > > perhaps see > >> >> > > > > > > > > > > > ARROW-1059.) > >> >> > > > > > > > > > > > > >> >> > > > > > > > > > > > >> >> > > > > > > > > > > KeyValue unfortunately is string to string. In > >> >> > > > > > > > > > > flatbuffer > >> >> > > > > strings are > >> >> > > > > > > > > only > >> >> > > > > > > > > > > UTF-8 or 7-bit ASCII. The app_metadata on the other > >> >> > > > > > > > > > > hand is > >> >> > > > > opaque > >> >> > > > > > > > > bytes. > >> >> > > > > > > > > > > The latter is a bit more useful. > >> >> > > > > > > > > > > > >> >> > > > > > > > > > > -- > >> >> > > > > > > > > > > > >> >> > > > > > > > > > > >> >> > > > > > > > > > >> >> > > > > > > > >> >> > > > > > >> >> > > > > >> >> > > > > >> >> > > > -- > >> >> > > >
