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. > >> > > > > > > > > > > > >> > > > > > > > > > > -- > >> > > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > >> > > > > > > > >> > > > > > >> > > > > >> > > > > >> > > > -- > >> > > > >
