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