Interesting that Cap'n Proto[1] is not even mentioned given that this library has similar if not the same goals and lineage.
"Cap'n'Proto promises to reduce Protocol Buffers much like FlatBuffers does, though with a more complicated binary encoding and less flexibility (no optional fields to allow deprecating fields or serializing with missing fields for which defaults exist). It currently also isn't fully cross-platform portable (lack of VS support)."
"Optional" fields (ones that don't take space on the wire at all) are not clearly much of an advantage. The usual use of optional fields in Protobufs was to emulate unions, which Protobufs itself never supported directly, but Cap'n Proto does. Also, when wire size really matters, Cap'n Proto offers a very fast compression pass which deflates zeros, and unset fields are always zero, largely mitigating this issue.
The lack of VS support in Cap'n Proto is of course a very fair criticism, and a huge sore point in Cap'n Proto adoption. MSVC is finally catching up in its C++11 support so I hope this can be solved soon.
(Disclosure: I'm the author of Cap'n Proto, and also the former maintainer of Protocol Buffers.)
You've heard of Valve Software right? Their game engine, being a distant relative of Quake, used the old quake message packing format up until 1 or 2 years ago. Since then they have retrofitted all their popular games to use protocol buffers instead since it allows them to more easily extend their net code without breaking backwards compatibility (up to a point of course). In the context of games, optional fields are very important as are default values.
Also in this context; the author of flatbuffers is Wouter van Oortmerssen, author of the Cube and Cube2 game engines: http://sauerbraten.org
From the white paper:
> In particular, FlatBuffers focus is on mobile hardware (where memory size and memory bandwidth is even more constrained than on desktop hardware), and applications that have the highest performance needs: games.
The question is how a field is represented on the wire if it's defined in the schema but you don't assign it a value before serializing. With FlatBuffers, it won't take any space on the wire (although it still takes space in the vtable, but that can be amortized with many instances of the same structure). With Cap'n Proto, all fields are located at fixed offsets from the start of the struct, therefore they take space even if they aren't set. You can truncate unused fields off the end, but not from the middle.
Also, note that Cap'n Proto uses pointers for variable-width fields. The pointer can be null if the field is not set. So the maximum wasted space for an unset field is 8 bytes, all of which are zero and therefore compress nicely.
Cap'n Proto's approach is exactly the approach used by in-memory data structures in C, C++, and all other high-performance programming languages. If your C struct contains a field you don't use, it still takes space in memory. I would think this would be preferable for games.
I really don't see how you can even pretend that cap'n proto is less complicated. Yes there's an extra indirection at runtime, but the benefit is great simplicity, and quite possibly better performance when the vtable is hot in the cache.
Guessing from the name I expected FlatBuffers to be a unified file format for on-disk and on-RAM presence, so that you can just mmap the disk file and you have the data-structure in memory. No need to translate between on disk and on disk representations. I am not claiming that it is so, this is what I expected it to be, given the name. But after spending some time looking around I am still confused if that's what it is. Would appreciate confirmation either way. Have to look at the code when I get around to it.
I have an use case in mind for a tree of appendable/insertable matrices. HDf5 comes close, but isnt quite what I am looking for.
EDIT @vertex-four thanks for the clarification much appreciated.
No. It's a serialisation format that's specifically designed so that data structures in this format can be used as-is by programs, without unnecessarily copying data into or out of the format to use it. The RPC layer is then built on top of that, but the format could also be used for mmapped files, etc.
But if anything it was the serialization that distracted me from the RPC. Cap'n Proto was originally (briefly) a capability-based RPC system on top of Protobufs. :)