Deeper in the site they have a benchmarks page[1] that has a short "why not Cap'n Proto":

"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)."

[1] http://google.github.io/flatbuffers/md__benchmarks.html

Hmm, my personal (biased) opinion is that using variable offsets and vtables (as FlatBuffer does) is a lot more complicated than Cap'n Proto's fixed offsets.

"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.)

I think you have maybe misunderstood the purpose of why flatbuffers exists; message packing for real time high performance games. It's not a replacement for Protocol Buffers or Cap'n Proto.

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.

I think there's some confusion here. To be clear, Cap'n Proto allows you to add new fields to a message without breaking backwards compatibility, and Cap'n Proto implements default values.

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.

IME, optional fields are used for...optional fields. Like when you have a 50-field data object but only want to transmit 5 of those fields.

I think if you have a 50 field data object, and you have cases where you transmit only 5 of those fields, the problem is your data modelling, not the serialization library.

I think you haven't worked on games. 99% of the fields in game objects are the defaults, with some minor customization (e.g. position).

Games are all I've worked on for over a decade. See my other comments.

I think looking up values based on a simple offset is far simpler to all the clever rules and "parsing" that happens in capn' proto (some of which no doubt caused by the desire to support streaming "automatically", an anti-feature IMO, streaming should happen at a higher level to take advantage of domain knowledge).

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.

I dont know full details of Cap'n Proto, so please correct me if I am wrong. My impression is that Cap'n Proto is a re-implementation of protocol buffers with additional functionality for RPC wrapped in a futures/promises abstraction. Would that be a reasonable summary of Cap'n Proto ?

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.

> My impression is that Cap'n Proto is a re-implementation of protocol buffers with additional functionality for RPC wrapped in a futures/promises abstraction.

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.

I always felt that the RPC should live in an extra project to not distract from the format part.

They are compiled as separate libraries, so you can use the serialization layer without linking in the RPC layer.

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. :)