The actual language itself is very nice https://capnproto.org/language.html, and it has a bunch of really great security features. I actually think of it the other way around and don't understand why it's not used more.
- It has yet to escape the ZeroVer versioning scheme and produce a “stable” API.
- Maturity for languages other than C++ can be a bit spotty.
- The RPC protocol is cool, but apparently most of it was never implemented. The C++ implementation only implements “level 1.”
Cap’n Proto is undeniably cooler than Protobuf, although in practice I worry primarily about maturity and support.
The biggest issue is undeniably that the language maturity isn't quite as solid as the alterntaives. You've got C++, Rust, Python, C#, maybe one or two others. Only C++ is first class. So gRPC and Protobuf win here. But the actual RPC design and IDL design of Cap'n is solid and way more well engineered than the alternatives, IMO.
I have no opinions on the ZeroVer thing. Cap'n Proto is very stable in my experience but I see why others would raise their eyebrows.
A lot of their tests were checking for Exception raises and were basically testing AttributeError due to a change in interface instead of the initial error they were testing for.
With it being fundamental to Cloudflare Workers at this point, you should assume it's support will be solid for at least as long as Cloudflare Workers is a significant part of Cloudflare's business.
Having worked with it over years, the API stability is there. Backwards compatibility is given very strong emphasis, , even with early versions.
> Maturity for languages other than C++ can be a bit spotty.
The language support is a pain at times, especially as the more esoteric language support tends to be built with a specific use case in mind and may not use and implement all the features of the protocol in sometimes undocumented ways.
People can and do write implementations and contribute to existing ones. The C++ and its Javascript and Python bindings work well enough for many use cases. The Rust ones are also fine. There are probably others that have good support now, too.
> The RPC protocol is cool, but apparently most of it was never implemented. The C++ implementation only implements “level 1.”
The RPC protocol is perfectly usable, just not for the higher levels – which all do things not found in other RPC systems. Level 2 can be trivially implemented between client and server at the app level, as anything similar would need to be in any other RPC system. Level 3 sounds way beyond standard RPC needs.
The language maturity one, I can concur though. The rust implementation has been fine, but pycapnp was plagued with memory leaks until recently, and curre tly has performance problems[1] that makes it unsuitable for my use case.
As for the level 2+ protocol, it looks cool on paper, but it looks like a lot of added complexity to address an advanced use case.
The language itself has generics and sum types, which makes it much better at modelling the problem domain than protobuf.