Hey that's me. And yeah, if you don't hear much about Cap'n Proto, it's not because it isn't advancing, but more because I don't really have any reason to advertise it. Cap'n Proto's goal right now -- for me at least -- is not to take over the world or anything, but rather to support the needs of my main project, which is Cloudflare Workers. If other people find it useful to, great! But that's not my focus.
Can you use Capn’proto outside of C++? Last time I looked into it there seemed to be lots of dependencies.
The C++ implementation is the reference implementation which is by far the most complete, and is the only one I maintain personally. The other implementations have varying levels of completion and quality, ranging from "pretty solid" (Rust, Go) to "someone's weekend project one time in 2015". I suppose number of dependencies would depend entirely on the particular implementation.
Correct. It's extensively used at Cloudflare.
I've played with using it to interopt some python data science code with some C++ code efficiently without writing the project in Cython or using a tool like Pybind11. It worked pretty well in my test scenario, but I'm not sure how great of an idea that truly is.
Is the capn'n proto use case similar to something like ZeroMQ or NNG? I'm still not fully sure.
ZeroMQ/NNG are the transport layer. Cap'n Proto is mostly data serialization.