Similarly does anyone use https://capnproto.org/? It's a project I was really interested in a few years back, but I haven't heard much in the way of it lately.
IMO it's the best available option if you can choose to use it; the foundations are well researched, it's well designed, it has many good features that alternatives lack (cough sum types) and overall is robust and has a good, fast C++ and Rust implementation. From a technical perspective it's by far the best option both in design and implementation, if you ask me.
The problem, of course, is that technically worse solutions win for various reasons. One of those reasons is language support. Even if Protocol Buffers is worse in many metrics, it has a lot of language bindings and tools available.
I find myself primarily working in the apache arrow domain these days, so I've done more work with the Apache Flight RPC than anything else (https://arrow.apache.org/docs/format/Flight.html). I'll make sure to keep an eye on the capn'n proto project though. Seems well thought out and very interesting.
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.
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.
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.
