Why was gRPC nicer to work with in my opinion?
- Comprehensive and readable documentation. Cap'n'Proto has a high level overview on the main page but the documentation is far from complete. For example regarding I/O, a core part of many RPC systems, "Function calls that do I/O must do so asynchronously, and must return a “promise” for the result." is said, then no explanation is given on how to implement a function that does it (all the examples assume an existing library returns kj promises and since that's custom, no such libraries exist). For serialization, how do I build a list of integers? The "Lists" section on the serialization page doesn't explain this at all, it only mentions the init method for lists of lists or lists of blobs. The gRPC docs on the other hand are quite good, with numerous easy to read tutorials and examples.
- Built in support for threading. An event loop is limited to the throughput of a single core and there'll always be a point at which you need either threading or multiple processes. Since I don't want to have a bunch of different copies of a particular thing in memory, it works better for me to have a bunch of threads with access to shared memory. Again, the docs say "While multiple threads are allowed, each thread must have its own event loop." but don't document how to do it (and particularly, do it while sharing a port). Plus, there's no suggestions on how to return a promise that's fulfilled by another thread (this was and still is a big problem for me, I still haven't figured it out). gRPC makes this dead simple by just using threads (or a queue system I haven't looked into much yet).
- No kj. I might be more okay with it if the documentation was better but at the moment it's near-nonexistent and wandering around the code base for ~30 mins just to figure out how to construct and return a promise was pretty painful.
Really, you can sum up most of my problems in one word: Documentation.
Oh and as you said, language support. The Python driver is really important to me yet seems unfinished.