Just to let you know that Kenton Varda an ex-Googler working on Protocol buffer released CapNProto.
http://kentonv.github.io/capnproto There are some really nice features like 'Time-traveling RPC' (aka RPC with continuations) that allows several calls server side without paying for the round trip. Some ports are underway (C, Erlang, Go, Ruby, Rust) but no D for now (just saying :) ). Guillaume