Capnp_rpc_lwtSourceCap'n Proto RPC using the Cap'n Proto serialisation and Lwt for concurrency.
A promise for a response structure. You can use the generated _get_pipelined functions on a promise to get a promise for a capability inside the promise, and then pipeline messages to that promise without waiting for the response struct to arrive.
A capability is a reference to an object, or to a promise for an object. You can invoke methods on a capability even while it is still only a promise.