[kentonv]

> support message passing asynchronously, even when the other side is not currently up.

That's probably something that could be implemented in Cap'n Proto as some sort of a persistent transport layer. But since the connections are stateful, it does require that when one end goes down, it comes back up with its state still intact. I have a lot of ideas for how to make this possible it big systems but it's a long way off.

Of course, in the simple case where you do have a defined client and server and the server is only exporting one stateless global service object that the client is using -- which is roughly what 0mq req/rep sockets are for -- then it should be no problem to support this.

> whether RPCs are going to be synchronous or asynchronous

The interface will be asynchronous based on E-style promises (similar to futures). In fact, say you call an RPC which returns a remote object reference, and you immediately want to call another method on that reference. With Cap'n Proto's approach, you will be able to do this whole interaction in one round trip instead of two. This is called "Promise Pipelining". There's a bit (just a bit) more detail here:

http://kentonv.github.io/capnproto/rpc.html