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by btilly
4669 days ago
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The killer feature that I like for 0MQ is that you can support message passing asynchronously, even when the other side is not currently up. For instance in a request/response pattern, one side might go away, get restarted, reinitialize, and then they carry on as if there wasn't a period in the middle where there was no connection. This kind of robust handling of network interruptions is very convenient for many use cases. However what you describe isn't necessarily going to fit into that. The #1 thing that your description makes me wonder about is whether RPCs are going to be synchronous or asynchronous. So, for instance, if you hand me a data structure with a list objects that are references to data that I want to have, and I decide that I need 10 of them, do I have to pay for the overhead of 10 round trips, or can I say, "I need these 10" and get them all at once? |
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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