Custom Protocols

For two peers to be able to communicate, they must share a contract. In the world of communication, these are called protocols. ZeroMQ enables programmer to create protocols that suit their needs by removing most of the boilerplate.

You might have realized by now that there is no strict concept of request-reply as a socket operation. Indeed the library does not enforce a client socket to follow a send call by a recv call. This does't mean however that this strict type of request-reply could not be achieved. To do so, a programmer could easily write the following code:

#![allow(unused_variables)]
fn main() {
// Client side
fn request_reply(&mut self, msg: Msg) -> Result<Msg, Error> {
    self.client.send(msg)?;
    self.client.recv_msg()?
}

// Server side
fn run(&mut self) -> Result<(), Error> {
    loop {
        let request = self.server.recv_msg()?;
        let reply = self.on_request(request)?;
        self.server.send(reply)
    }
}
}

This creates an implicit contract between the client and the server. We will disregard the error handling and timeouts for simplicity.

• The client must send one request at a time and wait for one reply.
• The server must wait for a request and send one reply.

Since contract must be ensured at the application level, it must be properly documentated for developpers to be able to respect it.

ZeroMQ does not enforce a particular messaging protocol, instead it offers all the tools to build one.