ZeroMQ

ØMQ’s Built-in Proxy Function

It turns out that the core loop in the previous section’s rrbroker is very useful, and reusable. It lets us build pub-sub forwarders and shared queues and other little intermediaries with very little effort. ØMQ wraps this up in a single method, zmq_proxy():

zmq_proxy (frontend, backend, capture);

The two sockets (or three if we want to capture data) must be properly connected, bound, and configured. When we call the zmq_proxy() method, it’s exactly like starting the main loop of rrbroker. Let’s rewrite the request-reply broker to call zmq_proxy(), and re-badge this as an expensive-sounding “message queue” (people have charged houses for code that did less). Example 2-6 shows the result.

Example 2-6. Message queue broker (msgqueue.c)

//
//  Simple message queuing broker
//  Same as request-reply broker but using QUEUE device
//
#include "zhelpers.h"

int main (void) 
{
    void *context = zmq_ctx_new ();

    //  Socket facing clients
    void *frontend = zmq_socket (context, ZMQ_ROUTER);
    int rc = zmq_bind (frontend, "tcp://*:5559");
    assert (rc == 0);

    //  Socket facing services
    void *backend = zmq_socket (context, ZMQ_DEALER);
    rc = zmq_bind (backend, "tcp://*:5560");
    assert (rc == 0);

    //  Start the proxy
    zmq_proxy (frontend, backend, NULL);

    //  We never get here...
    zmq_close (frontend);
    zmq_close (backend);
    zmq_ctx_destroy (context);
    return 0;
}

If you’re like most ØMQ users, at this stage you’re starting to think, “What kind of evil stuff can I do if ...