ZeroMQ

Getting the Message Out

The second classic pattern is one-way data distribution, in which a server pushes updates to a set of clients. Let’s look at an example that pushes out weather updates consisting of a zip code, temperature, and relative humidity. We’ll generate random values, just like the real weather stations do.

Example 1-6 shows the code for the server. We’ll use port 5556 for this application.

Example 1-6. Weather update server (wuserver.c)

//
//  Weather update server
//  Binds PUB socket to tcp://*:5556
//  Publishes random weather updates
//
#include "zhelpers.h"

int main (void)
{
    //  Prepare our context and publisher
    void *context = zmq_ctx_new ();
    void *publisher = zmq_socket (context, ZMQ_PUB);
    int rc = zmq_bind (publisher, "tcp://*:5556");
    assert (rc == 0);
    rc = zmq_bind (publisher, "ipc://weather.ipc");
    assert (rc == 0);

    //  Initialize random number generator
    srandom ((unsigned) time (NULL));
    while (1) {
        //  Get values that will fool the boss
        int zipcode, temperature, relhumidity;
        zipcode     = randof (100000);
        temperature = randof (215) - 80;
        relhumidity = randof (50) + 10;

        //  Send message to all subscribers
        char update [20];
        sprintf (update, "%05d %d %d", zipcode, temperature, relhumidity);
        s_send (publisher, update);
    }
    zmq_close (publisher);
    zmq_ctx_destroy (context);
    return 0;
}

There’s no start and no end to this stream of updates; it’s like a never-ending broadcast (Figure 1-4).

Figure 1-4. Publish-subscribe

Example 1-7 shows the client application, which listens to the ...