ZeroMQ

ROUTER Broker and DEALER Workers

Anywhere you can use REQ, you can use DEALER. There are two specific differences:

• The REQ socket always sends an empty delimiter frame before any data frames; the DEALER does not.

• The REQ socket will send only one message before it receives a reply; the DEALER is fully asynchronous.

The synchronous versus asynchronous behavior has no effect on our example since we’re doing strict request-reply. It is more relevant when we come to recovering from failures, which we’ll address in Chapter 4.

Now let’s look at exactly the same example, but with the REQ socket replaced by a DEALER socket (Example 3-4).

Example 3-4. ROUTER-to-DEALER (rtdealer.c)

//
//  ROUTER-to-DEALER example
//
#include "zhelpers.h"
#include <pthread.h>

#define NBR_WORKERS 10

static void *
worker_task (void *args)
{
    void *context = zmq_ctx_new ();
    void *worker = zmq_socket (context, ZMQ_DEALER);
    s_set_id (worker);          //  Set a printable identity
    zmq_connect (worker, "tcp://localhost:5671");

    int total = 0;
    while (1) {
        //  Tell the broker we're ready for work
        s_sendmore (worker, "");
        s_send (worker, "Hi Boss");

        //  Get workload from broker, until finished
        free (s_recv (worker));     //  Envelope delimiter
        char *workload = s_recv (worker);
...

The code is almost identical, except that the worker uses a DEALER socket and reads and writes that empty frame before the data frame. This is the approach I use when I want to maintain compatibility with REQ workers.

However, remember the reason for that empty ...