ZeroMQ

Pub-Sub Tracing (Espresso Pattern)

Let’s start this chapter by looking at a way to trace pub-sub networks. In Chapter 2 we saw a simple proxy that used these to do transport bridging. The zmq_proxy() method has three arguments: a frontend and backend socket that it bridges together, and a capture socket to which it will send all messages.

The code is deceptively simple, as you can see in Example 5-1.

Example 5-1. Espresso pattern (espresso.c)

//
//  Espresso pattern
//  This shows how to capture data using a pub-sub proxy
//
#include "czmq.h"

//  The subscriber thread requests messages starting with
//  A and B, then reads and counts incoming messages.

static void
subscriber_thread (void *args, zctx_t *ctx, void *pipe)
{
    //  Subscribe to "A" and "B"
    void *subscriber = zsocket_new (ctx, ZMQ_SUB);
    zsocket_connect (subscriber, "tcp://localhost:6001");
    zsockopt_set_subscribe (subscriber, "A");
    zsockopt_set_subscribe (subscriber, "B");

    int count = 0;
    while (count < 5) {
        char *string = zstr_recv (subscriber);
        if (!string)
            break;              //  Interrupted
        free (string);
        count++;
    }
    zsocket_destroy (ctx, subscriber);
}

The publisher sends random messages starting with A–J, as seen in Example 5-2.

Example 5-2. Espresso pattern (espresso.c): publisher thread

static void
publisher_thread (void *args, zctx_t *ctx, void *pipe)
{
    void *publisher = zsocket_new (ctx, ZMQ_PUB);
    zsocket_bind (publisher, "tcp://*:6000");

    while (!zctx_interrupted) {
        char string [10];
        sprintf (string, "%c-%05d", randof (10) +