ZeroMQ

A Load-Balancing Message Broker

The previous example is half-complete. It can manage a set of workers with dummy requests and replies, but it has no way to talk to clients.

If we add a second frontend ROUTER socket that accepts client requests, and turn our example into a proxy that can switch messages from frontend to backend, we get a useful and reusable tiny load-balancing message broker (Figure 3-7).

Figure 3-7. Load-balancing broker

This broker does the following:

• Accepts connections from a set of clients

• Accepts connections from a set of workers

• Accepts requests from clients and holds these in a single queue

• Sends these requests to workers using the load-balancing pattern

• Receives replies back from workers

• Sends these replies back to the original requesting client

The broker code (listed in Example 3-5) is fairly long, but worth understanding.

Example 3-5. Load-balancing broker (lbbroker.c)

//
//  Load-balancing broker
//  Clients and workers are shown here in-process
//
#include "zhelpers.h"
#include <pthread.h>

#define NBR_CLIENTS 10
#define NBR_WORKERS 3

//  Dequeue operation for queue implemented as array of anything
#define DEQUEUE(q) memmove (&(q)[0], &(q)[1], sizeof (q) - sizeof (q [0]))

//  Basic request-reply client using REQ socket.
//  Since s_send and s_recv can't handle 0MQ binary identities we
//  set a printable text identity to allow routing.
//
static void *
client_task ...