ZeroMQ

Handling Multiple Sockets

The main loop of most examples so far has been:

1. Wait for message on socket.

2. Process message.

3. Repeat.

What if we want to read from multiple endpoints at the same time? The simplest way is to connect one socket to all the endpoints and get ØMQ to do the fan-in for us. This is legal if the remote endpoints are in the same pattern, but it would be wrong to connect a PULL socket to a PUB endpoint, for example.

To actually read from multiple sockets all at once, use zmq_poll(). An even better way might be to wrap zmq_poll() in a framework that turns it into a nice event-driven reactor, but that involves significantly more work than we want to cover here.

Let’s start with a dirty hack, partly for the fun of not doing it right, but mainly because it lets me show you how to do non-blocking socket reads. Example 2-1 is a simple example of reading from two sockets using non-blocking reads. This rather confused program acts both as a subscriber to weather updates, and a worker for parallel tasks.

Example 2-1. Multiple socket reader (msreader.c)

//
//  Reading from multiple sockets
//  This version uses a simple recv loop
//
#include "zhelpers.h"

int main (void) 
{
    //  Prepare our context and sockets
    void *context = zmq_ctx_new ();

    //  Connect to task ventilator
    void *receiver = zmq_socket (context, ZMQ_PULL);
    zmq_connect (receiver, "tcp://localhost:5557");

    //  Connect to weather server
    void *subscriber = zmq_socket (context, ZMQ_SUB);
    zmq_connect (subscriber, "tcp://localhost:5556" ...