1. Connecting

We use the ZeroMQ ROUTER-DEALER combination with the load balancer to perform asynchronous request-reply pairs. If the balancer server does not respond in time then the client can resend the request to a different worker. A good timeout value to use is 30 seconds.

To connect to the server using ZeroMQ in C++, we use:

zmq::context_t context(1);
zmq::socket_t socket(context, ZMQ_DEALER);
socket.connect("tcp://host:port");
// Configure socket to not wait at close time.
int linger = 0;
socket.setsockopt(ZMQ_LINGER, &linger, sizeof (linger));

We then poll for replies from the server. The details of zmq_poll() may differ depending on the language.

void backend_cluster::update()
{
    //  Poll socket for a reply, with timeout
    zmq::pollitem_t items[] = { { socket_, 0, ZMQ_POLLIN, 0 } };
    zmq::poll(&items[0], 1, 0);
    //  If we got a reply, process it
    if (items[0].revents & ZMQ_POLLIN)
        receive_incoming();
    // Finally resend any expired requests that we didn't get
    // a response to yet.
    strand_.randomly_queue(
        &backend_cluster::resend_expired, this);
}

void backend_cluster::receive_incoming()
{
    incoming_message response;
    if (!response.recv(socket_))
        return;
    strand_.randomly_queue(
        &backend_cluster::process, this, response);
}