Using ZeroMQ together with Boost::ASIO

Question

I've got a C++ application that is using ZeroMQ for some messaging. But it also has to provide a SGCI connection for an AJAX / Comet based web service.

For this I need a normal TCP socket. I could do that by normal Posix sockets, but to stay cross platform portable and make my life easier (I hope...) I was thinking of using Boost::ASIO.

But now I have the clash of ZMQ wanting to use it's own zmq_poll() and ASIO it's io_service.run()...

Is there a way to get ASIO to work together with the 0MQ zmq_poll()?

Or is there an other recommended way to achieve such a setup?

Note: I could solve that by using multiple threads - but it's only a little single core / CPU box that'll run that program with a very low amount of SCGI traffic, so multithreading would be a waste of resources...

Answer 1

After reading the documentation here and here, specifically this paragraph

ZMQ_FD: Retrieve file descriptor associated with the socket The ZMQ_FD option shall retrieve the file descriptor associated with the specified socket. The returned file descriptor can be used to integrate the socket into an existing event loop; the ØMQ library shall signal any pending events on the socket in an edge-triggered fashion by making the file descriptor become ready for reading.

I think you can use null_buffers for every zmq_pollitem_t and defer the event loop to an io_service, completely bypassing zmq_poll() altogether. There appear to be some caveats in the aforementioned documentation however, notably

The ability to read from the returned file descriptor does not necessarily indicate that messages are available to be read from, or can be written to, the underlying socket; applications must retrieve the actual event state with a subsequent retrieval of the ZMQ_EVENTS option.

So when the handler for one of your zmq sockets is fired, you'll have to do a little more work before handling the event I think. Uncompiled pseudo-code is below

const int fd = getZmqDescriptorSomehow();
boost::asio::posix::stream_descriptor socket( _io_service, fd );
socket->async_read_some(
    boost::asio::null_buffers(),
    [=](const boost::system::error_code& error)
    {
       if (!error) {
           // handle data ready to be read
       }
     }
);

note you don't have to use a lambda here, boost::bind to a member function would be sufficient.