ZeroMQ

A Real-Life Example

In the “XPUB subscription notifications” email thread, Dan Goes asks how to make a publisher that knows when a new client subscribes and sends out previous matching messages. It’s a standard pub-sub technique called “last value caching.” Over a one-way transport like pgm (where subscribers literally send no packets back to publishers), this can’t be done. But over TCP, it can, if we use an XPUB socket and if that socket didn’t cleverly filter out duplicate subscriptions to reduce upstream traffic.

Though I’m not an expert contributor to libzmq, this seemed like a fun problem to solve. How hard could it be? I started by forking the libzmq repository to my own GitHub account, and then cloned it to my laptop, where I built it:

Git clone [e-mail]:hintjens/libzmq.git
cd libzmq
./autogen.sh
./configure
make

Because the libzmq code is neat and well organized, it was quite easy to find the main files to change (xpub.cpp and xpub.hpp). Each socket type has its own source file and class. They inherit from socket_base.cpp, which has this hook for socket-specific options:

//  First, check whether specific socket type overloads the option.
int rc = xsetsockopt (option_, optval_, optvallen_);
if (rc == 0 || errno != EINVAL)
    return rc;

//  If the socket type doesn't support the option, pass it to
//  the generic option parser
return options.setsockopt (option_, optval_, optvallen_);

Then I checked where the XPUB socket filters out duplicate subscriptions, in its ...