Over at my github account (see, I do have one) I just added a minimalist implementation of a Process Virtual Machine (PVM).
Wait, what ?
Let's start with what a PVM is. You can find a thorough description of it here, but in brief it is an engine which can drive automated business processes and workflows. It actually forms the core of JBPM and Activiti. (If you really feel like geeking out on workflow engines you should definitely head over to Joram Barrez's blog, which has all sorts of Activiti craziness in it.)
I really wanted to figure out what makes such an engine tick, and while I can look at the source code for JBPM and Activiti they were never meant as an educational example. These engines have real work to do, and so come with usual amount of hardening and boilerplating. Rather than deep-diving through all that I wanted to see what a minimalist implementation could look like.
So that's where I went. I've got a minimalist PVM capable of running in a Node.js environment. As you'll see there's very little code to it; and I actually spent most of the time getting everything into the form of a literate program.
Because it is very minimalist there are certain features it does not (yet) have. The big ones are wait states (e.g. for human interaction), asynchronous tasks (e.g. for doing IO or REST calls), and persistence. In a future version I'll see if I can add those in a minimalist way as well, but for now this will do to satisfy my curiousity.
If you like, let me know; and feel free to experiment further.