I'm trying to design a system in Node.js (an attempt at solving one of my earlier problems, using Node's concurrency) but I'm running into trouble figuring out how to draw a plan of how the thing should operate.

I'm getting very tripped up thinking in terms of callbacks instead of returned values. The flow isn't linear, and it's really boggling my ability to draft it. How does one draw out an operational flow for an event-driven system?

I need something I can look at and say "Ok, yes, that's how it will work. I'll start it over here, and it will give me back these results over here."

Pictures would be very helpful for this one. Thanks.

Edit: I'm looking for something more granular than UML, specifically, something that will help me transition from a blocking and object-oriented programming structure, where I'm comfortable, to a non-blocking and event driven structure, where I'm unfamiliar.