It’s looking like I’m going to need to build a reactive engine for work, so I’m going to prepare for that by writing down what I know about them. I want to look at three ways of building reactive engines: push reactivity, pull reactivity, and the hybrid push/pull combination that is used in a bunch of web frameworks. The Problem Statement The simplest way to visualise reactivity, in my opinion, is as a spreadsheet. You have a series of input cells, containing your initial data, and a series of output cells containing the final results. In between are many more cells containing intermediate computations that need to be done to figure out the final results.