Chapter 7. Reactive Programming

I once read a book that suggested that Newton came up with the idea for calculus when he was observing the flow of a river around a reed. I've never been able to find any other source which supports that assertion. It is, however, a nice picture to hold in your mind. Calculus deals with understanding how the state of a system changes over time. Most developers will rarely have to deal with calculus in their day to day work. They will, however, have to deal with systems changing. After all, having a system which doesn't change at all is pretty boring.

Over the last few years a number of different ideas have arisen in the area of treating change as a stream of events – just like the stream that Newton supposedly observed. Given a starting position and a stream of events it should be possible to figure out the state of the system. Indeed, this is the idea behind using an event store. Instead of keeping the final state of an aggregate...