Event sourcing primer

I'll try and keep the theory information a little lighter here, as there is a ton of practical, hands on stuff to go over with Akka Persistence. Still, it's worth going over the high-level concepts in a bit more detail first so that you're mentally prepared to attack the code changes that we'll make.

As I stated in the opener, event sourcing is a technique where, instead of storing the current state of the data explicitly, we store the event stream for a particular entity. Using that event stream, we can arrive at the current state when necessary by replaying the events in memory within our code. We also have the added benefit of having a full audit log for each entity instance that can provide all kinds of benefits, which I'll break out in this section.

At the heart of the event sourcing model, there is the simple concept of Action (command) and Reaction (event), depicted in the following diagram:

As users interact with your system, their actions (represented in the...