In the previous section, we talked about writing concurrent code in the JVM and how it can be both error prone and also inefficient due to the overhead of obtaining instance locks. Since multi-threaded apps and multi-core machines are a way of life, we can't really ignore concurrency in the apps that we write on the JVM. Thankfully, Akka actors take the pain out of writing concurrency-aware components. In fact, it's so simple and consistent that I often forget how complicated the underlying concept used to be without Akka.

A lot of that simplicity stems from the way actor components communicate with each other via message passing. If component A (an actor) needs to use some functionality represented by component B (another actor), then component A will send component B a message, and if necessary, asynchronously react to a response that B can route back to it.

This model of message passing is different than life in the synchronous world. In a synchronous...