Akka's clustering framework allows you to build fault-tolerant, decentralized, peer-to-peer based clusters without any single point of failure. It's a good system to build on top of if you want dynamic elasticity in your application services. It allows you to add and remove nodes on the fly, increasing or decreasing your total CPU power as the need arises.

The clustering system is somewhat complex, both in how it works and in the different things you can do with it. I definitely suggest that you read through the documentation for it on the Akka site if you plan on using it in your apps. I'll do my best to first give a high-level overview of the basics, such as how it works and how to enable it. Then, we'll shift gears into two specific features within—Clustering Sharding and Cluster Singleton—and how we will use them in the bookstore app.