Designing and creating a computer system is a balancing act: we constantly try to add features and capabilities while keeping the code simple and the system's performance reasonable. Data analysis systems are no different. In fact, they may be worse. Often, data is only partially consistent, and we need to employ a variety of strategies to extract usable data before we can even begin analysis.

This can get out of hand.

Clojure has a number of tools to help us manage our system's complexity. One of the most powerful of these is concurrent programming. This allows us to conceptualize our programs differently. Instead of having monolithic blocks of code that do many things and have direct, tight dependencies, we can structure our program more modularly by composing together many independent modules, each of which do one thing. These communicate by using simple, well-defined protocols. But they all work independently and concurrently—that is, at the same time. Clojure's primary tool...