Programming Scala, 2nd Edition

Chapter 17. Tools for Concurrency

The multicore problem was a growing concern in the early 2000s as we hit the end of Moore’s Law for single-core CPUs. We’ve continued to scale performance through increasing numbers of cores and servers, trading vertical scaling for horizontal scaling.

That’s created a challenge for developers, because it has meant writing concurrent software. Concurrency isn’t easy because it has traditionally required coordinated access to shared, mutable state, which means difficult multithreaded programming with tools like locks, mutexes, and semaphores. Failure to coordinate access correctly can result in the spooky action at a distance we mentioned in Chapter 2, where some mutable state you’re using suddenly and unexpectedly changes, due to activity on another thread. Or it can mean race conditions and lock contention.

Functional programming started going mainstream when we learned that embracing immutability and purity largely bypasses these problems. We also saw a renaissance of older approaches to concurrency, like the actor model.

This chapter explores concurrency tools for Scala. You can certainly use any mechanism you’ve used with Java, including the multithreading APIs, message queues, etc. We’ll just discuss Scala-specific tools, starting with an API for a very old approach: single-threaded processes that work together.

The scala.sys.process Package

In some cases, we can use small, synchronous processes that coordinate state through database transactions, ...