1.1. What is concurrency?

At the simplest and most basic level, concurrency is about two or more separate activities happening at the same time. We encounter concurrency as a natural part of life; we can walk and talk at the same time or perform different actions with each hand, and of course we each go about our lives independently of each other—you can watch football while I go swimming, and so on.

1.1.1. Concurrency in computer systems

When we talk about concurrency in terms of computers, we mean a single system performing multiple independent activities in parallel, rather than sequentially, or one after the other. It isn’t a new phenomenon: multitasking operating systems that allow a single computer to run multiple applications at the same time through task switching have been commonplace for many years, and high-end server machines with multiple processors that enable genuine concurrency have been available for even longer. What is new is the increased prevalence of computers that can genuinely run multiple tasks in parallel rather than just giving the illusion of doing so.