Concurrency and parallelism

There was probably a time when concurrency and parallelism meant the same thing in computer science. That time is long gone now. Many people will tell you what concurrency is not: “concurrency is not parallelism,” but when it comes to telling what concurrency is, a simple definition is usually elusive. Different definitions of concurrency give different aspects of the concept because concurrency is not how the real world works. The real world works with parallelism. I will try to summarize some of the core ideas behind concurrency, hoping you can understand the abstract nature of it well enough so that you can apply it to solve practical problems.

Many things around us act independently at the same time. There are probably people around you minding their own business, and sometimes, they interact with you and with each other. All these things happen in parallel, so parallelism is the natural way of thinking about multiple independent things...