What is concurrency?

Concurrency is often confused with actual methods of implementing it. Some programmers also think that it is a synonym for parallel processing. This is the reason why we need to start by properly defining concurrency. Only then will we be able to properly understand various concurrency models and their key differences.

First and foremost, concurrency is not the same as parallelism. Concurrency is also not a matter of application implementation. Concurrency is a property of a program, algorithm, or problem, whereas parallelism is just one of the possible approaches to problems that are concurrent.

In Leslie Lamport's 1976 paper Time, Clocks, and the Ordering of Events in Distributed Systems, he defines the concept of concurrency as follows:

"Two events are concurrent if neither can causally affect the other."

By extrapolating events to programs, algorithms, or problems, we can say that something is concurrent if it can be fully or...