Sequential, Concurrent, Parallel

The words sequential, concurrent, and parallel answer different questions. Sequential work has no overlap. Concurrent work has multiple tasks in progress during an overlapping period. Parallel work executes more than one operation at the same instant. A program can be concurrent without being parallel, and it can use both.

We will develop these distinctions with a batch downloader. Given several URLs, the program fetches each response and performs a small amount of processing. Later versions will use threads, async tasks, queues, and processes, but the first version is an ordinary loop.