Now we will extend the sequential crawler to download the web pages in parallel. Note that if misused, a threaded crawler could request content too fast and overload a web server or cause your IP address to be blocked. To avoid this, our crawlers will have a delay flag to set the minimum number of seconds between requests to the same domain.

The Alexa list example used in this chapter covers 1 million separate domains, so this problem does not apply here. However, a delay of at least one second between downloads should be considered when crawling many web pages from a single domain in future.

How threads and processes work

Here is a diagram of a process containing multiple threads of execution:

When a Python script or other computer program is run, a process is created containing the code and state. These processes are executed by the CPU(s) of a computer. However, each CPU can only execute a single process at a time and will quickly switch between them to give the impression...