Chapter 6. Writing Web Crawlers
So far, you’ve seen single static pages with somewhat artificial canned examples. In this chapter, you’ll start looking at real-world problems, with scrapers traversing multiple pages and even multiple sites.
Web crawlers are called such because they crawl across the web. At their core is an element of recursion. They must retrieve page contents for a URL, examine that page for other URLs, and retrieve those pages, ad infinitum.
Beware, however: just because you can crawl the web doesn’t mean that you always should. The scrapers used in previous examples work great in situations where all the data you need is on a single page. With web crawlers, you must be extremely conscientious of how much bandwidth you are using and make every effort to determine whether there’s a way to make the target server’s load easier.
Traversing a Single Domain
Even if you haven’t heard of Six Degrees of Wikipedia, you may have heard of its namesake, Six Degrees of Kevin Bacon.1 In both games, the goal is to link two unlikely subjects (in the first case, Wikipedia articles that link to each other, and in the second case, actors appearing in the same film) by a chain containing no more than six total (including the two original subjects).
For example, Eric Idle appeared in Dudley Do-Right with Brendan Fraser, who appeared in The Air I Breathe with Kevin Bacon.2 In this case, the chain from Eric Idle to Kevin Bacon is only three subjects long.
In this section, you’ll begin ...
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