What changed?

Overheard at a dinner party:

Guest 1: My browser has this feature that remembers what sites I've been visiting, then shows me which ones have changed since I last visited them.

Guest 2: Sounds great.

Guest 1: Yeah, except that most of the time all it means is there are new ads.

Guest 2: Oh.

Guest 1: At least it works with your site, since you don't carry any ads.

(No, the site wasn't Field Notes)

Two similar questions with significantly different answers:

• Have the bytes representing this page changed?
• Has this page changed in any meaningful way?

Computers have a pretty good handle on the first but not the second. The bytes can change without the page really changing, not just because of ads but because of trivial reformatting, or an upgrade in some unseen component, or for any of a number of other reasons. Now that everything's all AJAX-y and a page may really just be a script for fetching and displaying the real content, the page can change without the bytes changing.

The questions above are central to web caching, a whole big ball of wax which (to mix metaphors) I'd rather not wade into at the moment. Fortunately, if your job is just to make sure bytes get distributed in an efficient manner you can forget about the second question and concentrate on the first. And a good thing, that, otherwise there would be no Web As We Know It.

If the second question really is important to you, I have one word, son: metadata.