Comment by staunch | Hacker News Reader

Comment by staunch | Hacker News Reader

Parent Full thread

Yup. It's very hard to avoid getting stuck in accidental tar pits. One page with inexhaustible URL variations.

  http://example.com/article/story?storyID=19039&ref=329932&sessionID=9043275

  http://example.com/article/story?storyID=19039&ref=902932&sessionID=9023409

  http://example.com/article/story?storyID=19039&ref=904354&sessionID=8230235

  ... infinite ...

Canonical URL meta tags can help.

Filtering out certain query parameters (JSESSIONID, PHPSESSIONID, etc) can help.

Sitemaps help.

I'm rather impressed that search engines do it so well. I imagine the right approach involves examining the contents of the pages and doing checksums, but I'd love to know what the real search engines do.

I do know that a naive crawler will completely fail to crawl any significant portion of the real web without solving this. It's quite possible those 250 million "pages" were actually something like 1 million distinct pages.

>I'm rather impressed that search engines do it so well. I imagine the right approach involves examining the contents of the pages and doing checksums

Interesting point. Q: Is it possible for two different pages to give the same checksum? (Asking for my own info; I do know what checksums are, in overview, from way back, but haven't checked :) them out much.

Given two different pages getting the same checksum should be VERY unlikely. Getting a collision among billions of pages is fairly or very likely depending on the type of checksum. So it is likely to be important to only compare within domains or divide the space by some method.

Also it doesn't help you at all because pages returned that you want to treat as the same aren't actually the same because they include the request time or other unique element.

Edit: Added sentence about comparing with domains.

You could use an edit distance algorithm instead of using checksums. Although that would be really time intensive.

Also, some sites have a LOT of generated pages for a tiny section. For instance, crawling my GitHub account with my homemade crawler was impossible. There is an amazing level of depth there.

Reply on news.ycombinator.com