Crawl the web breadth-first from a seed url, statefully


There are already tens of Node.js scripts that spider / crawl sites, many more Python and Ruby ones. It isn't clear how they work, or that they even recurse.

This project is a work in progress. I intend to document at least its philosophy, if not methodology, better than the competition.

It's called ruthless because it does not respect robots.txt.

I wanted a stateful queue, in case of stops or restarts. Redis was the first choice, but I had too much metadata. So I'm using Postgres right now. Not sure if that's the best idea. The pages table has these columns:

id, parent_id, url, tag, depth, content, plaintext, queued, fetched, failed, error

The pages table:

  • Seed urls are accompanied by a freeform tag, and enter the database with a depth of 0.
  • The pages they link to inherit that tag, have a parent_id set to the linking page's id.
  • The child pages have a depth of depth + 1 if they are on the same domain (protocol and subdomain insensitive), or depth + 100 if they do not share the domain (defined by the hostname field that Node.js's Url.parse(urlStr) produces).
  • The crawl proceeds breadth-first, so the depth never needs to be updated.

Here is an sample of depths retrieved for a single seed site (a blog) that I let run for a couple of minutes:

SELECT depth, COUNT(depth) FROM pages GROUP BY depth ORDER BY depth;

Depth  Count
    0      1
    1     81
    2    319
    3    731
    4    851
  100     22
  101    151
  102   1071
  103   1593

As it was, I hadn't gotten through the 3-deep sites yet.

Question: should I even keep track of sites >10 deep? Do I care about other domains?

TODO: Add more than one worker! It's currently kind of slow, because most things happen in series. Basically, I think the seen cache can handle most locking issues; as soon as work() fetches a new url, add the url to seen. Even if a page is fetched twice, it's not a big deal!

If the supplied credentials have superuser privileges, the database and pages table will be created automatically.

Otherwise, run the following at your command line to initialize (and reset) everything to the defaults:

dropdb ruthless
createdb ruthless
psql ruthless < schema.sql

Copyright © 2012–2013 Christopher Brown. MIT Licensed.