One of the things I’ve always liked least about building web applications is dealing with mod_rewrite. It’s a very useful feature, but it’s quirky and the config languages for webservers are difficult to use (at least from my experience with Apache and Nginx). But like it or not, mod_rewrite is often a necessary part of a web app. Until now…

Recently I had to redo the rewrite rules for pivotallabs.com when we switched from Apache to Nginx, which we did when moving to EngineYard’s cloud hosting. Since then our Nginx config has grown to over 150 lines, mainly to deal with multiple virtual hosts.

Now, managing a custom Nginx config on the EY cloud system isn’t as simple as I’d like, especially when the configs are different on production and demo environments. (Demo is what we call our usual environment for doing feature acceptance.) It’s far easier to use the automatically generated config, but that doesn’t work when you need to support multiple domain names.

The obvious thing to do was to move the rewrite/redirect logic out of the Nginx config. I found a couple Rack middleware components that did something sort of like what we needed, but none of them were sufficient for what we needed. So we created our own.

Refraction is a Rack middleware replacement for mod_rewrite. With Refraction we were able to replace our 150+ line Nginx config with a 50 line Ruby file, and go back to using the standard automatically generated config on EY cloud.

Here’s an example Refraction config file:

Refraction.configure do |req|
  feedburner  = "http://feeds.pivotallabs.com/pivotallabs"

  if req.env['HTTP_USER_AGENT'] !~ /FeedBurner|FeedValidator/ && req.host =~ /pivotallabs.com/
    case req.path
    when %r{^/(talks|blabs|blog).(atom|rss)$}        ; req.found! "#{feedburner}/#{$1}.#{$2}"
    when %r{^/users/(chris|edward)/blog.(atom|rss)$} ; req.found! "#{feedburner}/#{$1}.#{$2}"
    end
  else
    case req.host
    when 'tweed.pivotallabs.com'
      req.rewrite! "http://pivotallabs.com/tweed#{req.path}"
    when /([-w]+.)?pivotallabs.com/
      # passthrough with no change
    else  # wildcard domains (e.g. pivotalabs.com)
      req.permanent! :host => "pivotallabs.com"
    end
  end
end

These rules are extracted from the full config file for pivotallabs.com. They redirect high-traffic syndication feeds to feedburner, rewrite a subdomain (tweed.pivotallabs.com) to a path for that sub-site (pivotallabs.com/tweed), and redirect some aliases to our standard domain name (pivotalabs anyone?).

Refraction is thread-safe, which means you can put it outside the Rack::Lock, something we felt was important for performance. It will never have the performance of mod_rewrite, but it will certainly be better than handling redirections in Rails itself.

Full documentation is available in the README. Contributions welcome.

And of course big thanks to Sam Pierson and Wai Lun Mang who both paired with me on developing Refraction.