Adam @ Bitscribe

There doesn’t seem to be any good documentation for creating custom Mongrel handlers. The Mongrel site seems to be completely silent on the subject. I was able to extract what I needed to know from this article and Ezra Zygmuntowicz’s slides.

It’s actually quite easy to make a mongrel handler, so here’s a quick tutorial that should tell you everything you need to know.

First, why would you want to make a mongrel handler? The main reason would be speed and scalability. Mongrel is multithreaded even though Rails isn’t. Mongrel is fast, Rails is slow. Of course, Rails is Rails, and Mongrel is just a webserver; but many types of apps may have some services that are hit far more than others, and thus it would be worth writing what amounts to a stripped-down version of the controller action in order to handle just those requests.

For example, if you were writing eBay in Rails, you might want to implement the API calls which are used to check the status of an auction as a custom mongrel handler. This part of the site may be accessed with great frequency by third party apps trying to keep a current status, and chances are generating the results are pretty simple (converting a row in the database to XML and printing it out).

Here’s what that might look like:

class StatusHandler < Mongrel::HttpHandler
   def initialize
      @mutex = Mutex.new
   end

   def process(request, response)
      id = request.params['PATH_INFO'].slice(1, 20)  # trim leading slash

      response.start(200) do |head, out|
         head["Content-Type"] = "application/xml"
         out.write status(id).to_xml
      end
   end

   def status(id)
      rows = @mutex.synchronize { ActiveRecord::Base.connection.select_all("select * from auctions where id=#{id.to_i}") }
      return { 'error' => ‘No such record’ } if rows.length < 1
      return rows.first
   end
end

uri "/status", :handler => StatusHandler.new, :in_front => true

Name this status_handler.rb and drop it into the root dir of your Rails project. Instead of running script/server, execute this command:

mongrel_rails start -S status_handler.rb

Assuming you’ve set up a little database with some sample data in the table named by the handler (”auctions” in my example), accessing the url http://localhost:3000/status/1 will show the data for the record with id=1.

Now what’s so hot about this? For one, it’s fast - see Erza’s slides for benchmarks. But more importantly - to my mind - is that a long request won’t hold up any others. Try putting a “sleep 10″ as the first line of the process method. Restart your server and hit the status url again. The connection will hang temporarily, but now open another tab and hit any other page in your Rails app. Notice that it displays right away, even though the other tab is still loading.

The downside is that you don’t have Rails, and as it turns out, we like Rails. So suddenly you’re stuck doing a lot of your own dirty work. Here, for example, I load up ActiveRecord and manage the database connection and raw sql manupulation. (This whole thing could be done in one line as a Rails controller: respond_to { |f| f.xml { Auction.find(params[:id]).to_xml } }) Parsing the request string can be time-consuming so I went for simplicity - String#slice instead of a regular expression or tokenization. You also have to protect against CGI parameter attacks, which I again simplify with a to_i.

I wasn’t able to figure out how to load a custom handler from a mongrel yaml config file. It seems like the keyword should be config_script, but it doesn’t seem to produce the same result as -S. Anyone knows how to make this work, please comment.

Now that you know how to write a mongrel handler, the real fun can begin. In my next post I’ll describe how this can be used for server-push connections.

Update: Rick Olson improved the code by trimming out the superfluous establish_connection, I’ve used his version above.

This entry was posted on Sunday, May 6th, 2007 at 11:39 pm and is filed under Ruby. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.