Plugging Rack into Rails

2009-03-03T13:30:27Z

The interwebs are all a’buzz about Rack. For those that haven’t been following along, Rack is a specification and a library for connecting web servers to Ruby libraries (a.k.a. “Middleware”). It’s basically the Web 2.0 version of CGI, except that it doesn’t suck and it’s just for Ruby.

Rails 2.3 has Rack baked in. It uses Rack for things like sessions and parameter parsing. But what if you want to add your own middleware to a Rails app?

It’s really easy! In the init.rb of a plugin, or just in a Rails initializer:

ActionController::Dispatcher.middleware.use MyMiddleware

This will append your middleware to the end of the “stack”, so it will be executed after all of Rails’ middleware, but before anything else in the Rails framework.

But what if you need to massage request parameters before Rails parses them? All you need to do is insert your middleware in the stack before Rails parses the params. Here is the current list of middleware used by Rails:

Rack::Lock
ActionController::Failsafe
ActionController::Session::CookieStore
ActionController::Session::MemCacheStore
ActiveRecord::SessionStore
Rails::Rack::Metal
ActionController::RewindableInput
ActionController::ParamsParser
Rack::MethodOverride
Rack::Head
ActiveRecord::QueryCache

So you would need to insert it before ActionController::ParamsParser:

ActionController::Dispatcher.middleware.insert_after 'ActionController::Failsafe', MyMiddleware

Unfortunately, there’s not an insert_before method (yet), so I just inserted after the Failsafe.

If you don’t like the way that one of the existing pieces of middleware handles things, you can swap it out for your own version:

ActionController::Dispatcher.middleware.swap 'Rails::Rack::Metal', HeavyMetal

So there you have it. Eat, drink, and go write middleware.

opensoul.org - Plugging Rack into Rails Changes

Plugging Rack into Rails