Action Pack – From request to response

Action Pack is a framework for handling and responding to web requests. It provides mechanisms for routing (mapping request URLs to actions), defining controllers that implement actions, and generating responses by rendering views, which are templates of various formats. In short, Action Pack provides the view and controller layers in the MVC paradigm.

It consists of several modules:

  • Action Dispatch, which parses information about the web request, handles routing as defined by the user, and does advanced processing related to HTTP such as MIME-type negotiation, decoding parameters in POST/PUT bodies, handling HTTP caching logic, cookies and sessions.

  • Action Controller, which provides a base controller class that can be subclassed to implement filters and actions to handle requests. The result of an action is typically content generated from views.

  • Action View, which handles view template lookup and rendering, and provides view helpers that assist when building HTML forms, Atom feeds and more. Template formats that Action View handles are ERB (embedded Ruby, typically used to inline short Ruby snippets inside HTML), and XML Builder.

With the Ruby on Rails framework, users only directly interface with the Action Controller module. Necessary Action Dispatch functionality is activated by default and Action View rendering is implicitly triggered by Action Controller. However, these modules are designed to function on their own and can be used outside of Rails.

A short rundown of some of the major features:

  • Actions grouped in controller as methods instead of separate command objects and can therefore share helper methods

    class CustomersController < ActionController::Base
      def show
        @customer = find_customer
      def update
        @customer = find_customer
        if @customer.update_attributes(params[:customer])
          redirect_to :action => "show"
          render :action => "edit"
        def find_customer
          Customer.find params[:id]

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  • ERB templates (static content mixed with dynamic output from ruby)

    <% @posts.each do |post| %>
      Title: <%= post.title %>
    <% end %>
    All post titles: <%= @posts.collect{ |p| p.title }.join(", ") %>
    <% unless @person.is_client? %>
      Not for clients to see...
    <% end %>

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  • “Builder” templates (great for XML content, like RSS)

    xml.rss("version" => "2.0") do do
        xml.description "Basecamp: Recent items"
        xml.language "en-us"
        xml.ttl "40"
        @recent_items.each do |item|
          xml.item do

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  • Filters for pre- and post-processing of the response

    class WeblogController < ActionController::Base
      # filters as methods
      before_filter :authenticate, :cache, :audit
      # filter as a proc
      after_filter { |c| c.response.body = Gzip::compress(c.response.body) }
      # class filter
      after_filter LocalizeFilter
      def index
        # Before this action is run, the user will be authenticated, the cache
        # will be examined to see if a valid copy of the results already
        # exists, and the action will be logged for auditing.
        # After this action has run, the output will first be localized then
        # compressed to minimize bandwidth usage
        def authenticate
          # Implement the filter with full access to both request and response

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  • Helpers for forms, dates, action links, and text

    <%= text_field_tag "post", "title", "size" => 30 %>
    <%= link_to "New post", :controller => "post", :action => "new" %>
    <%= truncate(post.title, :length => 25) %>

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  • Layout sharing for template reuse

    class WeblogController < ActionController::Base
      layout "weblog_layout"
      def hello_world
    Layout file (called weblog_layout):
      <html><body><%= yield %></body></html>
    Template for hello_world action:
      <h1>Hello world</h1>
    Result of running hello_world action:
      <html><body><h1>Hello world</h1></body></html>

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  • Routing makes pretty URLs incredibly easy

    match 'clients/:client_name/:project_name/:controller/:action'
    Accessing "/clients/37signals/basecamp/project/index" calls ProjectController#index with
    { "client_name" => "37signals", "project_name" => "basecamp" } in `params`
    From that action, you can write the redirect in a number of ways:
    redirect_to(:action => "edit") =>
    redirect_to(:client_name => "nextangle", :project_name => "rails") =>

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  • Easy testing of both controller and rendered template through ActionController::TestCase

    class LoginControllerTest < ActionController::TestCase
      def test_failing_authenticate
        process :authenticate, :user_name => "nop", :password => ""
        assert flash.has_key?(:alert)
        assert_redirected_to :action => "index"

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  • Automated benchmarking and integrated logging

    Started GET "/weblog" for at Fri May 28 00:41:55
      Processing by WeblogController#index as HTML
    Rendered weblog/index.html.erb within layouts/application (25.7ms)
    Completed 200 OK in 29.3ms
    If Active Record is used as the model, you'll have the database debugging
    as well:
    Started POST "/posts" for at Sat Jun 19 14:04:23
      Processing by PostsController#create as HTML
      Parameters: {"post"=>{"title"=>"this is good"}}
    SQL (0.6ms) INSERT INTO posts (title) VALUES('this is good')
    Redirected to
    Completed 302 Found in 221ms (Views: 215ms | ActiveRecord: 0.6ms)
    You specify a logger through a class method, such as:
    ActionController::Base.logger ="Application Log")
    ActionController::Base.logger ="Application Log")
  • Caching at three levels of granularity (page, action, fragment)

    class WeblogController < ActionController::Base
      caches_page :show
      caches_action :account
      def show
        # the output of the method will be cached as
        # ActionController::Base.page_cache_directory + "/weblog/show.html"
        # and the web server will pick it up without even hitting Rails
        # the output of the method will be cached in the fragment store
        # but Rails is hit to retrieve it, so filters are run
      def update
        List.update(params[:list][:id], params[:list])
        expire_page   :action => "show", :id => params[:list][:id]
        expire_action :action => "account"
        redirect_to   :action => "show", :id => params[:list][:id]

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  • Powerful debugging mechanism for local requests

    All exceptions raised on actions performed on the request of a local user
    will be presented with a tailored debugging screen that includes exception
    message, stack trace, request parameters, session contents, and the
    half-finished response.

    Learn more

Simple example (from outside of Rails)

This example will implement a simple weblog system using inline templates and an Active Record model. So let’s build that WeblogController with just a few methods:

require 'action_controller'
require 'post'

class WeblogController < ActionController::Base
  layout "weblog/layout"

  def index
    @posts = Post.all

  def show
    @post = Post.find(params[:id])

  def new
    @post =

  def create
    @post = Post.create(params[:post])
    redirect_to :action => "show", :id =>

WeblogController::Base.view_paths = [ File.dirname(__FILE__) ]
WeblogController.process_cgi if $0 == __FILE__

The last two lines are responsible for telling ActionController where the template files are located and actually running the controller on a new request from the web-server (e.g., Apache).

And the templates look like this:

  <%= yield %>

  <% @posts.each do |post| %>
    <p><%= link_to(post.title, :action => "show", :id => %></p>
  <% end %>

    <b><%= @post.title %></b><br/>
    <b><%= @post.content %></b>

  <%= form "post" %>

This simple setup will list all the posts in the system on the index page, which is called by accessing /weblog/. It uses the form builder for the Active Record model to make the new screen, which in turn hands everything over to the create action (that’s the default target for the form builder when given a new model). After creating the post, it’ll redirect to the show page using an URL such as /weblog/5 (where 5 is the id of the post).

Download and installation

The latest version of Action Pack can be installed with RubyGems:

% [sudo] gem install actionpack

Source code can be downloaded as part of the Rails project on GitHub


Action Pack is released under the MIT license.


API documentation is at

Bug reports and feature requests can be filed with the rest for the Ruby on Rails project here: