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Not too long ago (and to be sure, today still), developers used to construct so-called server pages. Such a page would have a bunch of code at the top of an otherwise static page, just above the opening HTML tag. The markup would be littered with all sorts of code, and it wouldn t be at all unusual to see the database being accessed, forms being processed, sessions being set, and all manner of logic being performed in-line. The web server would be responsible for controlling the application one page redirecting to another, running the code, and then dumping the results to the screen. We won t get into the multitude of reasons why this is a bad idea, except to say that it presents the problem of coupling. In this scenario, the business logic and the view are all mashed together, making things more difficult to maintain and debug. ASP and PHP pages are notable offenders, and if you re coming from either of these camps, the concept of separating concerns might be foreign at first. Here s a way to think about it that might help. Imagine taking all that code and logic from the top of each page and sticking it in one place, leaving only the HTML behind. Then instead of using the web server to invoke each page as you would with a static site, have the web server call on a single dispatcher that finds the code you want to execute and calls it. The code it would invoke the file that contains all your processing logic extracted from the server page would be called the controller. Instead of dividing logic among pages, it would be divided into actions. The single biggest advantage of this pattern is that the processing logic is decoupled from the view and safely contained in one place. As you ll see, it s a lot easier to work this

way. The interplay between actions is considerably easier to visualize and understand when it is not spread out over a host of locations. Your server pages become lightweight views, left to handle only the simplest of instructions, if any at all.

Instead of boring you with more theory about controllers, views, and MVC, we think it s best to dig right in and start writing some real-world code. We re going to continue building our events application, highlighting the finer points and describing the places where convention wins out over configuration. Along the way, we ll touch on some of the most essential controller and view concepts. By the end of this walk-through, you ll have a complete grasp of how the Rails request cycle works, and you ll have a working example to refer to and expand on in the subsequent chapters. The purpose of this walk-through is not to examine each and every aspect of Action Pack in detail, but rather to give you a practical overview of how the components routes, controllers, helpers, views, layouts, and partials work together to control your application and construct its interface.

Links and URLs are important in web applications. They serve as the entry point to the application and contain all the information required to route an incoming request to the controller and action that will handle it. Before we get into the meat of creating controllers and defining actions, we need to spend a few minutes talking about how we get from request to response. As we explained earlier, it all starts with routing.

In Rails, all the rules for mapping URLs to controllers are a matter of configuration. You ll find the routes.rb file in the config directory. If you open that file in your editor now and examine it, you ll see the default route defined last. We ve already introduced it, but it bears repeating:

map.connect ':controller/:action/:id'

Routes work based on pattern matching and can include variables to set directly within the pattern. Here, the pattern is a three-segment string partitioned by slashes (/) and containing variables to set, where the variables are prefixed by a colon (:). The first segment sets the :controller variable, the second the :action variable, and the third the :id variable. These variables will be used to determine the controller and action to invoke, and the parameters to send along. The pattern is matched from left to right, and the variables are optional. If the :id variable is empty, only the controller and action will be set. If the :action variable is empty, only the :controller variable will be set, and so on.