What are controllers?

A controller is the component of a web application that accepts input and controls the flow of requests. This generally includes accepting a specific content type, authenticating, matching request parameters, and so on.

In an MVC framework, a controller is generally used as an entry point to the web application. The controller first matches a request based on its parameters (such as request method, route, and so on), and uses the model layer to fetch any requested data. Finally, it uses the views to present the data in a consumable way. In many modern MVC frameworks such as Phoenix, there also exists an additional component, the router, which takes on the responsibility of matching an incoming request to a controller for further handling.

The request-response cycle described is better shown in the following diagram:

Figure 4.1 – The MVC flow

Now that you’ve looked at a brief overview of controllers in general,...