Revel is a batteries-included web framework in the spirit of Rails or Play! Framework. Many of the same (proven) ideas are incorporated in the framework design and interface.

Revel makes it easy to build web applications using the Model-View-Controller (MVC) pattern by relying on conventions that require a certain structure in your application. In return, it is very light on configuration and enables an extremely fast development cycle.

MVC

Here is a quick summary:

• Models are the essential data objects that describe your application domain. Models also contain domain-specific logic for querying and updating the data.
• Views describe how data is presented and manipulated. In our case, this is the template that is used to present data and controls to the user.
• Controllers handle the request execution. They perform the user’s desired action, they decide which View to display, and they prepare and provide the necessary data to the View for rendering.

There are many excellent overviews of MVC structure online. In particular, the one provided by Play! Framework matches our model exactly.

Life of a Request

Here is an overview of the request processing framework.

Concept summary:

• Revel exposes a single http.Handler, responsible for instantiating the Controller (the context for the request) and passing the request along to the Filter Chain.
• Filters are links in a request processing chain. They may be composed to implement horizontal concerns like request logging, cookie policies, authorization, etc. Most of the built-in functionality is implemented as Filters.
• Actions are the application-specific functions that process the input and produce a Result.

HTTP Handler

Revel builds on top of the Go HTTP server, which creates a go-routine (lightweight thread) to process each incoming request. The implication is that your code is free to block, but it must handle concurrent request processing.

The Revel handler does nothing except hand the request to the Filter chain for processing and, upon completion, apply the result to write the response.

By default, the Revel handler will be registered on "/" to receive all incoming connections. However, applications are free to override this behavior – for example, they may want to use existing http.Handlers rather than re-implementing them within the Revel framework. See the FAQ for more detail.

Filters

Filters implement most request processing functionality provided by Revel. They have a simple interface that allows them to be nested.

The “Filter Chain” is an array of functions, each one invoking the next, until the terminal filter stage invokes the action. For example, one of the first Filters in the chain is the RouterFilter, which decides which Action the request is meant for and saves that to the Controller.

Overall, Filters and the Filter Chain are the equivalent of Rack.

Controllers and Actions

Each HTTP request invokes an action, which handles the request and writes the response. Related actions are grouped into controllers. The Controller type contains relevant fields and methods and acts as the context for each request.

As part of handling a HTTP request, Revel instantiates an instance of your Controller, and it sets all of these properties on the embedded revel.Controller. Revel does not share Controller instances between requests.

---

A Controller is any type that embeds *revel.Controller (directly or indirectly).

type AppController struct {
  *revel.Controller
}

An Action is any method on a Controller that meets the following criteria: * is exported * returns a revel.Result

For example:

func (c AppController) ShowLogin(username string) revel.Result {
	..
	return c.Render(username)
}

The example invokes revel.Controller.Render to execute a template, passing it the username as a parameter. There are many methods on revel.Controller that produce revel.Result, but applications are also free to create their own.

Results

A Result is anything conforming to the interface:

type Result interface {
	Apply(req *Request, resp *Response)
}

Typically, nothing is written to the response until the action and all filters have returned. At that point, Revel writes response headers and cookies (e.g. setting the session cookie), and then invokes Result.Apply to write the actual response content.