Concepts

Revel is almost a full stack web framework in the spirit of Rails and Play! frameworks.

• Many proven ideas are incorporated into the framework, its design and interface
• Also using golang, it’s also hackable ;-)

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

Revel is very customizable, you can plug in your own template system http server engine or session engine. Revel also allows integration of custom HTTP Mux, so if you have an existing code base you can migrate to Revel at your own pace, or integrate generated code easily. It is the perfect middleware that is designed to work out of the box and provide the ability to implement almost any application.

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.

A detailed overview of the MVC approach is beyond the scope of this guide.

Revel Startup

An event model has been added to revel to make it easier to provide additional control to the application. Events can be added by calling revel.AddInitEventHandler(func(event int, value interface{}) (r int) {}) The event ID’s are called in the following sequence

The Life of a Request

Below is an overview of the request processing framework:

• Revel contains a Server Engine interface. It allows a developer to build their own “engine” to communicate to the MVC architecture of Revel. By default a single Go http.Server gets created to handle http.Requests.
• A revel.Controller ( which is the context for the request) is populated and the request is passed to the Filter chain.
• Filters are links in a request processing chain. They will implement horizontal concerns like request logging, cookie policies, authorization, etc. Most of Revel’s built-in functionality are implemented as Filters.
• Actions are the application-specific controller and method that process the input and produce a Result. An Action is defined as the controller name and method name. For example “App.Index” is an action

Server Engine

Revel allows the developer to build their own Server Engine. It comes with Go’s http.Server out of the box which works great for building up your project. However in production you may need more control so you can replace it (see modules/serverengine/fasthttp) or extend it (see modules/serverengine/newrelic).

Revel’s Server Engine can handle multiple 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.

When a request is received (through the EngineInit.Callback) it is composed into a revel.Controller which is passed to the Filter chain for processing and, upon completion, apply the result to write the response.

By default, the GoEngine will be registered at the "/" url 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 creates that controller.

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

Namespace, Controllers and Methods

Each request invokes an action. An action is defined as the combination of a Controller Name and a Controller Method. For example “App.Index” is an action. An action may also include a namespace like static\App.Index (Namespaces are used to prevent duplicate actions when using a module). This action handles the request and writes the response.

Good program practice would group related methods into one controllers.
The Controller type contains relevant fields and methods and acts as the context for each request.

As part of handling the request, Revel instantiates an instance of a Controller, and it sets all of these properties on the embedded Controller. Revel does not share Controller instances between requests (but it will reuse them).

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

type MyAppController struct {
    *revel.Controller
}

A Action Method is any method on a Controller that meets the following criteria:

• is exported
• returns a revel.Result

For example:

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

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

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.