Rocket/site/guide/overview.md

# Overview

Rocket provides primitives to build web servers and applications with Rust: the
rest is up to you. In short, Rocket provides routing, pre-processing of
requests, and post-processing of responses. Your application code instructs
Rocket on what to pre-process and post-process and fills the gaps between
pre-processing and post-processing.

## Lifecycle

Rocket's main task is to listen for incoming web requests, dispatch the request
to the application code, and return a response to the client. We call the
process that goes from request to response the "lifecycle". We summarize the
lifecycle as the following sequence of steps:

  1. **Routing**

     Rocket parses an incoming HTTP request into native structures that your
     code operates on indirectly. Rocket determines which request handler to
     invoke by matching against route attributes declared in your application.

  2. **Validation**

     Rocket validates the incoming request against types and guards present in
     the matched route. If validation fails, Rocket _forwards_ the request to
     the next matching route or calls an _error handler_.

  3. **Processing**

     The request handler associated with the route is invoked with validated
     arguments. This is the main business logic of an application. Processing
     completes by returning a `Response`.

  4. **Response**

     The returned `Response` is processed. Rocket generates the appropriate HTTP
     response and sends it to the client. This completes the lifecycle. Rocket
     continues listening for requests, restarting the lifecycle for each
     incoming request.

The remainder of this section details the _routing_ phase as well as additional
components needed for Rocket to begin dispatching requests to request handlers.
The sections following describe the request and response phases as well as other
components of Rocket.

## Routing

Rocket applications are centered around routes and handlers. A _route_ is a
combination of:

  * A set of parameters to match an incoming request against.
  * A handler to process the request and return a response.

A _handler_ is simply a function that takes an arbitrary number of arguments and
returns any arbitrary type.

The parameters to match against include static paths, dynamic paths, path
segments, forms, query strings, request format specifiers, and body data. Rocket
uses attributes, which look like function decorators in other languages, to make
declaring routes easy. Routes are declared by annotating a function, the
handler, with the set of parameters to match against. A complete route
declaration looks like this:

```rust
#[get("/world")]              // <- route attribute
fn world() -> &'static str {  // <- request handler
    "Hello, world!"
}
```

This declares the `world` route to match against the static path `"/world"` on
incoming `GET` requests. The `world` route is simple, but additional route
parameters are necessary when building more interesting applications. The
[Requests](/guide/requests) section describes the available options for
constructing routes.

## Mounting

Before Rocket can dispatch requests to a route, the route needs to be _mounted_.
Mounting a route is like namespacing it. Routes are mounted via the `mount`
method on a `Rocket` instance. A `Rocket` instance is typically created with the
`rocket::ignite()` static method.

The `mount` method takes:

   1. A path to namespace a list of routes under,
   2. A list of route handlers through the `routes!` macro, tying Rocket's code
      generation to your application.

For instance, to mount the `world` route we declared above, we can write the
following:

```rust
rocket::ignite().mount("/hello", routes![world]);
```

This creates a new `Rocket` instance via the `ignite` function and mounts the
`world` route to the `"/hello"` path. As a result, `GET` requests to the
`"/hello/world"` path will be directed to the `world` function.

### Namespacing

When a route is declared inside a module other than the root, you may find
yourself with unexpected errors when mounting:

```rust
mod other {
    #[get("/world")]
    pub fn world() -> &'static str {
        "Hello, world!"
    }
}

use other::world;

fn main() {
  // error[E0425]: cannot find value `static_rocket_route_info_for_world` in this scope
  rocket::ignite().mount("/hello", routes![world]);
}
```

This occurs because the `routes!` macro implicitly converts the route's name
into the name of a structure generated by Rocket's code generation. The solution
is to name the route by a module path instead:

```rust
rocket::ignite().mount("/hello", routes![other::world]);
```

## Launching

Now that Rocket knows about the route, you can tell Rocket to start accepting
requests via the `launch` method. The method starts up the server and waits for
incoming requests. When a request arrives, Rocket finds the matching route and
dispatches the request to the route's handler.

We typically call `launch` from the `main` function. Our complete _Hello,
world!_ application thus looks like:

```rust
#![feature(plugin, decl_macro)]
#![plugin(rocket_codegen)]

extern crate rocket;

#[get("/world")]
fn world() -> &'static str {
    "Hello, world!"
}

fn main() {
    rocket::ignite().mount("/hello", routes![world]).launch();
}
```

Note that we've added the `#![feature(plugin, decl_macro)]` and
`#![plugin(rocket_codegen)]` lines to tell Rust that we'll be using Rocket's
code generation plugin. We've also imported the `rocket` crate into our
namespace via `extern crate rocket`. Finally, we call the `launch` method in the
`main` function.

Running the application, the console shows:

```sh
🔧  Configured for development.
    => address: localhost
    => port: 8000
    => log: normal
    => workers: [logical cores * 2]
    => secret key: generated
    => limits: forms = 32KiB
    => tls: disabled
🛰  Mounting '/hello':
    => GET /hello/world
🚀  Rocket has launched from http://localhost:8000
```

If we visit `localhost:8000/hello/world`, we see `Hello, world!`, exactly as
we expected.

A version of this example's complete crate, ready to `cargo run`, can be found
on
[GitHub](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/hello_world).
You can find dozens of other complete examples, spanning all of Rocket's
features, in the [GitHub examples
directory](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/).