# 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 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 by your application.

  2. **Validation**

    Rocket validates the incoming request against types and request 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 the 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.

## Routing

Rocket applications are centered around routes and handlers.

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

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. Rocket instances can be created with the
`ignite()` static method.

The `mount` method takes **1)** a path to namespace a list of routes under, and
**2)** a list of route handlers through the `routes!` macro. The `routes!` macro
ties Rocket's code generation to your application.

For instance, to mount the `world` route we declared above, we would use the
following code:

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

Altogether, 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)]
#![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)]` 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}
🛰  Mounting '/world':
    => 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/v0.2.8/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/v0.2.8/examples/).

## Configuration

At any point in time, a Rocket application is operating in a given
_configuration environment_. There are three such environments:

   * `development` (short: `dev`)
   * `staging` (short: `stage`)
   * `production` (short: `prod`)

Without any action, Rocket applications run in the `development` environment.
The environment can be changed via the `ROCKET_ENV` environment variable. For
example, to launch the `Hello, world!` application in the `staging` environment,
we can run:

```sh
ROCKET_ENV=stage cargo run
```

You'll likely need `sudo` for the command to succeed since `staging` defaults to
listening on port `80`. Note that you can use the short or long form of the
environment name to specify the environment, `stage` _or_ `staging` here. Rocket
tells us the environment we have chosen and its configuration when it launches:

```sh
$ sudo ROCKET_ENV=staging cargo run

🔧  Configured for staging.
    => address: 0.0.0.0
    => port: 80
    => log: normal
    => workers: {logical cores}
🛰  Mounting '/':
    => GET /
🚀  Rocket has launched from http://0.0.0.0:80...
```

Configuration settings can be changed in one of two ways: via the `Rocket.toml`
configuration file, or via environment variables.

### Configuration File

A `Rocket.toml` file can be used to specify the configuration parameters for
each environment. The file is optional. If it is not present, the default
configuration parameters are used.

The file must be a series of TOML tables, at most one for each environment and
an optional "global" table, where each table contains key-value pairs
corresponding to configuration parameters for that environment. If a
configuration parameter is missing, the default value is used. The following is
a complete `Rocket.toml` file, where every standard configuration parameter is
specified with the default value:

```toml
[development]
address = "localhost"
port = 8000
workers = max(number_of_cpus, 2)
log = "normal"

[staging]
address = "0.0.0.0"
port = 80
workers = max(number_of_cpus, 2)
log = "normal"

[production]
address = "0.0.0.0"
port = 80
workers = max(number_of_cpus, 2)
log = "critical"
```

The `workers` parameter is computed by Rocket automatically; the value above is
not valid TOML syntax.

The "global" pseudo-environment can be used to set and/or override configuration
parameters globally. A parameter defined in a `[global]` table sets, or
overrides if already present, that parameter in every environment. For example,
given the following `Rocket.toml` file, the value of `address` will be
`"1.2.3.4"` in every environment:

```toml
[global]
address = "1.2.3.4"

[development]
address = "localhost"

[production]
address = "0.0.0.0"
```

### Extras

In addition to overriding default configuration parameters, a configuration file
can also define values for any number of _extra_ configuration parameters. While
these parameters aren't used by Rocket directly, other libraries, or your own
application, can use them as they wish. As an example, the
[Template](https://api.rocket.rs/rocket_contrib/struct.Template.html) type
accepts a value for the `template_dir` configuration parameter. The parameter
can be set in `Rocket.toml` as follows:

```toml
[development]
template_dir = "dev_templates/"

[production]
template_dir = "prod_templates/"
```

This sets the `template_dir` extra configuration parameter to `"dev_templates/"`
when operating in the `development` environment and `"prod_templates/"` when
operating in the `production` environment. Rocket will prepend the `[extra]` tag
to extra configuration parameters when launching:

```sh
🔧  Configured for development.
    => ...
    => [extra] template_dir: "dev_templates/"
```

### Environment Variables

All configuration parameters, including extras, can be overridden through
environment variables. To override the configuration parameter `{param}`, use an
environment variable named `ROCKET_{PARAM}`. For instance, to override the
"port" configuration parameter, you can run your application with:

```sh
ROCKET_PORT=3721 cargo run

🔧  Configured for staging.
    => ...
    => port: 3721
```

Environment variables take precedence over all other configuration methods: if a
variable is set, it will be used as that parameter's value.