Rocket/site/guide/fairings.md

# Fairings

Fairings are Rocket's approach to structured middleware. They allow for
interposition at various points in the application and request/response
lifecycle through callbacks issued by Rocket.

## Overview

A _fairing_ is any type that implements the [`Fairing`] trait. The `Fairing`
trait is composed of methods that represent callbacks that Rocket will run at
requested points in a program. Through these methods, fairings can rewrite or
record information about requests and responses as well as perform actions when
a Rocket application launches.

[`Fairing`]: https://api.rocket.rs/rocket/fairing/trait.Fairing.html

### Attaching

For a fairing to be active, it must first be _attached_ through the the
[`attach`] method on a [`Rocket`] instance. For instance, to attach fairings
named `req_fairing` and `res_fairing` to a new Rocket instance, you might write:

```rust
rocket::ignite()
    .attach(req_fairing)
    .attach(res_fairing)
    .launch();
```

Once a fairing is attached, Rocket will execute its callbacks at the appropiate
time.

[`attach`]: https://api.rocket.rs/rocket/struct.Rocket.html#method.attach
[`Rocket`]: https://api.rocket.rs/rocket/struct.Rocket.html

### Callbacks

A fairing can implement any combination of the following four callbacks:

  * **Attach**

    An attach callback is called when a fairing is first attached via the
    [`attach`](https://api.rocket.rs/rocket/struct.Rocket.html#method.attach)
    method. An attach callback can arbitrarily modify the `Rocket` instance
    being constructed and optionally abort launch.

  * **Launch**

    A launch callback is called immediately before the Rocket application has
    launched. A launch callback can inspect the `Rocket` instance being
    launched.

  * **Request**

    A request callback is called just after a request is received. A request
    callback can modify the request at will and peek into the incoming data. It
    may not, however, abort or respond directly to the request; these issues are
    better handled via request guards or via response callbacks.

  * **Response**

    A response callback is called when a response is ready to be sent to the
    client. A response callback can modify the response at will. For example, a
    response callback can provide a default response when the user fails to
    handle the request by checking for 404 responses.


### Execution Order

Fairings are executed in the order in which they are attached: the first
attached fairing has its callbacks executed before all others. Because fairing
callbacks may not be commutative, the order in which fairings are attached may
be significant.

### Ad-Hoc Fairings

For simple occasions, implementing the `Fairing` trait can be cumbersome. This
is why Rocket provides the [`AdHoc`] type, which creates a fairing from a simple
function or clusure.

Using the `AdHoc` type is easy: simply call the `on_attach`, `on_launch`,
`on_request`, or `on_response` constructors to create an `AdHoc` structure from
a function or closure. Then, attach the structure to a `Rocket` instance. Rocket
takes care of the rest.

As an example, the code below creates a `Rocket` instance with two attached
ad-hoc fairings. The first, a launch fairing, simply prints a message indicating
that the application is about to the launch. The second, a request fairing,
changes the method of all requests to `PUT`.

```rust
use rocket::fairing::AdHoc;
use rocket::http::Method;

rocket::ignite()
    .attach(AdHoc::on_launch(|_| {
        println!("Rocket is about to launch! Exciting!");
    }))
    .attach(AdHoc::on_request(|req, _| {
        req.set_method(Method::Put);
    }));
```

[`AdHoc`]: https://api.rocket.rs/rocket/fairing/enum.AdHoc.html

## Considerations

Fairings are a large hammer that can easily be abused and misused. If you
are considering writing a `Fairing` implementation, first consider if it is
appropriate to do so. While middleware is often the best solution to some
problems in other frameworks, it is often a suboptimal solution in Rocket.
This is because Rocket provides richer mechanisms such as [request guards]
and [data guards] that can be used to accomplish the same objective in a
cleaner, more composable, and more robust manner.

As a general rule of thumb, only _globally applicable actions_ should be
implemented via fairings. For instance, you should _not_ use a fairing to
implement authentication or authorization (preferring to use a [request
guard] instead) _unless_ the authentication or authorization applies to the
entire application. On the other hand, you _should_ use a fairing to record
timing and/or usage statistics or to implement global security policies.

[request guard]: https://api.rocket.rs/rocket/request/trait.FromRequest.html
[request guards]: https://api.rocket.rs/rocket/request/trait.FromRequest.html
[data guards]: https://api.rocket.rs/rocket/data/trait.FromData.html

## Implementing

A fairing must implement the [`Fairing`] trait. A `Fairing` implementation has
one required method: [`info`], which returns an [`Info`] structure. This
structure is used by Rocket to assign a name to the `Fairing` and determine
which callbacks to actually issue on the `Fairing`. A `Fairing` can also
implement any of the available callbacks: [`on_attach`], [`on_launch`],
[`on_request`], and [`on_response`].

[`Info`]: https://api.rocket.rs/rocket/fairing/struct.Info.html
[`info`]: https://api.rocket.rs/rocket/fairing/trait.Fairing.html#tymethod.info
[`on_attach`]: https://api.rocket.rs/rocket/fairing/trait.Fairing.html#method.on_attach
[`on_launch`]: https://api.rocket.rs/rocket/fairing/trait.Fairing.html#method.on_launch
[`on_request`]: https://api.rocket.rs/rocket/fairing/trait.Fairing.html#method.on_request
[`on_response`]: https://api.rocket.rs/rocket/fairing/trait.Fairing.html#method.on_response

### Restrictions

A `Fairing` must be `Send + Sync + 'static`. This means that the fairing must be
sendable across thread boundaries (`Send`), thread-safe (`Sync`), and have only
`'static` references, if any (`'static`). Note that these bounds _do not_
prohibit a `Fairing` from holding state: the state need simply be thread-safe
and statically available or heap allocated.

### Example

Imagine that we want to record the number of `GET` and `POST` requests that our
application has received. While we could do this with request guards and managed
state, it would require us to annotate every `GET` and `POST` request with
custom types, polluting handler signatures. Instead, we can create a simple
fairing that acts globally.

The `Counter` fairing below records the number of all `GET` and `POST` requests
received. It makes these counts available at a special `'/counts'` path.

```rust
struct Counter {
    get: AtomicUsize,
    post: AtomicUsize,
}

impl Fairing for Counter {
    fn info(&self) -> Info {
        Info {
            name: "GET/POST Counter",
            kind: Kind::Request | Kind::Response
        }
    }

    fn on_request(&self, request: &mut Request, _: &Data) {
        match request.method() {
            Method::Get => self.get.fetch_add(1, Ordering::Relaxed),
            Method::Post => self.post.fetch_add(1, Ordering::Relaxed),
            _ => return
        }
    }

    fn on_response(&self, request: &Request, response: &mut Response) {
        // Don't change a successful user's response, ever.
        if response.status() != Status::NotFound {
            return
        }

        if request.method() == Method::Get && request.uri().path() == "/counts" {
            let get_count = self.get.load(Ordering::Relaxed);
            let post_count = self.post.load(Ordering::Relaxed);
            let body = format!("Get: {}\nPost: {}", get_count, post_count);

            response.set_status(Status::Ok);
            response.set_header(ContentType::Plain);
            response.set_sized_body(Cursor::new(body));
        }
    }
}
```

For brevity, imports are not shown. The complete example can be found in the
[`Fairing`
documentation](https://api.rocket.rs/rocket/fairing/trait.Fairing.html#example).