Fairings

Fairings are Rocket's approach to structured middleware. With fairings, your application can hook into the request lifecycle to record or rewrite information about incoming requests and outgoing responses.

Overview

A fairing is a special name for a type that implements the Fairing trait. Fairings receive callbacks from Rocket when certain events, like that of an incoming request, occur. Rocket passes information about the event to the fairing, and the fairing can do what it wants with the information. This includes rewriting data when applicable, recording information about the event or data, or doing nothing at all.

Fairings are a lot like middleware in other frameworks with a few key distinctions:

Fairings cannot terminate or respond to an incoming request directly.
Fairings cannot inject arbitrary, non-request data into a request.
Fairings can prevent an application from launching.
Fairings can inspect and modify the application's configuration.

If you are familiar with middleware from other frameworks, you may find yourself reaching for fairings instinctively. Before doing so, consider whether Rocket provides a better solution to your problem: While middleware may be the best solution to a problem in another framework, it is often a suboptimal solution in Rocket. Rocket provides richer mechanisms such as request guards and data guards that can be used to solve problems in a cleaner, more composable, and more robust manner.

As a general rule of thumb, only globally applicable actions should be effected through fairings. 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 global security policies.

Attaching

Fairings are registered with Rocket via the attach method on a Rocket instance. Only when a fairing is attached will its callbacks fire. As an example, the following snippet attached two fairings, req_fairing and res_fairing, to a new Rocket instance:

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

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.

Callbacks

There are four events for which Rocket issues fairing callbacks. Each of these events is described below:

Attach (on_attach)

An attach callback is called when a fairing is first attached via the attach method. An attach callback can arbitrarily modify the Rocket instance being constructed and optionally abort launch. Attach fairings are commonly used to parse and validate configuration values, aborting on bad configurations, and inserting the parsed value into managed state for later retrieval.
Launch (on_launch)

A launch callback is called immediately before the Rocket application has launched. A launch callback can inspect the Rocket instance being launched. A launch callback can be a convenient hook for launching services related to the Rocket application being launched.
Request (on_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 (on_response)

A response callback is called when a response is ready to be sent to the client. A response callback can modify part or all of the response. As such, a response fairing can be used to provide a response when the greater applications fails to by rewriting 404 responses as desired. As another example, response fairings can also be used to inject headers into all outgoing responses.

Implementing

Recall that a fairing is any type that implements 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 the set of callbacks the fairing is registering for. A Fairing can implement any of the available callbacks: on_attach, on_launch, on_request, and on_response. Each callback has a default implementation that does absolutely nothing.

Requirements

A type implementing Fairing is required to 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 code for a Counter fairing below implements exactly this. The fairing receives a request callback, where it increments a counter on each GET and POST request. It also receives a response callback, where it responds to unrouted requests to the /counts path by returning the recorded number of counts.

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

impl Fairing for Counter {
    // This is a request and response fairing named "GET/POST Counter".
    fn info(&self) -> Info {
        Info {
            name: "GET/POST Counter",
            kind: Kind::Request | Kind::Response
        }
    }

    // Increment the counter for `GET` and `POST` requests.
    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
        }

        // Rewrite the response to return the current counts.
        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.

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 on AdHoc to create an AdHoc structure from a function or closure.

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.

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);
    }));

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