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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

Any type that implements the Fairing trait is a fairing. Fairings hook into Rocket's request lifecycle, receiving callbacks for events such as incoming requests and outgoing responses. Rocket passes information about these events to the fairing; the fairing can do what it wants with the information. This includes rewriting requests or responses, recording information about the event, or doing nothing at all.

Rocket’s fairings are a lot like middleware from other frameworks, but they bear 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, remember that Rocket provides a rich set of mechanisms such as request guards and data guards that can be used to solve problems in a clean, composable, and robust manner.

! warning

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 all or the overwhelming majority of the application. On the other hand, you should use a fairing to record timing and usage statistics or to enforce 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:

# use rocket::launch;
#[launch]
fn rocket() -> _ {
    # let req_fairing = rocket::fairing::AdHoc::on_request("example", |_, _| Box::pin(async {}));
    # let res_fairing = rocket::fairing::AdHoc::on_response("example", |_, _| Box::pin(async {}));
    rocket::build()
        .attach(req_fairing)
        .attach(res_fairing)
}

Fairings are executed in the order in which they are attached: the first attached fairing has its callbacks executed before all others. A fairing can be attached any number of times. Except for singleton fairings, all attached instances are polled at runtime. Fairing callbacks may not be commutative; the order in which fairings are attached may be significant.

Callbacks

There are five events for which Rocket issues fairing callbacks. Each of these events is breifly described below and in details in the Fairing trait docs:

Ignite (on_ignite)

An ignite callback is called during ignition An ignite callback can arbitrarily modify the Rocket instance being built. They are commonly used to parse and validate configuration values, aborting on bad configurations, and inserting the parsed value into managed state for later retrieval.
Liftoff (on_liftoff)

A liftoff callback is called immediately after a Rocket application has launched. A liftoff callback can inspect the Rocket instance being launched. A liftoff 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 application fails by rewriting 404 responses as desired. As another example, response fairings can also be used to inject headers into all outgoing responses.
Shutdown (on_shutdown)

A shutdown callback is called when shutdown is triggered. At this point, graceful shutdown has commenced but not completed; no new requests are accepted but the application may still be actively serving existing requests. All registered shutdown fairings are run concurrently; resolution of all fairings is awaited before resuming shutdown.

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_ignite, on_liftoff, on_request, on_response, and on_shutdown. 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

As an example, 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.

use std::io::Cursor;
use std::sync::atomic::{AtomicUsize, Ordering};

use rocket::{Request, Data, Response};
use rocket::fairing::{Fairing, Info, Kind};
use rocket::http::{Method, ContentType, Status};

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

#[rocket::async_trait]
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.
    async fn on_request(&self, request: &mut Request<'_>, _: &mut Data<'_>) {
        match request.method() {
            Method::Get => self.get.fetch_add(1, Ordering::Relaxed),
            Method::Post => self.post.fetch_add(1, Ordering::Relaxed),
            _ => return
        };
    }

    async fn on_response<'r>(&self, request: &'r Request<'_>, response: &mut Response<'r>) {
        // 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(body.len(), Cursor::new(body));
        }
    }
}

The complete example can be found in the Fairing documentation.

Ad-Hoc Fairings

For simpler cases, implementing the Fairing trait can be cumbersome. This is why Rocket provides the AdHoc type, which creates a fairing from a simple function or closure. Using the AdHoc type is easy: simply call the on_ignite, on_liftoff, on_request, on_response, or on_shutdown constructors on AdHoc to create a fairing from a function or closure.

As an example, the code below creates a Rocket instance with two attached ad-hoc fairings. The first, a liftoff fairing named "Liftoff Printer", prints a message indicating that the application has launched. The second named "Put Rewriter", a request fairing, rewrites the method of all requests to be PUT.

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

rocket::build()
    .attach(AdHoc::on_liftoff("Liftoff Printer", |_| Box::pin(async move {
        println!("...annnddd we have liftoff!");
    })))
    .attach(AdHoc::on_request("Put Rewriter", |req, _| Box::pin(async move {
        req.set_method(Method::Put);
    })))
    .attach(AdHoc::on_shutdown("Shutdown Printer", |_| Box::pin(async move {
        println!("...shutdown has commenced!");
    })));

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