Rocket is ready to launch.

Rocket's main task is to route incoming requests to the appropriate request handler using your application's declared routes. Routes are declared using Rocket's route attributes. The attribute describes the requests that match the route. The attribute is placed on top of a function that is the request handler for that route.

As an example, consider the simple route below:

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

This index route matches any incoming HTTP GET request to /, the index. The handler returns a String. Rocket automatically converts the string into a well-formed HTTP response that includes the appropriate Content-Type and body encoding metadata.

Rocket automatically parses dynamic data in path segments into any desired type. To illustrate, let's use the following route:

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#[get("/hello/<name>/<age>")]
fn hello(name: &str, age: u8) -> String {
    format!("Hello, {} year old named {}!", age, name)
}

This hello route has two dynamic parameters, identified with angle brackets, declared in the route URI: <name> and <age>. Rocket maps each parameter to an identically named function argument: name: &str and age: u8. The dynamic data in the incoming request is parsed automatically into a value of the argument's type. The route is called only when parsing succeeds.

Parsing is directed by the FromParam trait. Rocket implements FromParam for many standard types, including both &str and u8. You can implement it for your own types, too!

Rocket can automatically parse body data, too!

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#[post("/login", data = "<login>")]
fn login(login: Form<UserLogin>) -> String {
    format!("Hello, {}!", login.name)
}

The dynamic parameter declared in the data route attribute parameter again maps to a function argument. Here, login maps to login: Form<UserLogin>. Parsing is again trait-directed, this time by the FromData trait.

The Form type is Rocket's robust form data parser. It automatically parses the request body into the internal type, here UserLogin. Other built-in FromData types include Data, Json, and MsgPack. As always, you can implement FromData for your own types, too!

In addition to dynamic path and data parameters, request handlers can also contain a third type of parameter: request guards. Request guards aren't declared in the route attribute, and any number of them can appear in the request handler signature.

Request guards protect the handler from running unless some set of conditions are met by the incoming request metadata. For instance, if you are writing an API that requires sensitive calls to be accompanied by an API key in the request header, Rocket can protect those calls via a custom ApiKey request guard:

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#[get("/sensitive")]
fn sensitive(key: ApiKey) { ... }

ApiKey protects the sensitive handler from running incorrectly. In order for Rocket to call the sensitive handler, the ApiKey type needs to be derived through a FromRequest implementation, which in this case, validates the API key header. Request guards are a powerful and unique Rocket concept; they centralize application policy and invariants through types.

The return type of a request handler can be any type that implements Responder:

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#[get("/")]
fn route() -> T { ... }

Above, T must implement Responder. Rocket implements Responder for many of the standard library types including &str, String, File, Option, and Result. Rocket also implements custom responders such as Redirect, Flash, and Template.

The task of a Responder is to generate a Response, if possible. Responders can fail with a status code. When they do, Rocket calls the corresponding error catcher, a catch route, which can be declared as follows:

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#[catch(404)]
fn not_found() -> T { ... }

Finally, we get to launch our application! Rocket begins dispatching requests to routes after they've been mounted and the application has been launched. These two steps, usually wrtten in a rocket function, look like:

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#[launch]
fn rocket() -> _ {
    rocket::build().mount("/base", routes![index, another])
}

The mount call takes a base and a set of routes via the routes! macro. The base path (/base above) is prepended to the path of every route in the list, effectively namespacing the routes. #[launch] creates a main function that starts the server. In development, Rocket prints useful information to the console to let you know everything is okay.

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🚀 Rocket has launched from http://127.0.0.1:8000