Guide

The ins and outs of Rocket, in detail.

Requests #

Together, a route’s attribute and function signature specify what must be true about a request in order for the route’s handler to be called. You’ve already seen an example of this in action:

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#[get("/world")]
fn handler() { .. }

This route indicates that it only matches against GET requests to the /world route. Rocket ensures that this is the case before handler is called. Of course, you can do much more than specify the method and path of a request. Among other things, you can ask Rocket to automatically validate:

The route attribute and function signature work in tandem to describe these validations. Rocket’s code generation takes care of actually validating the properties. This section describes how to ask Rocket to validate against all of these properties and more.

Methods #

A Rocket route attribute can be any one of get, put, post, delete, head, patch, or options, each corresponding to the HTTP method to match against. For example, the following attribute will match against POST requests to the root path:

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#[post("/")]

The grammar for these attributes is defined formally in the rocket_codegen API docs.

HEAD Requests #

Rocket handles HEAD requests automatically when there exists a GET route that would otherwise match. It does this by stripping the body from the response, if there is one. You can also specialize the handling of a HEAD request by declaring a route for it; Rocket won’t interfere with HEAD requests your application handles.

Reinterpreting #

Because browsers can only send GET and POST requests, Rocket reinterprets request methods under certain conditions. If a POST request contains a body of Content-Type: application/x-www-form-urlencoded, and the form’s first field has the name _method and a valid HTTP method name as its value (such as "PUT"), that field’s value is used as the method for the incoming request. This allows Rocket applications to submit non-POST forms. The todo example makes use of this feature to submit PUT and DELETE requests from a web form.

Dynamic Segments #

You can declare path segments as dynamic by using angle brackets around variable names in a route’s path. For example, if we want to say Hello! to anything, not just the world, we can declare a route like so:

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#[get("/hello/<name>")]
fn hello(name: &RawStr) -> String {
    format!("Hello, {}!", name.as_str())
}

If we were to mount the path at the root (.mount("/", routes![hello])), then any request to a path with two non-empty segments, where the first segment is hello, will be dispatched to the hello route. For example, if we were to visit /hello/John, the application would respond with Hello, John!.

Any number of dynamic path segments are allowed. A path segment can be of any type, including your own, as long as the type implements the FromParam trait. Rocket implements FromParam for many of the standard library types, as well as a few special Rocket types. For the full list of supplied implementations, see the FromParam API docs. Here’s a more complete route to illustrate varied usage:

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#[get("/hello/<name>/<age>/<cool>")]
fn hello(name: String, age: u8, cool: bool) -> String {
    if cool {
        format!("You're a cool {} year old, {}!", age, name)
    } else {
        format!("{}, we need to talk about your coolness.", name)
    }
}

Raw Strings #

You may have noticed an unfamiliar RawStr type in the code example above. This is a special type, provided by Rocket, that represents an unsanitzed, unvalidated, and undecoded raw string from an HTTP message. It exists to separate validated string inputs, represented by types such as String, &str, and Cow<str> types, from unvalidated inputs, represented by &RawStr. It provides helpful methods to convert the unvalidated string into a validated one.

Because &RawStr implements FromParam, it can be used as the type of a dynamic segment, as in the example above. When used as the type of a dynamic segment, a RawStr points to a potentially undecoded string. By constrast, a String is guaranteed to be decoded. Which you should use depends on whether you want direct but potentially unsafe access to the string (&RawStr), or safe access to the string at the cost of an allocation (String).

Forwarding #

Let’s take a closer look at the route attribute and signature pair from the last example:

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#[get("/hello/<name>/<age>/<cool>")]
fn hello(name: String, age: u8, cool: bool) -> String { ... }

What if cool isn’t a bool? Or, what if age isn’t a u8? When a parameter type mismatch occurs, Rocket forwards the request to the next matching route, if there is any. This continues until a route doesn’t forward the request or there are no remaining routes to try. When there are no remaining routes, a customizable 404 error is returned.

Routes are attempted in increasing rank order. Rocket chooses a default ranking from -4 to -1, detailed in the next section, for all routes, but a route’s rank can also be manually set with the rank attribute. To illustrate, consider the following routes:

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#[get("/user/<id>")]
fn user(id: usize) -> T { ... }

#[get("/user/<id>", rank = 2)]
fn user_int(id: isize) -> T { ... }

#[get("/user/<id>", rank = 3)]
fn user_str(id: &RawStr) -> T { ... }

Notice the rank parameters in user_int and user_str. If we run this application with the routes mounted at the root, requests to /user/<id> will be routed as follows:

  1. The user route matches first. If the string at the <id> position is an unsigned integer, then the user handler is called. If it is not, then the request is forwarded to the next matching route: user_int.

  2. The user_int route matches next. If <id> is a signed integer, user_int is called. Otherwise, the request is forwarded.

  3. The user_str route matches last. Since <id> is a always string, the route always matches. The user_str handler is called.

Forwards can be caught by using a Result or Option type. For example, if the type of id in the user function was Result<usize, &RawStr>, then user would never forward. An Ok variant would indicate that <id> was a valid usize, while an Err would indicate that <id> was not a usize. The Err’s value would contain the string that failed to parse as a usize.

By the way, if you were to omit the rank parameter in the user_str or user_int routes, Rocket would emit an error and abort launch, indicating that the routes collide, or can match against similar incoming requests. The rank parameter resolves this collision.

Default Ranking #

If a rank is not explicitly specified, Rocket assigns a default ranking. By default, routes with static paths and query strings have lower ranks (higher precedence) while routes with dynamic paths and without query strings have higher ranks (lower precedence). The table below describes the default ranking of a route given its properties.

static pathquery stringrankexample
yesyes-4/hello?world=true
yesno-3/hello
noyes-2/<hi>?world=true
nono-1/<hi>

Multiple Segments #

You can also match against multiple segments by using <param..> in a route path. The type of such parameters, known as segments parameters, must implement FromSegments. Segments parameters must be the final component of a path: any text after a segments parameter will result in a compile-time error.

As an example, the following route matches against all paths that begin with /page/:

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#[get("/page/<path..>")]
fn get_page(path: PathBuf) -> T { ... }

The path after /page/ will be available in the path parameter. The FromSegments implementation for PathBuf ensures that path cannot lead to path traversal attacks. With this, a safe and secure static file server can be implemented in 4 lines:

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#[get("/<file..>")]
fn files(file: PathBuf) -> Option<NamedFile> {
    NamedFile::open(Path::new("static/").join(file)).ok()
}

Format #

A route can specify the data format it is willing to accept or respond with using the format route parameter. The value of the parameter is a string identifying an HTTP media type. For instance, for JSON data, the string application/json can be used.

When a route indicates a payload-supporting method (PUT, POST, DELETE, and PATCH), the format route parameter instructs Rocket to check against the Content-Type header of the incoming request. Only requests where the Content-Type header matches the format parameter will match to the route.

As an example, consider the following route:

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#[post("/user", format = "application/json", data = "<user>")]
fn new_user(user: Json<User>) -> T { ... }

The format parameter in the post attribute declares that only incoming requests with Content-Type: application/json will match new_user. (The data parameter is described in the next section.)

When a route indicates a non-payload-supporting method (GET, HEAD, and OPTIONS), the format route parameter instructs Rocket to check against the Accept header of the incoming request. Only requests where the preferred media type in the Accept header matches the format parameter will match to the route.

As an example, consider the following route:

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#[get("/user/<id>", format = "application/json")]
fn user(id: usize) -> Json<User> { ... }

The format parameter in the get attribute declares that only incoming requests with application/json as the preferred media type in the Accept header will match user.

Request Guards #

Request guards are one of Rocket’s most powerful instruments. As the name might imply, a request guard protects a handler from being called erroneously based on information contained in an incoming request. More specifically, a request guard is a type that represents an arbitrary validation policy. The validation policy is implemented through the FromRequest trait. Every type that implements FromRequest is a request guard.

Request guards appear as inputs to handlers. An arbitrary number of request guards can appear as arguments in a route handler. Rocket will automatically invoke the FromRequest implementation for request guards before calling the handler. Rocket only dispatches requests to a handler when all of its guards pass.

As an example, the following dummy handler makes use of three request guards, A, B, and C. An input can be identified as a request guard if it is not named in the route attribute. This is why param is not a request guard.

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#[get("/<param>")]
fn index(param: isize, a: A, b: B, c: C) -> ... { ... }

Request guards always fire in left-to-right declaration order. In the example above, the order will be A followed by B followed by C. Failure is short-circuiting; if one guard fails, the remaining are not attempted. To learn more about request guards and implementing them, see the FromRequest documentation.

Custom Guards #

You can implement FromRequest for your own types. For instance, to protect a sensitive route from running unless an ApiKey is present in the request headers, you might create an ApiKey type that implements FromRequest and then use it as a request guard:

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#[get("/sensitive")]
fn sensitive(key: ApiKey) -> &'static str { ... }

You might also implement FromRequest for an AdminUser type that authenticates an administrator using incoming cookies. Then, any handler with an AdminUser or ApiKey type in its argument list is assured to only be invoked if the appropriate conditions are met. Request guards centralize policies, resulting in a simpler, safer, and more secure applications.

Forwarding Guards #

Request guards and forwarding are a powerful combination for enforcing policies. To illustrate, we consider how a simple authorization system might be implemented using these mechanisms.

We start with two request guards:

We now use these two guards in combination with forwarding to implement the following three routes, each leading to an administrative control panel at /admin:

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#[get("/admin")]
fn admin_panel(admin: AdminUser) -> &'static str {
    "Hello, administrator. This is the admin panel!"
}

#[get("/admin", rank = 2)]
fn admin_panel_user(user: User) -> &'static str {
    "Sorry, you must be an administrator to access this page."
}

#[get("/admin", rank = 3)]
fn admin_panel_redirect() -> Redirect {
    Redirect::to("/login")
}

The three routes above encode authentication and authorization. The admin_panel route only succeeds if an administrator is logged in. Only then is the admin panel displayed. If the user is not an admin, the AdminUser route will forward. Since the admin_panel_user route is ranked next highest, it is attempted next. This route succeeds if there is any user signed in, and an authorization failure message is displayed. Finally, if a user isn’t signed in, the admin_panel_redirect route is attempted. Since this route has no guards, it always succeeds. The user is redirected to a log in page.

Cookies #

Cookies is an important, built-in request guard: it allows you to get, set, and remove cookies. Because Cookies is a request guard, an argument of its type can simply be added to a handler:

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use rocket::http::Cookies;

#[get("/")]
fn index(cookies: Cookies) -> Option<String> {
    cookies.get("message")
        .map(|value| format!("Message: {}", value))
}

This results in the incoming request’s cookies being accessible from the handler. The example above retrieves a cookie named message. Cookies can also be set and removed using the Cookies guard. The cookies example on GitHub illustrates further use of the Cookies type to get and set cookies, while the Cookies documentation contains complete usage information.

Private Cookies #

Cookies added via the Cookies::add() method are set in the clear. In other words, the value set is visible by the client. For sensitive data, Rocket provides private cookies.

Private cookies are just like regular cookies except that they are encrypted using authenticated encryption, a form of encryption which simultaneously provides confidentiality, integrity, and authenticity. This means that private cookies cannot be inspected, tampered with, or manufactured by clients. If you prefer, you can think of private cookies as being signed and encrypted.

The API for retrieving, adding, and removing private cookies is identical except methods are suffixed with _private. These methods are: get_private, add_private, and remove_private. An example of their usage is below:

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/// Retrieve the user's ID, if any.
#[get("/user_id")]
fn user_id(cookies: Cookies) -> Option<String> {
    cookies.get_private("user_id")
        .map(|cookie| format!("User ID: {}", cookie.value()))
}

/// Remove the `user_id` cookie.
#[post("/logout")]
fn logout(mut cookies: Cookies) -> Flash<Redirect> {
    cookies.remove_private(Cookie::named("user_id"));
    Flash::success(Redirect::to("/"), "Successfully logged out.")
}

Secret Key #

To encrypt private cookies, Rocket uses the 256-bit key specified in the secret_key configuration parameter. If one is not specified, Rocket will automatically generate a fresh key. Note, however, that a private cookie can only be decrypted with the same key with which it was encrypted. As such, it is important to set a secret_key configuration parameter when using private cookies so that cookies decrypt properly after an application restart. Rocket emits a warning if an application is run in production without a configured secret_key.

Generating a string suitable for use as a secret_key configuration value is usually done through tools like openssl. Using openssl, a 256-bit base64 key can be generated with the command openssl rand -base64 32.

For more information on configuration, see the Configuration section of the guide.

One-At-A-Time #

For safety reasons, Rocket currently requires that at most one Cookies instance be active at a time. It’s uncommon to run into this restriction, but it can be confusing to handle if it does crop up.

If this does happen, Rocket will emit messages to the console that look as follows:

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=> Error: Multiple `Cookies` instances are active at once.
=> An instance of `Cookies` must be dropped before another can be retrieved.
=> Warning: The retrieved `Cookies` instance will be empty.

The messages will be emitted when a violating handler is called. The issue can be resolved by ensuring that two instances of Cookies cannot be active at once due to the offending handler. A common error is to have a handler that uses a Cookies request guard as well as a Custom request guard that retrieves Cookies, as so:

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#[get("/")]
fn bad(cookies: Cookies, custom: Custom) { .. }

Because the cookies guard will fire before the custom guard, the custom guard will retrieve an instance of Cookies when one already exists for cookies. This scenario can be fixed by simply swapping the order of the guards:

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#[get("/")]
fn good(custom: Custom, cookies: Cookies) { .. }

Body Data #

At some point, your web application will need to process body data. Data processing, like much of Rocket, is type directed. To indicate that a handler expects data, annotate it with data = "<param>", where param is an argument in the handler. The argument’s type must implement the FromData trait. It looks like this, where T: FromData:

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#[post("/", data = "<input>")]
fn new(input: T) -> String { ... }

Any type that implements FromData is also known as data guard.

Forms #

Forms are the most common type of data handled in web applications, and Rocket makes handling them easy. Say your application is processing a form submission for a new todo Task. The form contains two fields: complete, a checkbox, and description, a text field. You can easily handle the form request in Rocket as follows:

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#[derive(FromForm)]
struct Task {
    complete: bool,
    description: String,
}

#[post("/todo", data = "<task>")]
fn new(task: Form<Task>) -> String { ... }

The Form type implements the FromData trait as long as its generic parameter implements the FromForm trait. In the example, we’ve derived the FromForm trait automatically for the Task structure. FromForm can be derived for any structure whose fields implement FromFormValue. If a POST /todo request arrives, the form data will automatically be parsed into the Task structure. If the data that arrives isn’t of the correct Content-Type, the request is forwarded. If the data doesn’t parse or is simply invalid, a customizable 400 - Bad Request or 422 - Unprocessable Entity error is returned. As before, a forward or failure can be caught by using the Option and Result types:

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#[post("/todo", data = "<task>")]
fn new(task: Option<Form<Task>>) -> String { ... }
Lenient Parsing #

Rocket’s FromForm parsing is strict by default. In other words, A Form<T> will parse successfully from an incoming form only if the form contains the exact set of fields in T. Said another way, a Form<T> will error on missing and/or extra fields. For instance, if an incoming form contains the fields “a”, “b”, and “c” while T only contains “a” and “c”, the form will not parse as Form<T>.

Rocket allows you to opt-out of this behavior via the LenientForm data type. A LenientForm<T> will parse successfully from an incoming form as long as the form contains a superset of the fields in T. Said another way, a LenientForm<T> automatically discards extra fields without error. For instance, if an incoming form contains the fields “a”, “b”, and “c” while T only contains “a” and “c”, the form will parse as LenientForm<T>.

You can use a LenientForm anywhere you’d use a Form. Its generic parameter is also required to implement FromForm. For instance, we can simply replace Form with LenientForm above to get lenient parsing:

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#[derive(FromForm)]
struct Task { .. }

#[post("/todo", data = "<task>")]
fn new(task: LenientForm<Task>) { .. }
Field Renaming #

By default, Rocket matches the name of an incoming form field to the name of a structure field. While this behavior is typical, it may also be desired to use different names for form fields and struct fields while still parsing as expected. You can ask Rocket to look for a different form field for a given structure field by using the #[form(field = "name")] field annotation.

As an example, say that you’re writing an application that receives data from an external service. The external service POSTs a form with a field named type. Since type is a reserved keyword in Rust, it cannot be used as the name of a field. To get around this, you can use field renaming as follows:

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#[derive(FromForm)]
struct External {
    #[form(field = "type")]
    api_type: String
}

Rocket will then match the form field named type to the structure field named api_type automatically.

Field Validation #

Fields of forms can be easily validated via implementations of the FromFormValue trait. For example, if you’d like to verify that some user is over some age in a form, then you might define a new AdultAge type, use it as a field in a form structure, and implement FromFormValue so that it only validates integers over that age:

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struct AdultAge(usize);

impl<'v> FromFormValue<'v> for AdultAge {
    type Error = &'v RawStr;

    fn from_form_value(form_value: &'v RawStr) -> Result<AdultAge, &'v RawStr> {
        match form_value.parse::<usize>() {
            Ok(age) if age >= 21 => Ok(AdultAge(age)),
            _ => Err(form_value),
        }
    }
}

#[derive(FromForm)]
struct Person {
    age: AdultAge
}

If a form is submitted with a bad age, Rocket won’t call a handler requiring a valid form for that structure. You can use Option or Result types for fields to catch parse failures:

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#[derive(FromForm)]
struct Person {
    age: Option<AdultAge>
}

The forms validation and forms kitchen sink examples on GitHub provide further illustrations.

JSON #

Handling JSON data is no harder: simply use the Json type:

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#[derive(Deserialize)]
struct Task {
    description: String,
    complete: bool
}

#[post("/todo", data = "<task>")]
fn new(task: Json<Task>) -> String { ... }

The only condition is that the generic type in Json implements the Deserialize trait from Serde. See the JSON example on GitHub for a complete example.

Streaming #

Sometimes you just want to handle incoming data directly. For example, you might want to stream the incoming data out to a file. Rocket makes this as simple as possible via the Data type:

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#[post("/upload", format = "text/plain", data = "<data>")]
fn upload(data: Data) -> io::Result<String> {
    data.stream_to_file("/tmp/upload.txt").map(|n| n.to_string())
}

The route above accepts any POST request to the /upload path with Content-Type: text/plain The incoming data is streamed out to tmp/upload.txt, and the number of bytes written is returned as a plain text response if the upload succeeds. If the upload fails, an error response is returned. The handler above is complete. It really is that simple! See the GitHub example code for the full crate.

Query Strings #

Query strings are handled just like forms. A query string can be parsed into any structure that implements the FromForm trait. They are matched against by appending a ? to the path followed by a static query string or a dynamic parameter <param>.

For instance, say you change your mind and decide to use query strings instead of POST forms for new todo tasks in the previous forms example, reproduced below:

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#[derive(FromForm)]
struct Task { .. }

#[post("/todo", data = "<task>")]
fn new(task: Form<Task>) -> String { ... }

Rocket makes the transition simple: simply declare <task> as a query parameter as follows:

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#[get("/todo?<task>")]
fn new(task: Task) -> String { ... }

Rocket will parse the query string into the Task structure automatically by matching the structure field names to the query parameters. If the parse fails, the request is forwarded to the next matching route. Parse failures can be captured on a per-field or per-form basis.

To catch failures on a per-field basis, use a type of Option or Result for the given field:

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#[derive(FromForm)]
struct Task<'r> {
    description: Result<String, &'r RawStr>,
    complete: Option<bool>
}

To catch failures on a per-form basis, change the type of the query string target to either Option or Result:

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#[get("/todo?<task>")]
fn new(task: Option<Task>) { ... }

For a concrete illustration on how to handle query parameters, see the query_params example.

Error Catchers #

Routing may fail for a variety of reasons. These include:

If any of these conditions occur, Rocket returns an error to the client. To do so, Rocket invokes the error catcher corresponding to the error’s status code. A catcher is like a route, except it only handles errors. Catchers are declared via the error attribute, which takes a single integer corresponding to the HTTP status code to catch. For instance, to declare a catcher for 404 errors, you’d write:

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#[error(404)]
fn not_found(req: &Request) -> String { ... }

As with routes, Rocket needs to know about a catcher before it is used to handle errors. The process is similar to mounting: call the catch method with a list of catchers via the errors! macro. The invocation to add the 404 catcher declared above looks like:

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rocket::ignite().catch(errors![not_found])

Unlike request handlers, error handlers can only take 0, 1, or 2 parameters of types Request and/or Error. At present, the Error type is not particularly useful, and so it is often omitted. The error catcher example on GitHub illustrates their use in full.

Rocket has a default catcher for all of the standard HTTP error codes including 404, 500, and more.