Rocket/site/guide/3-overview.md

Overview

Rocket provides primitives to build web servers and applications with Rust: Rocket provides routing, pre-processing of requests, and post-processing of responses; the rest is up to you. Your application code instructs Rocket on what to pre-process and post-process and fills the gaps between pre-processing and post-processing.

Lifecycle

Rocket's main task is to listen for incoming web requests, dispatch the request to the application code, and return a response to the client. We call the process that goes from request to response the "lifecycle". We summarize the lifecycle as the following sequence of steps:

1. Routing

   Rocket parses an incoming HTTP request into native structures that your code operates on indirectly. Rocket determines which request handler to invoke by matching against route attributes declared in your application.

2. Validation

   Rocket validates the incoming request against types and guards present in the matched route. If validation fails, Rocket forwards the request to the next matching route or calls an error handler.

3. Processing

   The request handler associated with the route is invoked with validated arguments. This is the main business logic of an application. Processing completes by returning a Response.

4. Response

   The returned Response is processed. Rocket generates the appropriate HTTP response and sends it to the client. This completes the lifecycle. Rocket continues listening for requests, restarting the lifecycle for each incoming request.

The remainder of this section details the routing phase as well as additional components needed for Rocket to begin dispatching requests to request handlers. The sections following describe the request and response phases as well as other components of Rocket.

Routing

Rocket applications are centered around routes and handlers. A route is a combination of:

• A set of parameters to match an incoming request against.
• A handler to process the request and return a response.

A handler is simply a function that takes an arbitrary number of arguments and returns any arbitrary type.

The parameters to match against include static paths, dynamic paths, path segments, forms, query strings, request format specifiers, and body data. Rocket uses attributes, which look like function decorators in other languages, to make declaring routes easy. Routes are declared by annotating a function, the handler, with the set of parameters to match against. A complete route declaration looks like this:

# #![feature(proc_macro_hygiene)]
# #[macro_use] extern crate rocket;

#[get("/world")]              // <- route attribute
fn world() -> &'static str {  // <- request handler
    "hello, world!"
}

This declares the world route to match against the static path "/world" on incoming GET requests. Instead of #[get], we could have used #[post] or #[put] for other HTTP methods, or #[catch] for serving custom error pages. Additionally, other route parameters may be necessary when building more interesting applications. The Requests chapter, which follows this one, has further details on routing and error handling.

Mounting

Before Rocket can dispatch requests to a route, the route needs to be mounted:

# #![feature(proc_macro_hygiene)]
# #[macro_use] extern crate rocket;

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

fn main() {
    rocket::ignite().mount("/hello", routes![world]);
}

The mount method takes as input:

1. A base path to namespace a list of routes under, here, "/hello".
2. A list of routes via the routes! macro: here, routes![world], with multiple routes: routes![a, b, c].

This creates a new Rocket instance via the ignite function and mounts the world route to the "/hello" path, making Rocket aware of the route. GET requests to "/hello/world" will be directed to the world function.

! note: In many cases, the base path will simply be "/".

Namespacing

When a route is declared inside a module other than the root, you may find yourself with unexpected errors when mounting:

# #![feature(proc_macro_hygiene)]
# #[macro_use] extern crate rocket;

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

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

use other::world;

fn main() {
    // error[E0425]: cannot find value `static_rocket_route_info_for_world` in this scope
    rocket::ignite().mount("/hello", routes![hello, world]);
}

This occurs because the routes! macro implicitly converts the route's name into the name of a structure generated by Rocket's code generation. The solution is to refer to the route using a namespaced path instead:

# #![feature(proc_macro_hygiene)]
# #[macro_use] extern crate rocket;

# #[get("/")] pub fn hello() {}
# mod other { #[get("/world")] pub fn world() {} }

rocket::ignite().mount("/hello", routes![hello, other::world]);

Launching

Now that Rocket knows about the route, you can tell Rocket to start accepting requests via the launch method. The method starts up the server and waits for incoming requests. When a request arrives, Rocket finds the matching route and dispatches the request to the route's handler.

We typically call launch from the main function. Our complete Hello, world! application thus looks like:

#![feature(proc_macro_hygiene)]

#[macro_use] extern crate rocket;

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

fn main() {
    # if false {
    rocket::ignite().mount("/hello", routes![world]).launch();
    # }
}

Note the #![feature] line: this tells Rust that we're opting in to compiler features available in the nightly release channel. This line must be in the crate root, typically main.rs. We've also imported the rocket crate and all of its macros into our namespace via #[macro_use] extern crate rocket. Finally, we call the launch method in the main function.

Running the application, the console shows:

🔧  Configured for development.
    => address: localhost
    => port: 8000
    => log: normal
    => workers: [logical cores * 2]
    => secret key: generated
    => limits: forms = 32KiB
    => keep-alive: 5s
    => tls: disabled
🛰  Mounting '/hello':
    => GET /hello/world (world)
🚀  Rocket has launched from http://localhost:8000

If we visit localhost:8000/hello/world, we see Hello, world!, exactly as we expected.

A version of this example's complete crate, ready to cargo run, can be found on GitHub. You can find dozens of other complete examples, spanning all of Rocket's features, in the GitHub examples directory.

Futures and Async

Rocket uses Rust Futures for concurrency. Asynchronous programming with Futures and async/await allows route handlers to perform wait-heavy I/O such as filesystem and network access while still allowing other requests to be processed. For an overview of Rust Futures, see Asynchronous Programming in Rust.

In general, you should prefer to use async-ready libraries instead of synchronous equivalents inside Rocket applications.

async appears in several places in Rocket:

• Routes and Error Catchers can be async fns. Inside an async fn, you can .await Futures from Rocket or other libraries
• Several of Rocket's traits, such as FromData and FromRequest, have methods that return Futures.
• Data and DataStream (incoming request data) and Response and Body (outgoing response data) are based on tokio::io::AsyncRead instead of std::io::Read.

You can find async-ready libraries on crates.io with the async tag.

! note

Rocket 0.5 uses the tokio (0.2) runtime. The runtime is started for you if you use #[rocket::launch] or #[rocket::main], but you can still launch() a rocket instance on a custom-built Runtime.

Cooperative Multitasking

Rust's Futures are a form of cooperative multitasking. In general, Futures and async fns should only .await on other operations and never block. Some common examples of blocking include locking mutexes, joining threads, or using non-async library functions (including those in std) that perform I/O.

If a Future or async fn blocks the thread, inefficient resource usage, stalls, or sometimes even deadlocks can occur.

! note

Sometimes there is no good async alternative for a library or operation. If necessary, you can convert a synchronous operation to an async one with tokio::task::spawn_blocking:

#[get("/blocking_task")]
async fn blocking_task() -> String {
    // In a real application, we would use rocket::response::NamedFile
    tokio::task::spawn_blocking(|| {
        std::fs::read_file("data.txt")
    }).await
}