Rocket/site/guide/6-state.md

State

Many web applications have a need to maintain state. This can be as simple as maintaining a counter for the number of visits or as complex as needing to access job queues and multiple databases. Rocket provides the tools to enable these kinds of interactions in a safe and simple manner.

Managed State

The enabling feature for maintaining state is managed state. Managed state, as the name implies, is state that Rocket manages for your application. The state is managed on a per-type basis: Rocket will manage at most one value of a given type.

The process for using managed state is simple:

1. Call manage on the Rocket instance corresponding to your application with the initial value of the state.
2. Add a State<T> type to any request handler, where T is the type of the value passed into manage.

! note: All managed state must be thread-safe.

Because Rocket automatically multithreads your application, handlers can concurrently access managed state. As a result, managed state must be thread-safe. Thanks to Rust, this condition is checked at compile-time by ensuring that the type of values you store in managed state implement Send + Sync.

Adding State

To instruct Rocket to manage state for your application, call the manage method on an instance of Rocket. For example, to ask Rocket to manage a HitCount structure with an internal AtomicUsize with an initial value of 0, we can write the following:

use std::sync::atomic::AtomicUsize;

struct HitCount {
    count: AtomicUsize
}

rocket::ignite().manage(HitCount { count: AtomicUsize::new(0) });

The manage method can be called any number of times as long as each call refers to a value of a different type. For instance, to have Rocket manage both a HitCount value and a Config value, we can write:

rocket::ignite()
    .manage(HitCount { count: AtomicUsize::new(0) })
    .manage(Config::from(user_input));

Retrieving State

State that is being managed by Rocket can be retrieved via the State type: a request guard for managed state. To use the request guard, add a State<T> type to any request handler, where T is the type of the managed state. For example, we can retrieve and respond with the current HitCount in a count route as follows:

use rocket::State;

#[get("/count")]
fn count(hit_count: State<HitCount>) -> String {
    let current_count = hit_count.count.load(Ordering::Relaxed);
    format!("Number of visits: {}", current_count)
}

You can retrieve more than one State type in a single route as well:

#[get("/state")]
fn state(hit_count: State<HitCount>, config: State<Config>) -> T { ... }

! warning

If you request a State<T> for a T that is not managed, Rocket won't call the offending route. Instead, Rocket will log an error message and return a 500 error to the client.

You can find a complete example using the HitCount structure in the state example on GitHub and learn more about the manage method and State type in the API docs.

Within Guards

It can also be useful to retrieve managed state from a FromRequest implementation. To do so, simply invoke State<T> as a guard using the Request::guard() method.

fn from_request(req: &'a Request<'r>) -> request::Outcome<T, ()> {
    let hit_count_state = req.guard::<State<HitCount>>()?;
    let current_count = hit_count_state.count.load(Ordering::Relaxed);
    ...
}

Request-Local State

While managed state is global and available application-wide, request-local state is local to a given request, carried along with the request, and dropped once the request is completed. Request-local state can be used whenever a Request is available, such as in a fairing, a request guard, or a responder.

Request-local state is cached: if data of a given type has already been stored, it will be reused. This is especially useful for request guards that might be invoked multiple times during routing and processing of a single request, such as those that deal with authentication.

As an example, consider the following request guard implementation for RequestId that uses request-local state to generate and expose a unique integer ID per request:

/// A global atomic counter for generating IDs.
static request_id_counter: AtomicUsize = AtomicUsize::new(0);

/// A type that represents a request's ID.
struct RequestId(pub usize);

/// Returns the current request's ID, assigning one only as necessary.
impl<'a, 'r> FromRequest<'a, 'r> for RequestId {
    fn from_request(request: &'a Request<'r>) -> request::Outcome {
        // The closure passed to `local_cache` will be executed at most once per
        // request: the first time the `RequestId` guard is used. If it is
        // requested again, `local_cache` will return the same value.
        Outcome::Success(request.local_cache(|| {
            RequestId(request_id_counter.fetch_add(1, Ordering::Relaxed))
        }))
    }
}

Note that, without request-local state, it would not be possible to:

1. Associate a piece of data, here an ID, directly with a request.
2. Ensure that a value is generated at most once per request.

For more examples, see the FromRequest request-local state documentation, which uses request-local state to cache expensive authentication and authorization computations, and the Fairing documentation, which uses request-local state to implement request timing.

Databases

Rocket includes built-in, ORM-agnostic support for databases. In particular, Rocket provides a procedural macro that allows you to easily connect your Rocket application to databases through connection pools. A database connection pool is a data structure that maintains active database connections for later use in the application. This implementation of connection pooling support is based on r2d2 and exposes connections through request guards. Databases are individually configured through Rocket's regular configuration mechanisms: a Rocket.toml file, environment variables, or procedurally.

Connecting your Rocket application to a database using this library occurs in three simple steps:

1. Configure the databases in Rocket.toml.
2. Associate a request guard type and fairing with each database.
3. Use the request guard to retrieve a connection in a handler.

Presently, Rocket provides built-in support for the following databases:

Kind	Driver	Poolable Type	Feature
MySQL	Diesel	diesel::MysqlConnection	diesel_mysql_pool
MySQL	rust-mysql-simple	mysql::conn	mysql_pool
Postgres	Diesel	diesel::PgConnection	diesel_postgres_pool
Postgres	Rust-Postgres	postgres::Connection	postgres_pool
Sqlite	Diesel	diesel::SqliteConnection	diesel_sqlite_pool
Sqlite	Rustqlite	rusqlite::Connection	sqlite_pool
Neo4j	rusted_cypher	rusted_cypher::GraphClient	cypher_pool
Redis	redis-rs	redis::Connection	redis_pool

Usage

To connect your Rocket application to a given database, first identify the "Kind" and "Driver" in the table that matches your environment. The feature corresponding to your database type must be enabled. This is the feature identified in the "Feature" column. For instance, for Diesel-based SQLite databases, you'd write in Cargo.toml:

[dependencies.rocket_contrib]
version = "0.4.0-rc.2"
default-features = false
features = ["diesel_sqlite_pool"]

Then, in Rocket.toml or the equivalent via environment variables, configure the URL for the database in the databases table:

[global.databases]
sqlite_logs = { url = "/path/to/database.sqlite" }

In your application's source code, create a unit-like struct with one internal type. This type should be the type listed in the "Poolable Type" column. Then decorate the type with the #[database] attribute, providing the name of the database that you configured in the previous step as the only parameter. Finally, attach the fairing returned by YourType::fairing(), which was generated by the #[database] attribute:

#[macro_use] extern crate rocket_contrib;

use rocket_contrib::databases::diesel;

#[database("sqlite_logs")]
struct LogsDbConn(diesel::SqliteConnection);

fn main() {
    rocket::ignite()
       .attach(LogsDbConn::fairing())
       .launch();
}

That's it! Whenever a connection to the database is needed, use your type as a request guard:

impl Logs {
    fn by_id(conn: &diesel::SqliteConnection, log_id: usize) -> Result<Logs> {
        logs.filter(id.eq(log_id)).load(conn)
    }
}

#[get("/logs/<id>")]
fn get_logs(conn: LogsDbConn, id: usize) -> Result<Logs> {
    Logs::by_id(&conn, id)
}

For more on Rocket's built-in database support, see the rocket_contrib::databases module documentation.