Rocket/site/guide/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`.

### Adding State

To instruct Rocket to manage state for your application, call the
[`manage`](https://api.rocket.rs/rocket/struct.Rocket.html#method.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:

```rust
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:

```rust
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`](https://api.rocket.rs/rocket/struct.State.html) type: a [request
guard](/guide/requests/#request-guards) 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:

```rust
#[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:

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

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](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/state) and
learn more about the [`manage`
method](https://api.rocket.rs/rocket/struct.Rocket.html#method.manage) and
[`State` type](https://api.rocket.rs/rocket/struct.State.html) 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.

```rust
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::guard()`]: https://api.rocket.rs/rocket/struct.Request.html#method.guard

### 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:

```rust
/// 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`] 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.

[`FromRequest`]: https://api.rocket.rs/rocket/request/trait.FromRequest.htmll#request-local-state
[`Fairing`]: https://api.rocket.rs/rocket/fairing/trait.Fairing.html#request-local-state

## Databases

While Rocket doesn't have built-in support for databases yet, you can combine a
few external libraries to get native-feeling access to databases in a Rocket
application. Let's take a look at how we might integrate Rocket with two common
database libraries: [`diesel`], a type-safe ORM and query builder, and [`r2d2`],
a library for connection pooling.

Our approach will be to have Rocket manage a pool of database connections using
managed state and then implement a request guard that retrieves one connection.
This will allow us to get access to the database in a handler by simply adding a
`DbConn` argument:

```rust
#[get("/users")]
fn handler(conn: DbConn) { ... }
```

[`diesel`]: http://diesel.rs/
[`r2d2`]: https://docs.rs/r2d2/

### Dependencies

To get started, we need to depend on the `diesel` and `r2d2` crates. For
detailed information on how to use Diesel, please see the [Diesel getting
started guide](http://diesel.rs/guides/getting-started/). For this example, we
use the following dependencies:

```
[dependencies]
rocket = "0.4.0-dev"
rocket_codegen = "0.4.0-dev"
diesel = { version = "<= 1.2", features = ["sqlite", "r2d2"] }
```

Your `diesel` dependency information may differ. The crates are imported as
well:

```rust
extern crate rocket;
#[macro_use] extern crate diesel;
```

### Managed Pool

The first step is to initialize a pool of database connections. The `init_pool`
function below uses `r2d2` to create a new pool of database connections. Diesel
advocates for using a `DATABASE_URL` environment variable to set the database
URL, and we use the same convention here. Excepting the long-winded types, the
code is fairly straightforward: the `DATABASE_URL` environment variable is
stored in the `DATABASE_URL` static, and an `r2d2::Pool` is created using the
default configuration parameters and a Diesel `SqliteConnection`
`ConnectionManager`.

```rust
use diesel::sqlite::SqliteConnection;
use diesel::r2d2::{ConnectionManager, Pool, PooledConnection};

// An alias to the type for a pool of Diesel SQLite connections.
type SqlitePool = Pool<ConnectionManager<SqliteConnection>>;

// The URL to the database, set via the `DATABASE_URL` environment variable.
static DATABASE_URL: &'static str = env!("DATABASE_URL");

/// Initializes a database pool.
fn init_pool() -> SqlitePool {
    let manager = ConnectionManager::<SqliteConnection>::new(DATABASE_URL);
    Pool::new(manager).expect("db pool")
}
```

We then use managed state to have Rocket manage the pool for us:

```rust
fn main() {
    rocket::ignite()
        .manage(init_pool())
        .launch();
}
```

### Connection Guard

The second and final step is to implement a request guard that retrieves a
single connection from the managed connection pool. We create a new type,
`DbConn`, that wraps an `r2d2` pooled connection. We then implement
`FromRequest` for `DbConn` so that we can use it as a request guard. Finally, we
implement `Deref` with a target of `SqliteConnection` so that we can
transparently use an `&*DbConn` as an `&SqliteConnection`.

```rust
use std::ops::Deref;
use rocket::http::Status;
use rocket::request::{self, FromRequest};
use rocket::{Request, State, Outcome};
use diesel::r2d2::{ConnectionManager, Pool, PooledConnection};

// Connection request guard type: a wrapper around an r2d2 pooled connection.
pub struct DbConn(pub PooledConnection<ConnectionManager<SqliteConnection>>);

/// Attempts to retrieve a single connection from the managed database pool. If
/// no pool is currently managed, fails with an `InternalServerError` status. If
/// no connections are available, fails with a `ServiceUnavailable` status.
impl<'a, 'r> FromRequest<'a, 'r> for DbConn {
    type Error = ();

    fn from_request(request: &'a Request<'r>) -> request::Outcome<Self, Self::Error> {
        let pool = request.guard::<State<SqlitePool>>()?;
        match pool.get() {
            Ok(conn) => Outcome::Success(DbConn(conn)),
            Err(_) => Outcome::Failure((Status::ServiceUnavailable, ()))
        }
    }
}

// For the convenience of using an &DbConn as an &SqliteConnection.
impl Deref for DbConn {
    type Target = SqliteConnection;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}
```

### Usage

With these two pieces in place, we can use `DbConn` as a request guard in any
handler or other request guard implementation, giving our application access to
a database. As a simple example, we might write a route that returns a JSON
array of some `Task` structures that are fetched from a database:

```rust
#[get("/tasks")]
fn get_tasks(conn: DbConn) -> QueryResult<Json<Vec<Task>>> {
    all_tasks.order(tasks::id.desc())
        .load::<Task>(&*conn)
        .map(|tasks| Json(tasks))
}
```