actix-website/content/docs/databases.md

title	menu	weight
Databases	docs_patterns	1010

Diesel

At the moment, Diesel 1.0 does not support asynchronous operations, but it's possible to use the actix synchronous actor system as a database interface api.

Technically, sync actors are worker style actors. Multiple sync actors can be run in parallel and process messages from same queue. Sync actors work in mpsc mode.

Let's create a simple database api that can insert a new user row into a SQLite table. We must define a sync actor and a connection that this actor will use. The same approach can be used for other databases.

use actix::prelude::*;

struct DbExecutor(SqliteConnection);

impl Actor for DbExecutor {
    type Context = SyncContext<Self>;
}

This is the definition of our actor. Now, we must define the create user message and response.

struct CreateUser {
    name: String,
}

impl Message for CreateUser {
    type Result = Result<User, Error>;
}

We can send a CreateUser message to the DbExecutor actor, and as a result, we will receive a User model instance. Next, we must define the handler implementation for this message.

impl Handler<CreateUser> for DbExecutor {
    type Result = Result<User, Error>;

    fn handle(&mut self, msg: CreateUser, _: &mut Self::Context) -> Self::Result
    {
        use self::schema::users::dsl::*;

        // Create insertion model
        let uuid = format!("{}", uuid::Uuid::new_v4());
        let new_user = models::NewUser {
            id: &uuid,
            name: &msg.name,
        };

        // normal diesel operations
        diesel::insert_into(users)
            .values(&new_user)
            .execute(&self.0)
            .expect("Error inserting person");

        let mut items = users
            .filter(id.eq(&uuid))
            .load::<models::User>(&self.0)
            .expect("Error loading person");

        Ok(items.pop().unwrap())
    }
}

That's it! Now, we can use the DbExecutor actor from any http handler or middleware. All we need is to start DbExecutor actors and store the address in a state where http handler can access it.

/// This is state where we will store *DbExecutor* address.
struct State {
    db: Addr<Syn, DbExecutor>,
}

fn main() {
    let sys = actix::System::new("diesel-example");

    // Start 3 parallel db executors
    let addr = SyncArbiter::start(3, || {
        DbExecutor(SqliteConnection::establish("test.db").unwrap())
    });

    // Start http server
    HttpServer::new(move || {
        App::with_state(State{db: addr.clone()})
            .resource("/{name}", |r| r.method(Method::GET).a(index))})
        .bind("127.0.0.1:8080").unwrap()
        .start().unwrap();

    println!("Started http server: 127.0.0.1:8080");
    let _ = sys.run();
}

We will use the address in a request handler. The handle returns a future object; thus, we receive the message response asynchronously. Route::a() must be used for async handler registration.

/// Async handler
fn index(req: HttpRequest<State>) -> Box<Future<Item=HttpResponse, Error=Error>> {
    let name = &req.match_info()["name"];

    // Send message to `DbExecutor` actor
    req.state().db.send(CreateUser{name: name.to_owned()})
        .from_err()
        .and_then(|res| {
            match res {
                Ok(user) => Ok(HttpResponse::Ok().json(user)),
                Err(_) => Ok(HttpResponse::InternalServerError().into())
            }
        })
        .responder()
}

A full example is available in the examples directory.

More information on sync actors can be found in the actix documentation.