Rocket REST API return global object

Question

I'm starting to learn Rust and the rocket framework https://crates.io/crates/rocket.

I have a dumb question that I can't figure out.

How do I return my_universe that I created on the first line of main() when calling GET /universe/ports/21?

fn main() {
    let my_universe = universe::model::Universe::new();
    rocket::ignite().mount("/universe", routes![ports]).launch();
}

#[get("/ports/<id>")]
fn ports(id: u16) -> universe::model::Universe {
    // need to return my_universe here
}

The issue I'm having is that if I define my_universe within the route controller ports(), it'll recreate the my_universe object on each request. Instead, I need the route to return the same my_universe object on each request

Answer 1

Sharing non-mutable state in rocket is fairly easy. You can add the state with manage during the build of rocket.

rocket::build()
  .manage(my_universe) // put the shared state here
  .mount("/universe", routes![ports])

If you want to return this state in a route you will have to add both serde as a dependency and the json feature of rocket.

[dependencies]
rocket = { version = "0.5.0-rc.2", features = ["json"]}
serde = "1.0.147"

You can now annotate your struct with Serialize so we can send it as a JSON response later.

#[derive(Serialize)]
struct Universe {
    /* ... */
}

And access this state in your route with a &State parameter.

#[get("/ports/<id>")]
fn ports(id: u16, universe: &State<Universe>) -> Json<&Universe> {
    Json(universe.inner())
}

Here we can access the inner value of the state and return it as Json.

---

So far, the state is immutable and can not be changed in the route which might not be what you want. Consider wrapping your state into a Mutex to allow for interior mutability.