main.rs

//! Run with
//!
//! ```not_rust
//! cargo run -p example-dependency-injection
//! ```

use std::{
    collections::HashMap,
    sync::{Arc, Mutex},
};

use axum::{
    extract::{Path, State},
    http::StatusCode,
    routing::{get, post},
    Json, Router,
};
use serde::{Deserialize, Serialize};
use tokio::net::TcpListener;
use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
use uuid::Uuid;

#[tokio::main]
async fn main() {
    tracing_subscriber::registry()
        .with(
            tracing_subscriber::EnvFilter::try_from_default_env()
                .unwrap_or_else(|_| format!("{}=debug", env!("CARGO_CRATE_NAME")).into()),
        )
        .with(tracing_subscriber::fmt::layer())
        .init();

    let user_repo = InMemoryUserRepo::default();

    // We generally have two ways to inject dependencies:
    //
    // 1. Using trait objects (`dyn SomeTrait`)
    //     - Pros
    //         - Likely leads to simpler code due to fewer type parameters.
    //     - Cons
    //         - Less flexible because we can only use object safe traits
    //         - Small amount of additional runtime overhead due to dynamic dispatch.
    //           This is likely to be negligible.
    // 2. Using generics (`T where T: SomeTrait`)
    //     - Pros
    //         - More flexible since all traits can be used.
    //         - No runtime overhead.
    //     - Cons:
    //         - Additional type parameters and trait bounds can lead to more complex code and
    //           boilerplate.
    //
    // Using trait objects is recommended unless you really need generics.

    let using_dyn = Router::new()
        .route("/users/{id}", get(get_user_dyn))
        .route("/users", post(create_user_dyn))
        .with_state(AppStateDyn {
            user_repo: Arc::new(user_repo.clone()),
        });

    let using_generic = Router::new()
        .route("/users/{id}", get(get_user_generic::<InMemoryUserRepo>))
        .route("/users", post(create_user_generic::<InMemoryUserRepo>))
        .with_state(AppStateGeneric { user_repo });

    let app = Router::new()
        .nest("/dyn", using_dyn)
        .nest("/generic", using_generic);

    let listener = TcpListener::bind("127.0.0.1:3000").await.unwrap();
    tracing::debug!("listening on {}", listener.local_addr().unwrap());
    axum::serve(listener, app).await.unwrap();
}

#[derive(Clone)]
struct AppStateDyn {
    user_repo: Arc<dyn UserRepo>,
}

#[derive(Clone)]
struct AppStateGeneric<T> {
    user_repo: T,
}

#[derive(Debug, Serialize, Clone)]
struct User {
    id: Uuid,
    name: String,
}

#[derive(Deserialize)]
struct UserParams {
    name: String,
}

async fn create_user_dyn(
    State(state): State<AppStateDyn>,
    Json(params): Json<UserParams>,
) -> Json<User> {
    let user = User {
        id: Uuid::new_v4(),
        name: params.name,
    };

    state.user_repo.save_user(&user);

    Json(user)
}

async fn get_user_dyn(
    State(state): State<AppStateDyn>,
    Path(id): Path<Uuid>,
) -> Result<Json<User>, StatusCode> {
    match state.user_repo.get_user(id) {
        Some(user) => Ok(Json(user)),
        None => Err(StatusCode::NOT_FOUND),
    }
}

async fn create_user_generic<T>(
    State(state): State<AppStateGeneric<T>>,
    Json(params): Json<UserParams>,
) -> Json<User>
where
    T: UserRepo,
{
    let user = User {
        id: Uuid::new_v4(),
        name: params.name,
    };

    state.user_repo.save_user(&user);

    Json(user)
}

async fn get_user_generic<T>(
    State(state): State<AppStateGeneric<T>>,
    Path(id): Path<Uuid>,
) -> Result<Json<User>, StatusCode>
where
    T: UserRepo,
{
    match state.user_repo.get_user(id) {
        Some(user) => Ok(Json(user)),
        None => Err(StatusCode::NOT_FOUND),
    }
}

trait UserRepo: Send + Sync {
    fn get_user(&self, id: Uuid) -> Option<User>;

    fn save_user(&self, user: &User);
}

#[derive(Debug, Clone, Default)]
struct InMemoryUserRepo {
    map: Arc<Mutex<HashMap<Uuid, User>>>,
}

impl UserRepo for InMemoryUserRepo {
    fn get_user(&self, id: Uuid) -> Option<User> {
        self.map.lock().unwrap().get(&id).cloned()
    }

    fn save_user(&self, user: &User) {
        self.map.lock().unwrap().insert(user.id, user.clone());
    }
}