rust

# Rust Backend Patterns

## Overview

Rust patterns for building backend services with Axum.

## Project Structure

```
project/
├── src/
│   ├── main.rs               # Entry point
│   ├── lib.rs                # Library root
│   ├── config.rs             # Configuration
│   ├── error.rs              # Error types
│   ├── routes/               # Route handlers
│   │   ├── mod.rs
│   │   └── users.rs
│   ├── services/             # Business logic
│   ├── repositories/         # Data access
│   ├── models/               # Domain models
│   └── middleware/           # HTTP middleware
├── migrations/               # SQLx migrations
├── tests/                    # Integration tests
├── Cargo.toml
└── .env
```

## Axum Application

### Main Application

```rust
// src/main.rs
use axum::{
    routing::{get, post},
    Router,
};
use sqlx::postgres::PgPoolOptions;
use std::sync::Arc;
use tower_http::cors::CorsLayer;

mod config;
mod error;
mod routes;
mod services;
mod repositories;

use config::Config;

#[derive(Clone)]
pub struct AppState {
    pub db: sqlx::PgPool,
    pub config: Arc<Config>,
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    dotenvy::dotenv().ok();
    tracing_subscriber::init();

    let config = Config::from_env()?;

    let pool = PgPoolOptions::new()
        .max_connections(config.database.max_connections)
        .connect(&config.database.url)
        .await?;

    sqlx::migrate!().run(&pool).await?;

    let state = AppState {
        db: pool,
        config: Arc::new(config),
    };

    let app = Router::new()
        .route("/health", get(|| async { "ok" }))
        .nest("/api/users", routes::users::router())
        .with_state(state)
        .layer(CorsLayer::permissive());

    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await?;
    tracing::info!("listening on {}", listener.local_addr()?);
    axum::serve(listener, app).await?;

    Ok(())
}
```

### Configuration

```rust
// src/config.rs
use serde::Deserialize;

#[derive(Debug, Deserialize)]
pub struct Config {
    pub database: DatabaseConfig,
    pub jwt: JwtConfig,
}

#[derive(Debug, Deserialize)]
pub struct DatabaseConfig {
    pub url: String,
    #[serde(default = "default_max_connections")]
    pub max_connections: u32,
}

#[derive(Debug, Deserialize)]
pub struct JwtConfig {
    pub secret: String,
    #[serde(default = "default_expiry")]
    pub expiry_hours: u64,
}

fn default_max_connections() -> u32 { 10 }
fn default_expiry() -> u64 { 24 }

impl Config {
    pub fn from_env() -> Result<Self, config::ConfigError> {
        config::Config::builder()
            .add_source(config::Environment::default().separator("__"))
            .build()?
            .try_deserialize()
    }
}
```

## Error Handling

```rust
// src/error.rs
use axum::{
    http::StatusCode,
    response::{IntoResponse, Response},
    Json,
};
use serde_json::json;
use thiserror::Error;

#[derive(Error, Debug)]
pub enum AppError {
    #[error("Not found: {0}")]
    NotFound(String),

    #[error("Validation error: {0}")]
    Validation(String),

    #[error("Unauthorized")]
    Unauthorized,

    #[error("Forbidden")]
    Forbidden,

    #[error("Database error")]
    Database(#[from] sqlx::Error),

    #[error("Internal error")]
    Internal(#[from] anyhow::Error),
}

impl IntoResponse for AppError {
    fn into_response(self) -> Response {
        let (status, code, message) = match &self {
            AppError::NotFound(msg) => (StatusCode::NOT_FOUND, "NOT_FOUND", msg.clone()),
            AppError::Validation(msg) => (StatusCode::BAD_REQUEST, "VALIDATION_ERROR", msg.clone()),
            AppError::Unauthorized => (StatusCode::UNAUTHORIZED, "UNAUTHORIZED", "Unauthorized".into()),
            AppError::Forbidden => (StatusCode::FORBIDDEN, "FORBIDDEN", "Forbidden".into()),
            AppError::Database(e) => {
                tracing::error!("Database error: {:?}", e);
                (StatusCode::INTERNAL_SERVER_ERROR, "DATABASE_ERROR", "Database error".into())
            }
            AppError::Internal(e) => {
                tracing::error!("Internal error: {:?}", e);
                (StatusCode::INTERNAL_SERVER_ERROR, "INTERNAL_ERROR", "Internal error".into())
            }
        };

        (
            status,
            Json(json!({
                "error": {
                    "code": code,
                    "message": message
                }
            })),
        ).into_response()
    }
}

pub type Result<T> = std::result::Result<T, AppError>;
```

## Models and DTOs

```rust
// src/models/user.rs
use chrono::{DateTime, Utc};
use serde::{Deserialize, Serialize};
use sqlx::FromRow;
use uuid::Uuid;
use validator::Validate;

#[derive(Debug, FromRow, Serialize)]
pub struct User {
    pub id: Uuid,
    pub name: String,
    pub email: String,
    #[serde(skip_serializing)]
    pub password_hash: String,
    pub created_at: DateTime<Utc>,
    pub updated_at: Option<DateTime<Utc>>,
}

#[derive(Debug, Deserialize, Validate)]
pub struct CreateUser {
    #[validate(length(min = 2, max = 100))]
    pub name: String,
    #[validate(email)]
    pub email: String,
    #[validate(length(min = 8))]
    pub password: String,
}

#[derive(Debug, Deserialize, Validate)]
pub struct UpdateUser {
    #[validate(length(min = 2, max = 100))]
    pub name: Option<String>,
    #[validate(email)]
    pub email: Option<String>,
}

#[derive(Debug, Serialize)]
pub struct UserResponse {
    pub id: Uuid,
    pub name: String,
    pub email: String,
    pub created_at: DateTime<Utc>,
}

impl From<User> for UserResponse {
    fn from(user: User) -> Self {
        Self {
            id: user.id,
            name: user.name,
            email: user.email,
            created_at: user.created_at,
        }
    }
}
```

## Repository Pattern

```rust
// src/repositories/user.rs
use sqlx::PgPool;
use uuid::Uuid;
use crate::error::{AppError, Result};
use crate::models::user::{User, CreateUser,