API development in Rust combines performance with safety, leveraging frameworks like Actix Web or Rocket to build robust, high-throughput web services. This tutorial covers REST API development from route handling to database integration.

1. Choosing a Web Framework

Popular Options:

Actix Web: High-performance actor framework
Rocket: Developer-friendly with macros
Axum: Tokio-based from Tower ecosystem

Actix Web Setup

# Cargo.toml
[dependencies]
actix-web = "4"
serde = { version = "1.0", features = ["derive"] }

// Basic server
use actix_web::{get, App, HttpServer, Responder};

#[get("/")]
async fn hello() -> impl Responder {
    "Hello from Rust API!"
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    HttpServer::new(|| {
        App::new().service(hello)
    })
    .bind(("127.0.0.1", 8080))?
    .run()
    .await
}

2. RESTful Route Design

Resource-oriented endpoints with proper HTTP methods.

use actix_web::{web, HttpResponse};

// Path parameters
#[get("/users/{user_id}")]
async fn get_user(user_id: web::Path<u32>) -> HttpResponse {
    HttpResponse::Ok().json(format!("User {}", user_id))
}

// Query parameters
#[get("/search")]
async fn search(query: web::Query<SearchQuery>) -> HttpResponse {
    HttpResponse::Ok().json(query.into_inner())
}

// JSON body
#[post("/users")]
async fn create_user(user: web::Json<User>) -> HttpResponse {
    HttpResponse::Created().json(user.into_inner())
}

// Register routes
App::new()
    .service(get_user)
    .service(search)
    .service(create_user)

HTTP Status Codes:

200 OK - Successful GET
201 Created - Successful POST
400 Bad Request - Invalid input
404 Not Found - Resource doesn't exist

3. Request/Response Models

Type-safe data handling with Serde serialization.

use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)]
struct User {
    id: u32,
    name: String,
    email: String,
}

#[derive(Deserialize)]
struct SearchQuery {
    q: String,
    limit: Option<u32>,
}

// Example JSON input:
// {
//   "id": 1,
//   "name": "Alice",
//   "email": "alice@example.com"
// }

Validation Tips:

Use validator crate for field validation
Implement FromRequest for custom parsing
Add schema documentation with utoipa or schemars

4. Database Access

SQL and NoSQL options with connection pooling.

SQLx (PostgreSQL Example)

# Cargo.toml
[dependencies]
sqlx = { version = "0.6", features = [
    "postgres",
    "runtime-tokio-native-tls"
] }

use sqlx::postgres::PgPoolOptions;

struct AppState {
    db: sqlx::PgPool,
}

async fn get_user(
    state: web::Data<AppState>,
    user_id: web::Path<i32>,
) -> Result<HttpResponse, Error> {
    let user = sqlx::query_as!(
        User,
        "SELECT id, name, email FROM users WHERE id = $1",
        user_id.into_inner()
    )
    .fetch_one(&state.db)
    .await?;

    Ok(HttpResponse::Ok().json(user))
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    let pool = PgPoolOptions::new()
        .connect("postgres://user:pass@localhost/db")
        .await?;

    HttpServer::new(move || {
        App::new()
            .app_data(web::Data::new(AppState { db: pool.clone() }))
            .service(web::resource("/users/{id}").to(get_user))
    })
    .bind(("127.0.0.1", 8080))?
    .run()
    .await
}

5. API Error Handling

Consistent error responses with proper status codes.

use thiserror::Error;

#[derive(Error, Debug)]
enum ApiError {
    #[error("Not found")]
    NotFound,
    #[error("Database error")]
    DatabaseError(#[from] sqlx::Error),
    #[error("Validation error: {0}")]
    ValidationError(String),
}

impl actix_web::error::ResponseError for ApiError {
    fn error_response(&self) -> HttpResponse {
        match self {
            ApiError::NotFound => HttpResponse::NotFound().json("Not found"),
            ApiError::DatabaseError(_) => 
                HttpResponse::InternalServerError().json("Database error"),
            ApiError::ValidationError(msg) => 
                HttpResponse::BadRequest().json(msg),
        }
    }
}

async fn get_user(
    state: web::Data<AppState>,
    user_id: web::Path<i32>,
) -> Result<HttpResponse, ApiError> {
    let user = sqlx::query_as!(
        User,
        "SELECT * FROM users WHERE id = $1",
        user_id.into_inner()
    )
    .fetch_one(&state.db)
    .await
    .map_err(|e| match e {
        sqlx::Error::RowNotFound => ApiError::NotFound,
        _ => ApiError::DatabaseError(e),
    })?;

    Ok(HttpResponse::Ok().json(user))
}

6. Adding Middleware

Cross-cutting concerns like logging and auth.

use actix_web::middleware::Logger;
use actix_web_httpauth::middleware::HttpAuthentication;

// 1. Request logging
App::new()
    .wrap(Logger::default())

// 2. JWT Authentication
let auth = HttpAuthentication::bearer(validator);

App::new()
    .wrap(auth)
    .service(
        web::resource("/protected")
            .to(protected_handler)
    )

// 3. CORS
use actix_cors::Cors;

App::new()
    .wrap(
        Cors::default()
            .allow_any_origin()
            .allowed_methods(["GET", "POST"])
    )

7. API Testing

Automated verification of endpoints.

#[cfg(test)]
mod tests {
    use super::*;
    use actix_web::test;

    #[actix_web::test]
    async fn test_hello() {
        let app = test::init_service(
            App::new().service(hello)
        ).await;

        let req = test::TestRequest::get()
            .uri("/")
            .to_request();
        let resp = test::call_service(&app, req).await;

        assert_eq!(resp.status(), StatusCode::OK);
        let body = test::read_body(resp).await;
        assert_eq!(body, "Hello from Rust API!");
    }
}

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Building APIs with Rust