Building a Simple Web Server in Rust

Building a web server with Rust doesn’t need to be complex. With frameworks like Axum, you can write a web server without hassle. Leveraging Rust allows you to easily take on the job of web services written in other languages and more. In this post we’re going to talk about how you can build and deploy a simple web server using Axum.

What Rust framework should I use?

While there’s a lot of options we can use, our personal choice is Axum for a few reasons:

Axum uses generics and traits. This allows you to leverage Rust language tooling in ways that other frameworks may not.
Axum has familiar syntax (using handler functions for routing).
It has an extremely high level of compatibility with tower crates. This allows you to go very low-level if required.

Additionally, we also have an article about what framework you should use here.

Getting started

To get started, you’ll want Rust installed on your system. Don’t have it installed? You can get it from this install page.

Next, you’ll want to use cargo shuttle init (requires cargo-shuttle installed). and then pick Axum for our framework.

Hello world!

When creating a project with cargo init, you will need to manually create (or copy!) the initial boilerplate. This may look something like this:

use axum::{Router, routing::get};
use std::net::SocketAddr;
use tokio::net::TcpListener;

async fn hello_world() -> &'static str {
    "Hello world!"
}

#[tokio::main]
async fn main() {
    let router = Router::new().route("/", get(hello_world));

    let addr = SocketAddr::from(([127,0,0,1], 8000));
    let tcp = TcpListener::bind(&addr).await.unwrap();

    axum::serve(tcp, router).await.unwrap();
}

As you can see from the above, we do a few things:

We set up a router with given routes and the functions that need to be called.
We define an address for our web server to receive requests at, bind it to a TCP listener.
The TCP listener then responds to requests and responds accordingly.

If you used cargo run to load this program up then go to [localhost:8000](http://localhost:8000) in the browser, it would return “Hello world!” as a raw text response.

When initialising a project with Shuttle, all of this gets set up for you. The basic project comes with a native integration so that you don’t need to set up the socket binding manually. The integration code is quite short and revolves around the usage of the shuttle_runtime::Service trait. If you have a non-standard service you want to run, you can run the service in a struct that implements the Service trait and you’ll be ready to go!

See below for an example of how a Shuttle “Hello World” project looks like:

use axum::{Router, routing::get};

async fn hello_world() -> &'static str {
    "Hello world!"
}

#[shuttle_runtime::main]
async fn main() -> shuttle_axum::ShuttleAxum {
    let router = Router::new().route("/", get(hello_world));

    Ok(router.into())
}

Routing

HTTP responses from routing within Rust frameworks can be done by anything that implements a trait that represents a HTTP response. In Axum, this would be the axum::response::IntoResponse trait (or axum::response::IntoResponseParts for headers and other non-response body parts). Web servers can only return things that are valid HTTP responses. Implementing IntoResponse (and IntoResponseParts respectively) ensures this!

It is possible to use impl IntoResponse as the return type for a function (for convenience). However, we would need to make sure all of the responses are of the same type. This can lead to confusion later down the line, particularly if you’re working in a team.

For JSON responses, Axum provides the handy Json<T> struct we can use as a response type by wrapping a type with it. For example, this snippet below shows how you can return some raw JSON:

use serde_json::{json, Value};
use axum::Json;

async fn return_some_json() -> Json<Value> {
    let json = json!({"hello":"world"})

    Json(json)
}

#[shuttle_runtime::main]
async fn main() -> shuttle_axum::ShuttleAxum {
    let router = Router::new()
        .route("/", get(hello_world))
        .route("/json/", get(return_some_json));

    Ok(router.into())
}

However, a more likely situation is that you’ll want to return data that follows a schema. We can use the Deserialize and Serialize traits from the serde crate to do this. We can easily apply these traits by adding the derive feature and then adding it as a derive macro to a struct:

use serde::{Deserialize, Serialize};

#[derive(Deserialize, Serialize)]
struct MyStruct {
    my_field: String
}

Now we can wrap it in axum::Json and return the struct in our HTTP response.

async fn return_a_struct_as_json() -> Json<MyStruct> {
     let my_struct = MyStruct { my_field: "Hello world!".to_string() };
     
     Json(my_struct)
}

Extractors

Extractors are handler function arguments. They extract parts of the HTTP request and turn it into simple variables that we can use with our application. We can use extractors for a lot of things:

Accessing shared mutable state (by adding state to our application then accessing it in the function)
Extracting a typed header for authorization purposes
Consuming the request body to extract a JSON or Form body, depending on what you want.
If there’s nothing readily available for our use case, we can just implement it ourselves!

Here is an example of how you can use extractors. Note here that we use de-structuring to automatically get the inner variable in Json<MyStruct> as it looks cleaner.

use axum::routing::post;

async fn function_with_extractors(
    Json(json): Json<MyStruct>,
) -> impl IntoResponse {
    format!("The contents of my_field is: {}", json.my_field)
}

#[shuttle_runtime::main]
async fn main() -> shuttle_axum::ShuttleAxum {
    let router = Router::new()
        .route("/", get(hello_world))
        .route("/json/", get(return_some_json))
        .route("/json-struct/", post(function_with_extractors));

    Ok(router.into())
}

Databases

Using databases generally isn’t much different in Rust than any other language. The main difference is Rust web frameworks generally use shared mutable state to pass around data. This means that you’ll want to first initialise your database connection (pool) and pass it around using state. In Axum, state is required to implement Clone. If your type cannot implement Clone because one or more types don’t implement Clone, you can wrap the state struct in a std::sync::Arc which does implement Clone.

Here’s an example of how you can do exactly that, using SQLx with Postgres as example. We initialise the PgPool, initialise our state struct and attach it to the router.

cargo add sqlx -F postgres

use sqlx::PgPool;
use axum::{extract::State, Router, routing::get, http::StatusCode};

#[derive(Clone)]
struct MyState {
    db: PgPool
}

#[tokio::main]
async fn main() {
    let db: PgPool = PgPool::connect("<your-db-connection-url-here>").await.unwrap();
    let state = MyState { db };
    let router = Router::new().route("/", get(hello_world)).with_state(state);

    // the rest of your code...
}

With Shuttle, we can provision a database by simply adding a database macro. This saves time both locally and in production! To get started, we’ll need to add the shuttle_shared_db dependency.

cargo add shuttle-shared-db -F postgres,sqlx

Then we simply add it to our main function and initialise the state struct again like before.

use sqlx::PgPool;
use axum::{Router, routing::get};

#[shuttle_runtime::main]
async fn main(
    #[shuttle_shared_db::Postgres] db: PgPool,
) -> shuttle_axum::ShuttleAxum {
    let state = MyState { db };
    let router = Router::new().route("/", get(hello_world)).with_state(state);

    Ok(router.into())
}

To use our state struct, we can use the axum::extract::State extractor:

use axum::{extract::State, http::StatusCode};

async fn hello_world(
    State(state): State<MyState>
) -> StatusCode {
    sqlx::query("SELECT 'Hello world!'")
         .execute(&state.db)
         .await
         .unwrap();

    StatusCode::OK
}

Static files

To get started with static files on Axum, we’ll create a folder in the root of our project called static. We’ll then define it in a file named Shuttle.toml in the project root:

assets = ["static/*"]

Using the wildcard tag allows us to serve any file contained within the folder by using the file path.

We can write a HTML file in the static folder called index.html:

<h1>Hello world</h1>

To serve this static folder on Axum, we’ll need to import some things from tower-http. We will run the following shell snippet:

cargo add tower-http -F fs

Then we can add it like below:

use tower_http::services::{ServeDir};

let router = Router::new()
    .route_service("/", ServeDir::new("static"));

If you are using a SPA framework like React or Vue for your static files, you will want to additionally set up a .not_found_service() to be able to serve index.html:

let router = Router::new()
    .route_service("/", ServeDir::new("static")
        .not_found_service(ServeFile::new("static/index.html") 
     )
);

Deployment

To deploy Rust to Shuttle, you can use cargo shuttle deploy and watch the magic happen! Don’t forget to add the --allow-dirty flag if on a Git branch with uncommitted changes. Besides web server development, Shuttle is a lifesaver when it comes to easily deploying and making your web service public.

Interested? You can find our docs here.