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Axum - Tokio's Web Framework

Axum is a web framework built on top of Tokio. It is inspired by the likes of Rocket, Actix and Warp. It is lightweight and relatively easy to use. It also includes a number of features that make it a good choice for building enterprise web services.

Hello Web

This example is in 03_async/hello_web

Let's make a really trivial webserver that just returns "Hello World" with no formatting, nor even HTML decoration.

To start, you need two services: tokio and axum. Add them to your Cargo.toml:

cargo add tokio -F full
cargo add axum

Now in our main.rs file, we can build a minimal webserver quite easily:

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

#[tokio::main]
async fn main() {
    let app = Router::new().route("/", get(say_hello_text));
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr)
        .serve(app.into_make_service())
        .await
        .unwrap();
}

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

Let's unpack this:

  • Router is an Axum service that matches URLs to services. In this case, we're matching the root URL (/) to the say_hello_text service.
  • addr is set with a SocketAddr to bind to localhost on port 3000.
  • axum::Server uses the builder pattern
    • bind sets the address to bind to.
    • serve accepts the Router you created, and launches a webserver.
    • You have to await the server, because it's an asynchronous task.
    • unwrap is used to handle any errors that might occur.

The say_hello_text function is relatively straightforward. Don't worry about 'static --- we'll talk about it in next week's talk about memory management, resource management and variables.

Run the program with cargo run. It won't output anything. Point a browser at http://localhost:3000 and be amazed by the Hello, world! message.

You've made a basic webserver in 16 lines of code. It's also very fast---which is extra easy since it doesn't really do anything yet.

Let's return "Hello World" as HTML

A webserver that doesn't return HTML is a bit odd, so let's turn "Hello World" into a proper HTML page.

use axum::response::Html;

async fn say_hello_html() -> Html<&'static str> {
    Html("<h1>Hello, world!</h1>")
}

And change your route to call the new function. Run the program and go to http://localhost:3000 again. You should see a big, bold "Hello, world!".

HTML in Static Files

It's easier to write large amounts of HTML in a separate file. You can then import the file into your program. Let's do that.

First, in your src directory create a file named hello.html:

<html>
<head>
    <title>Hello World</title>
</head>
<body>
    <p>Greetings, oh lovely world.</p>
</body>
</html>

I'm not great at HTML!

Now, in your main.rs file, you can import the file and return it as HTML:

async fn say_hello_html_included() -> Html<&'static str> {
    const HTML: &str = include_str!("hello.html");
    Html(HTML)
}

Change the route again, and your file will be included when you run the webserver.

HTML in Dynamic Files

There's a very real performance benefit to statically loading your pages, but it makes editing them a pain. Let's make a dynamic page that loads the HTML from a file.

This only requires a small change:

async fn say_hello_file() -> Html<String> {
    let path = Path::new("src/hello.html");
    let content = tokio::fs::read_to_string(path).await.unwrap();
    Html(content)
}

Now run your webserver. Change the HTML file and reload the page. You should see the changes.

Note: You probably want some cache in the real world---but this is great for rapid development.

Add a JSON Get Service

Let's add serde to our project with cargo add serde -F derive.

Now we'll add a structure and make it serializable:

#[derive(Serialize)]
struct HelloJson {
    message: String,
}

And we can add a handler function to return some JSON:

async fn say_hello_json() -> axum::Json<HelloJson> {
    axum::Json(HelloJson {
        message: "Hello, World!".to_string(),
    })
}

Lastly, we need to add a route to use it:

let app = Router::new()
    .route("/", get(say_hello_file))
    .route("/json", get(say_hello_json));

Now run the server, and connect to http://localhost:3000/json. You'll see a JSON response.

Responding to Other Verbs

Axum supports get, post, put, delete, head, options, trace, connect and patch. Let's add a post route.

async fn say_hello_post() -> &'static str {
    "Hello, POST!"
}

Now add it to your routes:

let app = Router::new()
    .route("/", get(say_hello_file))
    .route("/json", get(say_hello_json))
    .route("/post", post(say_hello_post));

Let's update the HTML page to perform the POST for us:

<html>

<head>
    <title>Hello World</title>
</head>

<body>
    <p>Greetings, oh lovely world.</p>
    <p id="result"></p>
</body>

<script>
    function doPost() {
        fetch('/post', {
            method: 'POST',
            headers: {
                'Content-Type': 'application/json'
            },
            body: ''
        })
            .then(response => response.text())
            .then(result => {
                document.getElementById('result').innerHTML = result;
            })
            .catch(error => {
                console.error('Error:', error);
            });

    }

    doPost();
</script>

</html>

As you can see, I'm not a JavaScript programmer either!

The same techniques work for all of the HTTP verbs.