Crate axum

Expand description

axum is a web application framework that focuses on ergonomics and modularity.

High level features

Route requests to handlers with a macro free API.
Declaratively parse requests using extractors.
Simple and predictable error handling model.
Generate responses with minimal boilerplate.
Take full advantage of the tower and tower-http ecosystem of middleware, services, and utilities.

In particular the last point is what sets axum apart from other frameworks. axum doesn’t have its own middleware system but instead uses tower::Service. This means axum gets timeouts, tracing, compression, authorization, and more, for free. It also enables you to share middleware with applications written using hyper or tonic.

Compatibility

axum is designed to work with tokio and hyper. Runtime and transport layer independence is not a goal, at least for the time being.

Example

The “Hello, World!” of axum is:

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

#[tokio::main]
async fn main() {
    // build our application with a single route
    let app = Router::new().route("/", get(|| async { "Hello, World!" }));

    // run it with hyper on localhost:3000
    axum::Server::bind(&"0.0.0.0:3000".parse().unwrap())
        .serve(app.into_make_service())
        .await
        .unwrap();
}

Routing

Router is used to setup which paths goes to which services:

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

// our router
let app = Router::new()
    .route("/", get(root))
    .route("/foo", get(get_foo).post(post_foo))
    .route("/foo/bar", get(foo_bar));

// which calls one of these handlers
async fn root() {}
async fn get_foo() {}
async fn post_foo() {}
async fn foo_bar() {}

See Router for more details on routing.

Handlers

In axum a “handler” is an async function that accepts zero or more “extractors” as arguments and returns something that can be converted into a response.

Handlers is where your application logic lives and axum applications are built by routing between handlers.

See handler for more details on handlers.

Extractors

An extractor is a type that implements FromRequest. Extractors is how you pick apart the incoming request to get the parts your handler needs.

use axum::extract::{Path, Query, Json};
use std::collections::HashMap;

// `Path` gives you the path parameters and deserializes them.
async fn path(Path(user_id): Path<u32>) {}

// `Query` gives you the query parameters and deserializes them.
async fn query(Query(params): Query<HashMap<String, String>>) {}

// Buffer the request body and deserialize it as JSON into a
// `serde_json::Value`. `Json` supports any type that implements
// `serde::Deserialize`.
async fn json(Json(payload): Json<serde_json::Value>) {}

See extract for more details on extractors.

Responses

Anything that implements IntoResponse can be returned from handlers.

use axum::{
    body::Body,
    routing::get,
    response::Json,
    Router,
};
use serde_json::{Value, json};

// `&'static str` becomes a `200 OK` with `content-type: text/plain; charset=utf-8`
async fn plain_text() -> &'static str {
    "foo"
}

// `Json` gives a content-type of `application/json` and works with any type
// that implements `serde::Serialize`
async fn json() -> Json<Value> {
    Json(json!({ "data": 42 }))
}

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

See response for more details on building responses.

Error handling

axum aims to have a simple and predictable error handling model. That means it is simple to convert errors into responses and you are guaranteed that all errors are handled.

See error_handling for more details on axum’s error handling model and how to handle errors gracefully.

Middleware

There are several different ways to write middleware for axum. See middleware for more details.

It is common to share some state between handlers for example to share a pool of database connections or clients to other services.

The two most common ways of doing that is:

Using request extensions
Using closure captures

Using request extensions

The easiest way to extract state in handlers is using AddExtension middleware (applied with AddExtensionLayer) and the Extension extractor:

use axum::{
    AddExtensionLayer,
    extract::Extension,
    routing::get,
    Router,
};
use std::sync::Arc;

struct State {
    // ...
}

let shared_state = Arc::new(State { /* ... */ });

let app = Router::new()
    .route("/", get(handler))
    .layer(AddExtensionLayer::new(shared_state));

async fn handler(
    Extension(state): Extension<Arc<State>>,
) {
    // ...
}

The downside to this approach is that you’ll get runtime errors (specifically a 500 Internal Server Error response) if you try and extract an extension that doesn’t exist, perhaps because you forgot add the middleware or because you’re extracting the wrong type.

Using closure captures

State can also be passed directly to handlers using closure captures:

use axum::{
    AddExtensionLayer,
    Json,
    extract::{Extension, Path},
    routing::{get, post},
    Router,
};
use std::sync::Arc;
use serde::Deserialize;

struct State {
    // ...
}

let shared_state = Arc::new(State { /* ... */ });

let app = Router::new()
    .route(
        "/users",
        post({
            let shared_state = Arc::clone(&shared_state);
            move |body| create_user(body, Arc::clone(&shared_state))
        }),
    )
    .route(
        "/users/:id",
        get({
            let shared_state = Arc::clone(&shared_state);
            move |path| get_user(path, Arc::clone(&shared_state))
        }),
    );

async fn get_user(Path(user_id): Path<String>, state: Arc<State>) {
    // ...
}

async fn create_user(Json(payload): Json<CreateUserPayload>, state: Arc<State>) {
    // ...
}

#[derive(Deserialize)]
struct CreateUserPayload {
    // ...
}

The downside to this approach is that its a little more verbose than using extensions.

Building integrations for axum

Libraries authors that want to provide FromRequest or IntoResponse implementations should depend on the axum-core crate, instead of axum if possible. axum-core contains core types and traits and is less likely to receive breaking changes.

Required dependencies

To use axum there are a few dependencies you have pull in as well:

[dependencies]
axum = "<latest-version>"
hyper = { version = "<latest-version>", features = ["full"] }
tokio = { version = "<latest-version>", features = ["full"] }
tower = "<latest-version>"

The "full" feature for hyper and tokio isn’t strictly necessary but it’s the easiest way to get started.

Note that hyper::Server is re-exported by axum so if thats all you need then you don’t have to explicitly depend on hyper.

Tower isn’t strictly necessary either but helpful for testing. See the testing example in the repo to learn more about testing axum apps.

Examples

The axum repo contains a number of examples that show how to put all the pieces together.

Feature flags

axum uses a set of feature flags to reduce the amount of compiled and optional dependencies.

The following optional features are available:

Name	Description	Default?
`headers`	Enables extracting typed headers via `TypedHeader`	No
`http1`	Enables hyper’s `http1` feature	Yes
`http2`	Enables hyper’s `http2` feature	No
`json`	Enables the `Json` type and some similar convenience functionality	Yes
`multipart`	Enables parsing `multipart/form-data` requests with `Multipart`	No
`tower-log`	Enables `tower`’s `log` feature	Yes
`ws`	Enables WebSockets support via `extract::ws`	No