The Hello World of WebSocket

The websocket stuff will be built on top of the HTTP server, so we don't need a new package this time. We're mostly going to work in paekli-http.

Axum WebSocket feature flag

Axum has built-in support for websockets, but it is gated behind a feature flag. Add it with the following command:

cargo add axum --features ws

Opening a WebSocket connection

First of all, we'll need a new route where clients can request to open a websocket connection:

#![allow(unused)]
fn main() {
axum::Router::new()
    // ...
    .route("/notifications", get(subscribe_to_notifications))
}

Next, we actually have to write that handler. Don't worry, everything will be explained in a second.

#![allow(unused)]
fn main() {
#[axum::debug_handler]
async fn subscribe_to_notifications(ws: WebSocketUpgrade) -> axum::response::Response {
    ws.on_upgrade(|mut socket| async {
        socket
            .send(axum::extract::ws::Message::Text("Hello, world!".into()))
            .await
            .unwrap();
        socket.close().await.unwrap();
    })
}
}

• WebSocketUpgrade is a type from axum, assuming the feature ws is enabled. Axum will only call this handler with requests for websocket connections.
• In the body, we immediately pass a closure to ws.on_upgrade(), which is run once the connection is successfully established.
• Once we have the socket in the closure, we send a single message "Hello, world!" and close it again.
• Don't worry about async and await.

Websockets can be tricky to debug. One option is to use websocat:

websocat ws://127.0.0.1:4200/notifications