Does LWA work with websockets? #586

BrnGuenther · 2025-03-25T14:27:59Z

BrnGuenther
Mar 25, 2025

So, I tried to deploy a simple example I found in the FastAPI documentation, in the WebSockets section.

from fastapi import FastAPI, WebSocket
from fastapi.responses import HTMLResponse

app = FastAPI()

html = """
<!DOCTYPE html>
<html>
    <head>
        <title>Chat</title>
    </head>
    <body>
        <h1>WebSocket Chat</h1>
        <form action="" onsubmit="sendMessage(event)">
            <input type="text" id="messageText" autocomplete="off"/>
            <button>Send</button>
        </form>
        <ul id='messages'>
        </ul>
        <script>
            var ws = new WebSocket("ws://LAMBDA-FUNCTION-URL/ws");
            ws.onmessage = function(event) {
                var messages = document.getElementById('messages')
                var message = document.createElement('li')
                var content = document.createTextNode(event.data)
                message.appendChild(content)
                messages.appendChild(message)
            };
            function sendMessage(event) {
                var input = document.getElementById("messageText")
                ws.send(input.value)
                input.value = ''
                event.preventDefault()
            }
        </script>
    </body>
</html>
"""


@app.get("/")
async def get():
    return HTMLResponse(html)


@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    while True:
        data = await websocket.receive_text()
        await websocket.send_text(f"Message text was: {data}")
        
if __name__ == '__main__':
    import uvicorn
    uvicorn.run(app, host="0.0.0.0", port=int(os.environ.get("PORT", "8080")))

With Uvicorn running, I can access my HTML page without any problem by directly using the Lambda Function URL. However, I can't access the WebSocket endpoint. I started wondering if LWA only supports HTTP/HTTPS connections.

bnusunny · 2025-04-02T06:06:16Z

bnusunny
Apr 2, 2025
Maintainer

No, LWA does not support WebSockets. Only http/1.0 and http/1.1 are supported.

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bnusunny Apr 2, 2025 Maintainer

BrnGuenther
Mar 25, 2025

bnusunny
Apr 2, 2025
Maintainer