[FIXED] Can I make a Quart app do a redirect after a background task finishes?

Issue

I am currently building an app with Quart that web scrapes an external site for data. Because this can take several minutes, I made the scraping a background task to avoid request timeouts.

After the user provides a user ID to a form on the home page, the app sets a background task to scrape the data and in the meantime, the user is redirected to a loading page.

@app.route('/')
async def index():

    form = await request.form      
    id = int(form['user_id'])

    # set the background task
    app.add_background_task(task_to_do, user_id=id)

    return redirect('/loading')

@app.route('/loading')
async def loading():    
    return await render_template('loading.html')

Currently, the user is successfully redirected to the loading page and the scraping background task finishes no problem.

At the completion of my task_to_do background task, I want the user to be redirected to a new page. How would one go about doing that?

Solution

I think you want to utilise Server Sent Events. This is where the client creates a connection to the server and waits for events from it. You could first send a loading event, and then when the computation completes send a result event. There is a guide in the Quart docs for Server Sent Events.

E.g. something like the code below (using the ServerSentEvent class from the docs). Note the user_id needs to be a path parameter as SSE works over GET requests.

from quart import abort, make_response

@app.get('/<int:id_>')
async def index(id_):
    if "text/event-stream" not in request.accept_mimetypes:
        abort(400)

    async def send_events():
        event = ServerSentEvent("loading")
        yield event.encode()
        data = await task_to_do(user_id=id_)
        event = ServerSentEvent(json.dumps(data))
        yield event.encode()

    response = await make_response(
        send_events(),
        {
            'Content-Type': 'text/event-stream',
            'Cache-Control': 'no-cache',
            'Transfer-Encoding': 'chunked',
        },
    )
    response.timeout = None
    return response

Answered By - pgjones