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Peter Hoffmann

Software Engineer
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How to implement server push in Flask framework?

Posted on September 2, 2012
#stackoverflow #python

This my Answer to the stackoverflow question: How to implement server push in Flask framework?:

Have a look at Server-Sent Events. Server-Sent Events is a browser API that lets you keep open a socket to your server, subscribing to a stream of updates. For more Information read Alex MacCaw (Author of Juggernaut) post on why he kills juggernaut and why the simpler Server-Sent Events are in manny cases the better tool for the job than Websockets.

The protocol is really easy. Just add the mimetype text/event-stream to your response. The browser will keep the connection open and listen for updates. An Event sent from the server is a line of text starting with data: and a following newline.

data: this is a simple message
<blank line>

If you want to exchange structured data, just dump your data as json and send the json over the wire.

An advantage is that you can use SSE in Flask without the need for an extra Server. There is a simple chat application example on github which uses redis as a pub/sub backend.

def event_stream():
    pubsub = red.pubsub()
    pubsub.subscribe('chat')
    for message in pubsub.listen():
        print message
        yield 'data: %s\n\n' % message['data']


@app.route('/post', methods=['POST'])
def post():
    message = flask.request.form['message']
    user = flask.session.get('user', 'anonymous')
    now = datetime.datetime.now().replace(microsecond=0).time()
    red.publish('chat', u'[%s] %s: %s' % (now.isoformat(), user, message))


@app.route('/stream')
def stream():
    return flask.Response(event_stream(),
                          mimetype="text/event-stream")

You do not need to use gunicron to run the example app. Just make sure to use threading when running the app, because otherwise the SSE connection will block your development server:

if __name__ == '__main__':
    app.debug = True
    app.run(threaded=True)

On the client side you just need a javascipt handler function which will be called when a new message is pushed from the server.

var source = new EventSource('/stream');
source.onmessage = function (event) {
     alert(event.data);
};

Server-Sent Events are supported by recent Firefox, Chrome and Safari browsers. Internet Explorer does not yet support Server-Sent Events, but is expected to support them in Version 10. There are two recommended Polyfills to support older browsers