I'm using this generator function to implement server-sent events:
def event_stream():
while True:
gevent.sleep(3)
yield 'data: some data\n\n'
...and returning it like this:
return Response(event_stream(), mimetype='text/event-stream')
If I use time.sleep(3) instead of gevent.sleep(3), it blocks everything else (as expected). So, when I use gevent and run the Flask app locally, it works fine.
However, when I run it on remote server, it doesn't send messages in 3-second intervals like it did when I ran the app locally, but instead seems to keep on piling up the messages so that when I press Ctrl+C to end the server, all the messages that kept on piling up are dumped at once at the client side.
So, after about 30 seconds of waiting without any messages, when I kill the remote server, the ten messages that should have been received in the 3-second intervals get dumped at once.
I'm assuming this has something to do with how I'm running gunicorn. Here's how I'm currently starting the server:
gunicorn -c config.py server:app -k gevent
The only thing configured inside config file is the bind variable.
After two days of trying out so many things, it turned out that nginx configuration had to be changed for Server-Sent Events.
Merely adding these three lines did the trick:
proxy_set_header Connection '';
proxy_http_version 1.1;
chunked_transfer_encoding off;
Source: First Google Hit for "nginx server sent events" (SO answer)