Menu
This is based on the answer posted at https://stackoverflow.com/a/13388915/819544
I would like to monitor a stream of data and push it to a front-end similar to the answer above, but where the stream starts generating/monitoring data as soon as the app kicks off, and the client always sees the current state of the data steam (which keeps running whether or not they are requesting data from the server).
I'm pretty sure I need to separate the data stream from the front-end via threading, but I'm not very practiced with threaded/asynchronous programming and believe I'm doing it wrong. Maybe instead of threading I need to be using multiprocessing? Here's roughly what I'm trying to do (modified from the answer linked above):
app.py
#!/usr/bin/env python
from __future__ import division
import itertools
import time
from flask import Flask, Response, redirect, request, url_for
from random import gauss
import threading
app = Flask(__name__)
# Generate streaming data and calculate statistics from it
class MyStreamMonitor(object):
def __init__(self):
self.sum = 0
self.count = 0
@property
def mu(self):
try:
outv = self.sum/self.count
except:
outv = 0
return outv
def generate_values(self):
while True:
time.sleep(.1) # an artificial delay
yield gauss(0,1)
def monitor(self, report_interval=1):
print "Starting data stream..."
for x in self.generate_values():
self.sum += x
self.count += 1
stream = MyStreamMonitor()
@app.route('/')
def index():
if request.headers.get('accept') == 'text/event-stream':
def events():
while True:
yield "data: %s %d\n\n" % (stream.count, stream.mu)
time.sleep(.01) # artificial delay. would rather push whenever values are updated.
return Response(events(), content_type='text/event-stream')
return redirect(url_for('static', filename='index.html'))
if __name__ == "__main__":
# Data monitor should start as soon as the app is started.
t = threading.Thread(target=stream.monitor() )
t.start()
print "Starting webapp..." # we never get to this point.
app.run(host='localhost', port=23423)
static/index.html
<!doctype html>
<title>Server Send Events Demo</title>
<style>
#data {
text-align: center;
}
</style>
<script src="http://code.jquery.com/jquery-latest.js"></script>
<script>
if (!!window.EventSource) {
var source = new EventSource('/');
source.onmessage = function(e) {
$("#data").text(e.data);
}
}
</script>
<div id="data">nothing received yet</div>
This code doesn't work. The "Starting webapp..." message never prints, nor do the normal flask messages, and visiting the served URL confirms the app isn't running.
How do I get the data monitor to run in the background in such a way that flask can access the values it's seeing and push the current state up to the client (better even: as long as the client is listening, push the current state when the relevant values change)?
731
SocketIo connection will get establish and video stream captured using webcam will be send to server frames by frames. These frames will be then processed at the backend and emit back to the client. Processed frames coming form the server can be shown in img tag.
Today, there are two ways to expose your flask application to the internet. Deploy the web application in your office server which has a public IP address and domain name. Deploy the web application in the cloud such as AWS, MS Azure, GCP or web hosting companies like GoDaddy, SiteGround, A2Hosting etc.
Flask isn't suitable for complex systems If you're trying to build a complex web app with lots of dynamic pages, then Flask isn't the right framework to use. Over time, the maintenance costs and efforts will be much higher compared to a framework like Django, which is better suited for large projects.
I just needed to change this line
t = threading.Thread(target=stream.monitor())
to this:
t = threading.Thread(target=stream.monitor)
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With