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 Javascript 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

• EventSource.js
• jquery.eventsource

As a follow-up to @peter-hoffmann's answer, I've written a Flask extension specifically to handle server-sent events. It's called Flask-SSE, and it's available on PyPI. To install it, run:

$ pip install flask-sse

You can use it like this:

from flask import Flask
from flask_sse import sse


app = Flask(__name__)
app.config["REDIS_URL"] = "redis://localhost"
app.register_blueprint(sse, url_prefix='/stream')


@app.route('/send')
def send_message():
sse.publish({"message": "Hello!"}, type='greeting')
return "Message sent!"

And to connect to the event stream from Javascript, it works like this:

var source = new EventSource("\{\{ url_for('sse.stream') }}");
source.addEventListener('greeting', function(event) {
var data = JSON.parse(event.data);
// do what you want with this data
}, false);

Documentation is available on ReadTheDocs. Note that you'll need a running Redis server to handle pub/sub.

Redis is overkill: use Server-Sent Events (SSE)

Late to the party (as usual), but IMHO using Redis may be overkill.

As long as you're working in Python+Flask, consider using generator functions as described in this excellent article by Panisuan Joe Chasinga. The gist of it is:

In your client index.html

var targetContainer = document.getElementById("target_div");
var eventSource = new EventSource("/stream")
eventSource.onmessage = function(e) {
targetContainer.innerHTML = e.data;
};
...
<div id="target_div">Watch this space...</div>

In your Flask server:

def get_message():
'''this could be any function that blocks until data is ready'''
time.sleep(1.0)
s = time.ctime(time.time())
return s


@app.route('/')
def root():
return render_template('index.html')


@app.route('/stream')
def stream():
def eventStream():
while True:
# wait for source data to be available, then push it
yield 'data: {}\n\n'.format(get_message())
return Response(eventStream(), mimetype="text/event-stream")

As a committer of https://github.com/WolfgangFahl/pyFlaskBootstrap4 i ran into the same need and created a flask blueprint for Server Sent Events that has no dependency to redis.

This solutions builds on the other answers that have been given here in the past.

https://github.com/WolfgangFahl/pyFlaskBootstrap4/blob/main/fb4/sse_bp.py has the source code (see also sse_bp.py below).

There are unit tests at https://github.com/WolfgangFahl/pyFlaskBootstrap4/blob/main/tests/test_sse.py

The idea is that you can use different modes to create your SSE stream:

• by providing a function
• by providing a generator
• by using a PubSub helper class
• by using the PubSub helper class and use pydispatch at the same time.

As of 2021-02-12 this is alpha code which i want to share nevertheless. Please comment here or as issues in the project.

There is a demo at http://fb4demo.bitplan.com/events and a description of the example use e.g. for a progress bar or time display at: http://wiki.bitplan.com/index.php/PyFlaskBootstrap4#Server_Sent_Events

example client javascript/html code

<div id="event_div">Watch this space...</div>
<script>
function fillContainerFromSSE(id,url) {
var targetContainer = document.getElementById(id);
var eventSource = new EventSource(url)
eventSource.onmessage = function(e) {
targetContainer.innerHTML = e.data;
};
};
fillContainerFromSSE("event_div","/eventfeed");
</script>

example server side code

def getTimeEvent(self):
'''
get the next time stamp
'''
time.sleep(1.0)
s=datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S')
return s


def eventFeed(self):
'''
create a Server Sent Event Feed
'''
sse=self.sseBluePrint
# stream from the given function
return sse.streamFunc(self.getTimeEvent)

sse_bp.py

'''
Created on 2021-02-06
@author: wf
'''
from flask import Blueprint, Response, request, abort,stream_with_context
from queue import Queue
from pydispatch import dispatcher
import logging


class SSE_BluePrint(object):
'''
a blueprint for server side events
'''
def __init__(self,app,name:str,template_folder:str=None,debug=False,withContext=False):
'''
Constructor
'''
self.name=name
self.debug=debug
self.withContext=False
if template_folder is not None:
self.template_folder=template_folder
else:
self.template_folder='templates'
self.blueprint=Blueprint(name,__name__,template_folder=self.template_folder)
self.app=app
app.register_blueprint(self.blueprint)
        

@self.app.route('/sse/<channel>')
def subscribe(channel):
def events():
PubSub.subscribe(channel)
self.stream(events)
                

def streamSSE(self,ssegenerator):
'''
stream the Server Sent Events for the given SSE generator
'''
response=None
if self.withContext:
if request.headers.get('accept') == 'text/event-stream':
response=Response(stream_with_context(ssegenerator), content_type='text/event-stream')
else:
response=abort(404)
else:
response= Response(ssegenerator, content_type='text/event-stream')
return response
        

def streamGen(self,gen):
'''
stream the results of the given generator
'''
ssegen=self.generateSSE(gen)
return self.streamSSE(ssegen)
            

def streamFunc(self,func,limit=-1):
'''
stream a generator based on the given function
Args:
func: the function to convert to a generator
limit (int): optional limit of how often the generator should be applied - 1 for endless
Returns:
an SSE Response stream
'''
gen=self.generate(func,limit)
return self.streamGen(gen)
                

def generate(self,func,limit=-1):
'''
create a SSE generator from a given function
Args:
func: the function to convert to a generator
limit (int): optional limit of how often the generator should be applied - 1 for endless
Returns:
a generator for the function
'''
count=0
while limit==-1 or count<limit:
# wait for source data to be available, then push it
count+=1
result=func()
yield result
        

def generateSSE(self,gen):
for result in gen:
yield 'data: {}\n\n'.format(result)
            

def enableDebug(self,debug:bool):
'''
set my debugging
        

Args:
debug(bool): True if debugging should be switched on
'''
self.debug=debug
if self.debug:
logging.basicConfig(level=logging.DEBUG, format='%(asctime)s.%(msecs)03d %(levelname)s:\t%(message)s', datefmt='%Y-%m-%d %H:%M:%S')
            

def publish(self, message:str, channel:str='sse', debug=False):
"""
Publish data as a server-sent event.
        

Args:
message(str): the message to send
channel(str): If you want to direct different events to different
clients, you may specify a channel for this event to go to.
Only clients listening to the same channel will receive this event.
Defaults to "sse".
debug(bool): if True  enable debugging
"""
return PubSub.publish(channel=channel, message=message,debug=debug)


def subscribe(self,channel,limit=-1,debug=False):
def stream():
for message in PubSub.subscribe(channel,limit,debug=debug):
yield str(message)
                

return self.streamGen(stream)
    

class PubSub:
'''
redis pubsub duck replacement
'''
pubSubByChannel={}
    

def __init__(self,channel:str='sse',maxsize:int=15, debug=False,dispatch=False):
'''
Args:
channel(string): the channel name
maxsize(int): the maximum size of the queue
debug(bool): whether debugging should be switched on
dispatch(bool): if true use the pydispatch library - otherwise only a queue
'''
self.channel=channel
self.queue=Queue(maxsize=maxsize)
self.debug=debug
self.receiveCount=0
self.dispatch=False
if dispatch:
dispatcher.connect(self.receive,signal=channel,sender=dispatcher.Any)
        

@staticmethod
def reinit():
'''
reinitialize the pubSubByChannel dict
'''
PubSub.pubSubByChannel={}
        

@staticmethod
def forChannel(channel):
'''
return a PubSub for the given channel
        

Args:
channel(str): the id of the channel
Returns:
PubSub: the PubSub for the given channel
'''
if channel in PubSub.pubSubByChannel:
pubsub=PubSub.pubSubByChannel[channel]
else:
pubsub=PubSub(channel)
PubSub.pubSubByChannel[channel]=pubsub
return pubsub
    

@staticmethod
def publish(channel:str,message:str,debug=False):
'''
publish a message via the given channel
        

Args:
channel(str): the id of the channel to use
message(str): the message to publish/send
Returns:
PubSub: the pub sub for the channel
            

'''
pubsub=PubSub.forChannel(channel)
pubsub.debug=debug
pubsub.send(message)
return pubsub
        

@staticmethod
def subscribe(channel,limit=-1,debug=False):
'''
subscribe to the given channel
        

Args:
channel(str): the id of the channel to use
limit(int): limit the maximum amount of messages to be received
debug(bool): if True debugging info is printed
'''
pubsub=PubSub.forChannel(channel)
pubsub.debug=debug
return pubsub.listen(limit)
    

def send(self,message):
'''
send the given message
'''
sender=object();
if self.dispatch:
dispatcher.send(signal=self.channel,sender=sender,msg=message)
else:
self.receive(sender,message)
        

def receive(self,sender,message):
'''
receive a message
'''
if sender is not None:
self.receiveCount+=1;
if self.debug:
logging.debug("received %d:%s" % (self.receiveCount,message))
self.queue.put(message)
        

def listen(self,limit=-1):
'''
listen to my channel
        

this is a generator for the queue content of received messages
        

Args:
limit(int): limit the maximum amount of messages to be received
        

Return:
generator: received messages to be yielded
'''
if limit>0 and self.receiveCount>limit:
return
yield self.queue.get()
    

def unsubscribe(self):
'''
unsubscribe me
'''
if self.dispatch:
dispatcher.disconnect(self.receive, signal=self.channel)
pass