drocco007/.gitignore

## .gitignore
*.html

## pushdemo.py
import psycopg2
from psycopg2.extensions import ISOLATION_LEVEL_AUTOCOMMIT

import eventlet
from eventlet import wsgi
from eventlet import websocket
from eventlet.hubs import trampoline

dsn = ""  # customise this

def dblisten(q):
    """
    Open a db connection and add notifications to *q*.
    """
    cnn = psycopg2.connect(dsn)
    cnn.set_isolation_level(ISOLATION_LEVEL_AUTOCOMMIT)
    cur = cnn.cursor()
    cur.execute("LISTEN data;")
    while 1:
        trampoline(cnn, read=True)
        cnn.poll()
        while cnn.notifies:
            n = cnn.notifies.pop()
            q.put(n)

@websocket.WebSocketWSGI
def handle(ws):
    """
    Receive a connection and send it database notifications.
    """
    q = eventlet.Queue()
    eventlet.spawn(dblisten, q)
    while 1:
        n = q.get()
        ws.send(unicode(n.payload))

def dispatch(environ, start_response):
    if environ['PATH_INFO'] == '/data':
        return handle(environ, start_response)
    else:
        start_response('200 OK',
            [('content-type', 'text/html')])
        return [page]

page = """
<html>
  <head><title>pushdemo</title>
    <!-- <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.1/jquery.min.js"></script> -->
    <script src="//code.jquery.com/jquery-1.11.0.min.js"></script>
    <style type="text/css">
      .bar {width: 20px; height: 20px;}
    </style>
    <script>
      window.onload = function() {
        ws = new WebSocket("ws://localhost:7000/data");
        ws.binaryType = "arraybuffer";
        ws.onmessage = function(msg) {
          bar = $('#' + msg.data);
          bar.width(bar.width() + 10);
        }
      }
    </script>
  </head>
  <body>
    <div style="width: 400px;">
      <div id="red" class="bar"
          style="background-color: red;">&nbsp;</div>
      <div id="green" class="bar"
          style="background-color: green;">&nbsp;</div>
      <div id="blue" class="bar"
          style="background-color: blue;">&nbsp;</div>
    </div>
  </body>
</html>
"""

if __name__ == "__main__":
    listener = eventlet.listen(('127.0.0.1', 7000))
    wsgi.server(listener, dispatch)


## pushdemo.rst

      
    Raw
  

              pushdemo.rst
            
          
    Postgres/WebSocket Demo

This is the runnable version of the Postgres/WebSocket idea that I presented in my PyOhio talk Pushy Postgres and Python. It is a slightly updated version of Daniele Varrazzo's example code from his blog post PostgreSQL notifications with Psycopg2 and Eventlet. If you want to try it out yourself,

Make sure you have a Postgres database you can connect to
Edit pushdemo.py and update the variable dsn to point to your database

dsn = 'dbname=testdb user=postgres password=secret'

You do not need to create a separate database for this demo.
pip install psycopg2 eventlet
python pushdemo.py
Open a browser to http://localhost:7000. You should see a stack of three small color bars.
Open a shell connection to the database:
psql testdb postgres
Notify!

NOTIFY data, 'blue';
NOTIFY data, 'green';


Notes, for the Curious

The only significant change I made to the original demo is on line 35:

ws.send(unicode(n.payload))

I added the Unicode conversion due to a defect in Eventlet's API: without this conversion, Eventlet treats the payload as binary, which the client side of the WebSocket is not prepared to handle.

The demo uses Eventlet's trampoline__ instead of the standard library select__ call that I presented in the case study, so the blocking call:

select.select([conn], [], [], 10) != ([], [], [])

is spelled with the Eventlet-specific (line 20):

trampoline(cnn, read=True)
	import psycopg2
	from psycopg2.extensions import ISOLATION_LEVEL_AUTOCOMMIT

	import eventlet
	from eventlet import wsgi
	from eventlet import websocket
	from eventlet.hubs import trampoline

	dsn = "" # customise this

	def dblisten(q):
	"""
	Open a db connection and add notifications to q.
	"""
	cnn = psycopg2.connect(dsn)
	cnn.set_isolation_level(ISOLATION_LEVEL_AUTOCOMMIT)
	cur = cnn.cursor()
	cur.execute("LISTEN data;")
	while 1:
	trampoline(cnn, read=True)
	cnn.poll()
	while cnn.notifies:
	n = cnn.notifies.pop()
	q.put(n)

	@websocket.WebSocketWSGI
	def handle(ws):
	"""
	Receive a connection and send it database notifications.
	"""
	q = eventlet.Queue()
	eventlet.spawn(dblisten, q)
	while 1:
	n = q.get()
	ws.send(unicode(n.payload))

	def dispatch(environ, start_response):
	if environ['PATH_INFO'] == '/data':
	return handle(environ, start_response)
	else:
	start_response('200 OK',
	[('content-type', 'text/html')])
	return [page]

	page = """
	<html>
	<head><title>pushdemo</title>
	<!-- <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.1/jquery.min.js"></script> -->
	<script src="//code.jquery.com/jquery-1.11.0.min.js"></script>
	<style type="text/css">
	.bar {width: 20px; height: 20px;}
	</style>
	<script>
	window.onload = function() {
	ws = new WebSocket("ws://localhost:7000/data");
	ws.binaryType = "arraybuffer";
	ws.onmessage = function(msg) {
	bar = $('#' + msg.data);
	bar.width(bar.width() + 10);
	}
	}
	</script>
	</head>
	<body>
	<div style="width: 400px;">
	<div id="red" class="bar"
	style="background-color: red;"> </div>
	<div id="green" class="bar"
	style="background-color: green;"> </div>
	<div id="blue" class="bar"
	style="background-color: blue;"> </div>
	</div>
	</body>
	</html>
	"""

	if __name__ == "__main__":
	listener = eventlet.listen(('127.0.0.1', 7000))
	wsgi.server(listener, dispatch)