Django middleware that sets a tracking id (fetched from a ticket server) as a cookie in the http response.

middleware.py

'''
Django middleware that sets a tracking id (fetched from a ticket server) as a 
cookie in the http response.

This assumes you have setup a couple k/v stores as ticket servers and that
you know the key. For demonstration purposes only, here is how one might do it 
with redis:

    # start a redis server on default port (6379)
    $ redis-server /usr/local/etc/redis.conf &> /dev/null &
    
    # start a redis server on another port (16379)
    $ redis-server /tmp/redis.16379.conf &> /dev/null &
    
    # initialize the first service with integer 0
    $ redis-cli
    redis> select 1
    OK
    redis> set trackerid 0
    OK
    redis> quit
    
    # initialize the second service with integer 1
    $ redis-cli -p 16379
    redis> select 1
    OK
    redis> set trackerid 1
    OK
    redis> quit
        
and then add this middleware to settings.MIDDLEWARE_CLASSES somewhere near the
top:

    MIDDLEWARE_CLASSES = (
        # middleware at the top of the stack
        'middleware.TrackingMiddleware',
        # middleware at the bottom of the stack
    )
    
    
Finally start your server and test

    $ manage.py runserver
    
    $ curl -i http://localhost:8000/tracker/
    
    $ ab -n 10000 -c 5 http://localhost:8000/tracker/

'''

import random
from collections import namedtuple
from datetime import datetime, timedelta

import redis

TicketServer = namedtuple('TicketServer', 'host, port, db')
ticket_servers = (
    TicketServer(host='localhost', port=6379, db=1),
    TicketServer(host='localhost', port=16379, db=1),
)

# format according to rc2109 - http://www.ietf.org/rfc/rfc2109.txt
# (Wdy, DD-Mon-YY HH:MM:SS GMT)
COOKIE_EXPIRES_FORMAT = '%a, %d-%b-%Y %H:%M:%S GMT'


def ring_generator(iterable):
    '''simple generator that moves around the iterable'''
    
    i = random.randint(0, len(iterable)-1)
    while True:
        yield iterable[i]
        i = (i+1)%len(iterable)


class TrackingMiddleware(object):
    '''
    middleware that sets a tracking id (fetched from a ticket server) as a 
    cookie in the http response.
    
    '''
    
    def process_response(self, request, response):
        
        value = request.COOKIES.get('trackerid', None)
        if not value:
            # assign a default value, just in-case all ticket servers are down
            value = 0
             
            # for high availability, loop over the ticket servers, starting 
            # at random location as determined by the ring generator
            ticket_server_generator = ring_generator(ticket_servers)
            for i in range(len(ticket_servers)):
                try:
                    # get a ticket server and setup a client for it
                    ticket_server = ticket_server_generator.next()
                    ticket_server_client = redis.Redis(host=ticket_server.host,
                                                       port=ticket_server.port,
                                                       db=ticket_server.db)
                    
                    # get next value by increasing it by number of servers 
                    value = ticket_server_client.incr('trackerid', amount=len(ticket_servers))
                except Exception, e:
                    import traceback
                    traceback.print_exc()
                else:
                    break 
            
        # set the cookie and return the response
        expires = (datetime.utcnow() + timedelta(days=365)).strftime(COOKIE_EXPIRES_FORMAT)
        response.set_cookie('trackerid', value=value, expires=expires)
        return response

urls.py

from django.conf.urls.defaults import *

urlpatterns = patterns('tracker.views',
    url(r'^tracker/',   'empty'),
)

views.py

from django.http import HttpResponse

def empty(request):
    return HttpResponse('')