I need help understanding what the advantage of using an asynch framework is. Suppose I want to develop a simple chat web app. Why cant I write python code in the Django framework that does long polling where I dont send a response back the server until someone enters a new msg. What does Twisted provide that gives it an advantage for real-time apps like the chat app?
Sorry I am obviously little confused about the need for an asynchronous framework.
Asynchronous servers support much larger numbers of simultaneous client connections. More conventional servers come up against thread and process limits when servicing large number of concurrent clients, particularly those with long-lived connections. Async servers can also provide better performance as they avoid the overheads of e.g. thread context switching.
See this recent post about the Tornado web server, which was designed for FriendFeed. This other post, also about Tornado, mentions performance numbers.
As well as the Twisted framework, there are also asynchronous server building blocks in Python's standard library: asyncore and asynchat.
First off Django is a framework for writing web apps so it provides ORM, html templating, it requires running an http server etc. Twisted helps to write much lower level code than that. You could use twisted to write the http server Django runs on. If you use Django you are limited to http model, with twisted it could be communicating in any protocol you like including push protocols. So for your chat example you get a server that scales better since it can push comments to people who have logged in VS with django every client having to poll repeatedly.
edited to reflect comments by: sos-skyl
You could use WHIFF instead of either :). Check out http://aaron.oirt.rutgers.edu/myapp/gfChat/nucularChatRoom which uses a javascript polling loop with json to check for server updates. You could probably do something similar in Django, but I don't know how because I gave up on Django.
btw: I'm hoping to move this demo onto the google app engine as a more complete service when my life calms down a bit.
If you'd like to look at some source for integrating Twisted and Django, have a look at Yardbird.
In twisted you can implement protocols of your own. Django certainly can't do this.
The biggest advantage for me is that Twisted gives me an application that has state, and can communicate with many different clients using many protocols. For more information, see my blog entry on Why I choose twisted over using a database as the core of my power monitoring system.
For me, my Twisted server communicates with a number of sensors installed in houses and businesses that monitor power usage. It stores the data and keeps recent data and state in handy-dandy python classes in memory. Requests via xmlrpc from django get this state and can present recent data to the user. My Gridspy stuff is still in development so the actual site at your.gridspy.co.nz is a bit pre-alpha.
The best part is that you need surprisingly little code to make an effective server. An amazing amount of the work is done for you.