2007-12-16

Asynchronous Django responses (comet), yes, for WSGI

Comet on python?

I was looking for comet server implementation in python. There are many projects that claim to provide this, but none of them is working for me. Most of the projects are just proofs of concepts that never been deployed for real usage.

Here is the list of projects that my friend found:


The Orbited project looks like the thing I was looking for. The problem is that even the examples from official documentation aren't working. Sorry, I don't want to work with undocumented projects.

Next candidate is Twisted, but I'm not the fan of it for religious reasons.

Fapws project also is looking good, but it supports only basics, no possibility of deferring (preeptimg) request.

The problem

I want to do an ajax request from browser that will be finished only when some event occured at the server side. The connection should stall and wait for the server. (this idea, of pushing events from server to browser is called comet)

Next thing is to allow django applications to use this technology. As far as I know no one has ever done this before, probably because django is based on WSGI stack which is, unfortunetally, fully synchronous.

Even though I think it's possible to allow WSGI stack to support asynchronous responses without modifying the WSGI stack at all.

The solution

Author of FAPWS suggested that it's possible to use libevent directly from python using ctypes. Libevent proved to be very reliable and fast enough in most cases, so it should be good idea to use it.

Using libevent glued with ctypes I created simple WSGI server (William examples helped me much).

As I mentioned the idea was to allow Django to do async responses. For the proof-of-concept I created website that produces output in five chunks, separated by delays of one second. Sounds easy, but it's not so simple to do without freezing server process for the whole time.

Going straight to the example, here's the view.py file from sample django app:
# Url handler of the basic url.
# Headers only from this response are used.
# The same with status code, this function
# response status code is served to the client.
def comet_main(request):
response = HttpResponse('First content<br /> ') # return something
response.stage = 0
return defer_in_time(response,
1, '/stage/') # after one second run the '/stage/' url handler

@deferred_view()
def comet_stage(request, response_prev=None):
response = HttpResponse('stage%i<br /> ' % (response_prev.stage + 1) )
response.stage = response_prev.stage + 1
if response.stage < 3: # loop here three times
url = '/stage/'
else:
url = '/stage_last/' # than end the connection
return defer_in_time(response,
1, url) # after one second run the url

@deferred_view()
def comet_stage_last(request, response_prev=None):
# no magic here. just regular response
return HttpResponse('last_stage%i<br /> ' % (response_prev.stage +1))
Here is how it looks from the browser:


And what really happens on the wire (notice that Transfer-Encoding: chunked is used):


The sources are here.

What's next
The next thing to do is to create the example of full comet application.


9 comments:

Michael Carter said...

Hey, great work. Send me an email, I'd like to discuss future work with you.

The Documentation at Orbited was broken with our 0.3 release and hasn't been fixed yet, so I can understand your hesitation to use the project.

Regardless, I'd like to talk.

Eric said...

Hi Majek,

The tarball seems like a dead link. Any chance it could be uploaded again?

majek said...

Our server is currently dead. I hope it would be online in few days. Sorry for inconvenience.

Thanks for interest!

majek said...

The server should be available now.

Eric said...

Great, thanks!

temoto said...

Thank you so much for WSGI implementation.
Thank you twice for ctypes_event.py. I wanted to write this file from scratch.

I will try to extract it for using aside from django.

gatoatigrado said...

sorry, what religious reasons?

Andru said...

Hello, i try to do real-time django comments, but i don't know what tehnology is good for it (orbited or smth else)???

majek said...

Andru,

Take a look at my recent project - SockJS.

http://www.rabbitmq.com/blog/category/web-messaging/

https://github.com/sockjs