Pushing data once a URL is requested

Question

Given, when a user requests /foo on my server, I send the following HTTP response (not closing the connection):

Content-Type: multipart/x-mixed-replace; boundary=-----------------------

-----------------------
Content-Type: text/html

<a href="/bar">foo</a>

When the user goes to /bar (which will send 204 No Content so the view doesn't change), I want to send the following data in the initial response.

-----------------------
Content-Type: text/html

bar

How would I get the second request to trigger this from the initial response? I'm planning on possibly creating a fancy [engines that support multipart/x-mixed-replace (currently only Gecko)]-only email webapp that does server-push and Ajax effects without any JavaScript, just for fun.

Answer 1

No complete answer, but:

In your question, you're describing a Comet-style architecture. Regarding support of Comet-style techniques in Python/WSGI, there is a StackOverflow question, which talks about various Python servers with support for long-running requests a la Comet.

Also interesting is this mail thread in the Python Web-SIG: "Could WSGI handle Asynchronous response?". In May 2008, there was a broad discussion in the Web-SIG about the topic of asynchronous requests in WSGI.

A recent development is evserver, a lightweight WSGI server, which implements the Asynchronous WSGI extension proposed by Christopher Stawarz in the Web-SIG in May 2008.

Finally, the Tornado web server supports non-blocking asynchronous requests. It has a chat example application using long polling, which has similarities with your requirements.

Answer 2

If the problem is to pass some command from /bar application to /foo application and you are using some servlet-like approach (the Python code is loaded once and not for each request as in CGI), you can just change some class property of the /foo application and be ready to react to the change in the /foo instance (by checking the property state).

Obviously the /foo application should not return right after the first request and yield content line by line.

Thought this is just theory, I have not tried that myself.

Answer 3

I have created some small example (just for fun, you know :))

import threading

num = 0
cond = threading.Condition()

def app(environ, start_response):
    global num

    cond.acquire()
    num += 1
    cond.notifyAll()
    cond.release()

    start_response("200 OK", [("Content-Type", "multipart/x-mixed-replace; boundary=xxx")])
    while True:
        n = num    
        s = "--xxx\r\nContent-Type: text/html\r\n\r\n%s\n" % n
        yield s
        # wait for num change:
        cond.acquire()
        while num == n:
            cond.wait()
        cond.release()


from cherrypy.wsgiserver import CherryPyWSGIServer
server = CherryPyWSGIServer(("0.0.0.0", 3000), app)

try:
    server.start()
except KeyboardInterrupt:
    server.stop()

# Now whenever you visit http://127.0.0.1:3000/, the number increases.
# It also automatically increases in all previously opened windows/tabs.

The idea of a shared variable and thread synchronization (using condition variable object) is based on the fact that WSGI server provided by CherryPyWSGIServer is threaded.