Question

The workflow I am dealing with (user-wise) looks like this:

• User submits information and files with a form
• Form is saved
• Additional post-save processing is done

This is fine, but the post-save processing takes quite a while, so I’m looking to do it in the background and issue an HttpResponseRedirect to a message informing the user that processing is happening and to please return later. Unfortunately, this doesn’t seem to be working; what I’ve got at the moment is this:

    if form.is_valid():
        p = multiprocessing.Process(target=form.save)
        p.start()
        return HttpResponseRedirect('/running')

But the error that I get back is this:

IOError at /content/script/new/
sys.stdout access restricted by mod_wsgi
...
/usr/lib/python2.6/multiprocessing/forking.py in __init__
    # We define a Popen class similar to the one from subprocess, but
    # whose constructor takes a process object as its argument.
    #
    class Popen(object):
        def __init__(self, process_obj):
    >>>>        sys.stdout.flush() ...
            sys.stderr.flush()
            self.returncode = None
            self.pid = os.fork()
            if self.pid == 0:
                if 'random' in sys.modules:
▼ Local vars
Variable    Value
process_obj 
<Process(Process-1, initial)>
self    
<multiprocessing.forking.Popen object at 0xb8a06dec>

Does python have a more magical way to do this? Does Django? If not, how can I go ahead and use multiprocessing?

Answer 1

Use celery.

The mod_wsgi environment can multi-threaded — depending on your configuration. You do not want to interfere with how Apache, mod_wsgi, and Django are already using threads and processes to manage web server throughput.

You have to assume that your Django operation is a single thread and cannot do anything except respond to Apache as quickly as possible.