More mod_wsgi weirdness: process restarts on redirect

[Graham Dumpleton]

On Jan 30, 11:01 am, Ron Garret <rNOSPA... at flownet.com> wrote:
> In article <mailman.8321.1233272610.3487.python-l... at python.org>,
>  Joshua Kugler <jos... at joshuakugler.com> wrote:
>
> > Ron Garret wrote:
> > > My question is: is this supposed to be happening?  Or is this an
> > > indication that something is wrong, and if so, what?
>
> > You are probably just hitting a different instance of Apache, thus the
> > different process ID.
>
> Yep, that's what it turned out to be.  I thought I had a
> WSGIDaemonProcess processes=1 directive in my config, but I had it in
> the wrong place (a different vhost) so it wasn't actually active.
> http://code.google.com/p/modwsgi/wiki/ProcessesAndThreading
> But that leaves me wondering why the redirect would reliably trigger
> switching processes.  The reason I thought that I had the correct
> configuration and only had one process is that when I reloaded the
> non-redirected page I *always* got the same process ID.  How doesmod_wsgidecide which process  (and which thread for that matter) to use?

Details on process/threading in mod_wsgi available at:

  http://code.google.com/p/modwsgi/wiki/ProcessesAndThreading

When using WSGIDaemonProcess directive, if you want a single process
it is better to allow it to default to a single process and not have
'processes=1'. As soon as you say 'processes=1' it will trigger
wsgi.multiprocess to be True rather than default of False. This may
sound counter intuitive, but is a little back door to allow
wsgi.multiprocess to be set to True somehow when distributing an
application across a cluster of machines where it does need to be True
even if each machine only has a single process for that application.
Tthat wsgi.multiprocess is True will not usually matter unless you are
trying to use debugging middleware that require that there only be a
single process.

As to why you were getting a different process, because you were
actually running in embedded mode due to WSGIDaemonProcess/
WSGIProcessGroup being in wrong context, then what process was used
was really up to Apache and how it works. Specifically it can have
multiple processes that can listen on the HTTP port (80). Because only
one should be listening at a time it uses a cross process mutex lock
to mediate access. When a process handles a request, it gives up the
lock. If using worker MPM then another thread in same process may get
lock, or for either worker MPM or prefork MPM, then another process
could get it. Which actually gets it is a bit indeterminate as simply
depends on which process the operating system lets have the lock next.
So, there is no strict rule one can say as to who would get it next.

Graham
Graham