[mod_python] Threading and apache.import_module

Nicolas Lehuen nicolas.lehuen at gmail.com
Sun Jun 19 04:19:21 EDT 2005

Just as a note, those issues have been thoroughly covered by Graham
Dumpleton in his Vampire project, and some of his ideas have been
retrofitted into the development version of mod_python.


2005/6/18, michael bayer <mike_mp at zzzcomputing.com>:
> I use the "two-check" approach to this, which allows multiple threads
> to get at an already-imported resource without blocking each other,
> while allowing the actual import to occur in a threadsafe
> manner...suppose every access of a module calls "verify_module" which
> reloads the module if needed, using the functions _needs_reload to
> perform a check and _reload_module to do the actual reload:
>     import threading
>     mutex = threading.RLock()
>     def verify_module(module):
>         if _needs_reload(module):
>            mutex.acquire()
>            try:
>                if _needs_reload(module):
>                     _reload_module(module)
>            finally:
>                mutex.release()
> in the normal case, threads will call _needs_reload() without
> synchronization and return, without any bottleneck occuring.  when
> the reload is actually required, threads will line up onto the
> mutex.acquire() and re-check once entering the critical section, thus
> allowing only the first thread inside to actually perform the reload.
> an additional optimization is to use a unique mutex for each
> module...that way multiple threads that are reloading several
> different modules also don't block each other.
> the issue of other threads executing code against the module while
> its being reloaded is not really addressed here.  as a normal reload
> () or imp.load_module() doesnt remove any of the old module values,
> its usually (but not always) the case that nothing really
> happens....unless there are persistent pointers to the module's
> globals lying around elsewhere, which then become outdated.    I did
> see a reload() implementation that loads a new module under a
> temporary name, then moved it's dictionary over, which attempts to be
> more atomic...I havent tried it but its over at http://
> lists.canonical.org/pipermail/kragen-hacks/2002-January/000302.html .
> On Jun 17, 2005, at 3:09 PM, Dan Eloff wrote:
> > Well this is the biggest problem in my otherwise threadsafe code. From
> > what I've read, importing/reloading modules in a threaded environment
> > is just plain dangerous. There could be any number of requests
> > executing code from and accessing global variables in a module when it
> > changes and gets reloaded. Then you have the two level problem of what
> > happens to the executing requests, as well as what happens when
> > multiple incoming requests all spot that the module has changed, and
> > reload it multiple times in quick succession, re-executing code in the
> > module scope (this is pretty much worst case)
> >
> > I know Graham knows a lot more about this than I do (help!).
> >
> > I do have a lot of flexibility here, since I'm using my own publisher
> > I do not have to use apache.import_module. In fact I would prefer to
> > not have the whole importing/reloading business affect my code outside
> > of the publisher, that way I can change things internally at will.
> > Clearly one option is to create a simple import function that imports
> > once only in a production environment, and every time in a development
> > env, but I'm still interested in exploring the options.
> >
> > -Dan
> >
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