[mod_python] Co-ordinating ccess to a single-use resource

Nicolas Lehuen wrote:

>2005/9/8, Graham Dumpleton <grahamd at dscpl.com.au>:
>  
>
>>On 09/09/2005, at 2:23 AM, John Simeone wrote:
>>
>>    
>>
>>>Hello to everyone.
>>>
>>>I am responsible for a project using Apache 2.0 with mod_python in a
>>>Win2k environment, the choice of OS being dictated by the fact that
>>>the purpose of the server is to supply access to a proprietary,
>>>supplier written Windows program to our user base via the web.
>>>
>>>That program is single threaded and provides for only 1 user at a
>>>time. Access to the program in Python is through  win32com.
>>>
>>>I am using a mod_python.publisher handler.
>>>
>>>All the http requests come through a single proxy server with a static
>>>address but not all are from browser clients. Some are originating
>>>from perl programs but all requests are directed to a single request
>>>handler.
>>>
>>>Can anyone suggest a method to control access to this single user
>>>resource. In a non-web programming environment, I'd just use a mutex
>>>semaphore to co-ordinate access among the threads but I am too much of
>>>newbie with mod_python and Apache to implement a workable solution so
>>>far.
>>>
>>>What mechanism can I use to create a shared resource that I can lock
>>>and have other requests wait until the resource is unlocked?
>>>      
>>>
>>When using mod_python under Win32 platform, there is effectively only
>>one process and requests are handled in separate threads. Because of
>>this, you should be able to mediate access to the shared resource
>>through a normal thread mutual exclusion locking mechanism just as
>>easily as you would in a non web environment.
>>
>>Graham
>>    
>>
>
>That is to say :
>
>import threading
>
># "R" stand for Re-entrant, i.e. a thread can acquire the lock multiple times
># make sure that the lock is instantiated only once, for example by
>putting it in
># a module which your published code imports.
>lock = threading.RLock() 
>
>lock.acquire()
>try:
>    # access the shared resource
>finally:
>    lock.release()
>
>See the documentation of the threading module for more information.
>
>Regards,
>Nicolas
>
>
>  
>
Thanks to Graham and Nicolas for their prompt responses.

I followed Nicolas' counsel and created a file 'tconstants.py' :

import threading
mutex = threading.RLock()

included an 'import' into my publisher handler and wrapped the access to 
the single resource with 'mutex.acquire()' and mutex.release(). When I 
got an error regarding the unavailability of the mutex global, I ran the 
following test in IDLE:

import threading
import tconstants
def setmutex():
    mutex.acquire()
    return
setmutex()

and received the same error:  "global name 'mutex' is not defined."

I ran the threading demo to verify there wasn't a problem with the 
thread module in my copy of Python 2.3. It ran fine.

The only way I could get this test to work was to alter the code to 
define the mutex not in the imported tconstants but in the code directly:

import threading
mutex = threading.RLock()
def setmutex():
    mutex.acquire()
    return
setmutex()
mutex.release()

Will defining the mutex this way cause problems in proper threading 
control for mod_python in its interaction with Apache?

Regards,

John


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