[Zope-dev] Re: Threads, Locks, Volatility, and Angst

[Tres Seaver]

Christian Theune wrote:
> On Thu, 2004-04-01 at 00:25, Ian Beatty wrote:
> 
>>I thought "Okay, no problem. I'll just make the lock variable volatile,
>>since I don't really care whether it persists when Zope shuts down." So I
>>renamed the lock to begin with '_v_'. That solves my "UnpickleableError"
>>problem nicely. Unfortunately, it introduces a different problem: the code
>>that calls acquire() on the lock now throws "AttributeError: 'NoneType'
>>object has no attribute 'acquire'". I'm sure I initialize that variable to a
>>threading.Lock() in the object's __init__ method. So now I'm worried that
>>Zope is doing all kinds of pickling-unpickling activity behind the scenes,
>>and anything I have that's volatile can disappear without warning.
> 
> 
> I'm not sure about this right now, but iirc _v_ attributes are not
> stored until "zope shutdown" but until the object is reloaded from the
> ZODB (for whatever reason it went out of the cache). This basically
> means no availability longer than until the end of one request can be
> guaranteed.

'_v_' variablews are not stored *at all* in the ZODB;  they are only 
useful for caching an expensive computed value.  In fact, the ZODB does 
not even guarantee that volatile won't go away during a single request, 
although objects are not normally ghosted until the end of a 
transaction.  Any use of volatiles needs careful thought.

An example:;

   class Bar:

       def __init__( self ):
           """ do something really expensive here.
           """

   class Foo( Persistent ):

       _v_bar = None

       def _getBar( self ):
           result = self._v_bar
           if result is None:
               result = self._v_bar = Bar()
           return result

       # Always use self._getBar() instead of self._v_bar.....

I don't think volatility is a good match for locks, which are tasked 
with ensuring consistency.

Tres.
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Tres Seaver                                tseaver at zope.com
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