[Zope] zope operations atomic?

[Chris McDonough]

AFAIK, a read conflict is raised in this circumstance, so there can't be 
a dirty read.

But again, since requests which raise conflicts are retried, there is 
never a consistency problem.

- C


Kapil Thangavelu wrote:
> even more generally the problem can be stated as
> transaction A starts reads a bunch of objects,
> transaction B starts and will read a set of objects
> that intersects with transaction A. transaction A
> commits changing objects that B is about to read
> (after B has read some objects). B finishes, and has
> possibly  commited or done operations on data that is
> no longer in sync because the set of data may not have
> been consistent for its operation, hence the dirty
> read.
> 
> this is a more interesting and IMO legitimate concern,
> i haven't done any testing recently, but i believe as
> it currently stands this should throw a ConflictError
> (i'm not sure, though.). Jim and pythonlabs have
> talked about setting up some sort MVCC for the zodb,
> whereby transaction B will see a snapshot of objects
> as it existed when it began. if you're interested in
> this further i would send mail to the
> zodb-dev@zope.org list, referencing this thread.
>  
> cheers
> 
> kapil
> 
> --- Clark OBrien
> <COBrien@isis-server.vuse.vanderbilt.edu> wrote:
> 
>>Here is the example I was thinking about.
>>
>>
>>1) Suppose an online store is selling product X and
>>at a 
>>given moment its inventory is n. Two shoppers hit
>>the buy
>>button at about the same time. 
>>
>>Then either:
>>
>>a) No conflict is detected because both threads set
>>the inventory
>>value to the same value, n-1. This would cause a
>>problem if n = 1
>>
>>Or:
>>b) A conflict is detected when the last thread
>>writes because the inventory
>>is not 
>>n as it was when he read it. request is resubmitted
>>but again another thread
>>has 
>>changed made the value dirty- thread starves.
>>
>>
>>
>>
>>
>>
>> 
>>
>>
>>
>>-----Original Message-----
>>From: Chris McDonough [mailto:chrism@zope.com]
>>Sent: Thursday, November 01, 2001 6:50 AM
>>To: Clark OBrien; k_vertigo@yahoo.com; zope@zope.org
>>Subject: Re: [Zope] zope operations atomic?
>>
>>
>>What is "the dirty read problem"?
>>
> 
> 
> 
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-- 
Chris McDonough                    Zope Corporation
http://www.zope.org             http://www.zope.com
"Killing hundreds of birds with thousands of stones"