[ZODB-Dev] newbie ZEO first try. ConflictError.

[tsmiller]

Christian,
Thanks. How painfully obvious.  I have written the words
'transaction.begin()' about a thousand times or so!  But I get
discombobulated when I start looking at something new ( ZEO ) and forget the
obvious.  Final code for this little test that works perfectly:

while True:
        transaction.begin()
        root[ "one" ] = "program 2 - " + time.asctime()
        while True:
                try:
                        transaction.commit()
                except POSException.ConflictError:
                        time.sleep(.2)
                else:
                        break

        time.sleep(5)

tom



Christian Theune-2 wrote:
> 
> Hi,
> 
> On 11/26/2009 10:08 PM, James Bergstra wrote:
>> On Thu, Nov 26, 2009 at 1:51 PM, tsmiller<tsmiller at gnixterhouse.com> 
>> wrote:
>>>
>>> Laurence,
>>> Thank you for your very quick reply.  I did as you suggested and now the
>>> ConflictError is now handled.  And that is a most excellent thing
>>> because
>>> now ConflictError is raised every time.  So it still seems to be
>>> confused.
>>> I am trying to figure out if I will be able to use ZEO in my
>>> application.
>>>
>>> Now my program reads:
>>>
>>> while True:
>>>         root[ "one" ] = time.asctime()
>>>         while True:
>>>                 try:
>>>                         print "Try to commit transaction"
>>>                         transaction.commit()
>>>                         print "root is",  root
>>>                 except POSException.ConflictError:
>>>                         print "we have a conflict"
>>>                         transaction.abort()
>>>                         time.sleep(.2)
>>>                 else:
>>>                         break
>>>
>>>         time.sleep(10)
>>>
>>
>> I have a related question about this code... in the inner loop tom is
>> calling abort() ; sleep(); commit().  Does that make sense?
>>
>> I thought that abort() would revert the root to the database's version
>> of things, and discard any change that the client had tried to make.
>> So what does it mean to call commit() again immediately and why that
>> commit cause a conflict when the client hasn't actually changed
>> anything since abort() ?
> 
> The abort() actually also marks the point when the next transaction 
> begins implicitly.
> 
> Your code would work perfectly if whenever you start a transaction, you 
> simply call transaction.begin():
> 
> while True:
>      transaction.begin()
>      root[....
> 
> This will cause the time.sleep(2) to not be included in the transaction 
> and your conflict rate will quickly go down for your example. If you 
> also use a random factor for the sleep (e.g. between 0.2 and 0.3) then 
> you'll have a lesser chance of subsequent conflicts in your example 
> because all your transactions are very similar. In real life 
> transactions would be distinct enough to cause different offsets in
> runtime.
> 
> Hope this helps,
> Christian
> 
> -- 
> Christian Theune · ct at gocept.com
> gocept gmbh & co. kg · forsterstraße 29 · 06112 halle (saale) · germany
> http://gocept.com · tel +49 345 1229889 0 · fax +49 345 1229889 1
> Zope and Plone consulting and development
> 
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