[ZODB-Dev] Re: RelStorage now in Subversion

[Tres Seaver]

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Shane Hathaway wrote:
> Tres Seaver wrote:
>> I've got a question for you on the removal of the LISTEN / NOTIFY stuff
>> in the Postgresql adapter:  it looks to me as though your code was not
>> doing the async check-for-notifies as shown in the 'notify.py' in the
>> psycopg2 examples:
>>
>> http://www.initd.org/tracker/psycopg/browser/psycopg2/trunk/examples/notify.py
>>
>> So, any notifies you got were only capture on the basis of "syncrhonous"
>> activity (a cursor doing some other work).  Could that explain the
>> unreliability which caused you to remove it?  The symptom would be like
>> the time that the ZEO async stuff got dropped in Zope3 during the
>> Twisted integration[1]:  a "busy" client would stay up to date, but an
>> idle one would serve stale objects from cache.
> 
> It does appear that a short select() loop is required.  However, I tried
> that and learned that notifications have no timeliness guarantees.  The
> current model requires strict timeliness of change notification in order
> to maintain ZODB consistency.
> 
> However, I think we can still take advantage of listen and notify in a
> slightly different way.  Currently, the Postgres adapter rolls back the
> load transaction on every connection close, but if it instead leaves the
> session idle with a transaction open, it would be safe for RelStorage to
> sometimes ignore new changes and only roll back and poll after an async
> notification arrives.  I could see scalability going way up that way, as
> long as Postgres doesn't mind all the idle sessions with an open
> transaction.

Hmm, I think I read that NOTIFY events are only delivered between
transactions, which would make that strategy unworkable.  It might
satisfy the timeliness / ordering problem you mention above, though.


Tres.
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Tres Seaver          +1 540-429-0999          tseaver at palladion.com
Palladion Software   "Excellence by Design"    http://palladion.com
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