[ZODB-Dev] Savepoint release support

[Laurence Rowe]

2010/1/17 Jim Fulton <jim at zope.com>:
> On Sat, Jan 16, 2010 at 1:03 PM, Laurence Rowe <l at lrowe.co.uk> wrote:
>> I've had a request to add savepoint release support to zope.sqlalchemy
>> as some databases seem to limit the number of savepoints in a
>> transaction.
>>
>> I've added this in a branch of transaction here:
>> svn+ssh://svn.zope.org/repos/main/transaction/branches/elro-savepoint-release
>>
>> >From the changelog:
>>
>> * Add support for savepoint.release(). Some databases only support a limited
>>  number of savepoints or subtransactions, this provides an opportunity for a
>>  data manager to free those resources.
>>
>> * Rename InvalidSavepointRollbackError to InvalidSavepointError (BBB provided.)
>>
>> If there are no objections, I shall merge this to trunk.
>
> I'll review and merge.

Great, thanks!

> What does it mean to "release" a savepoint? How is this different from aborting
> a save point? I ask particularly in light of:
>
> On Sat, Jan 16, 2010 at 2:26 PM, Laurence Rowe <l at lrowe.co.uk> wrote:
>> 2010/1/16 Laurence Rowe <l at lrowe.co.uk>:
>>> I'm still not sure this will allow me to add savepoint release support
>>> to zope.sqlalchemy, as SQLAlchemy has a concept of nested transactions
>>> rather than savepoints.
>>> http://groups.google.com/group/sqlalchemy/browse_thread/thread/7a4632587fd97724
>>
>> Michael Bayer noted on the sqlalchemy group that on RELEASE SAVEPOINT
>> Postgresql destroys all subsequent savepoints. My branch now
>> implements this behaviour.

For zope.sqlalchemy I commit the sqlalchemy substransaction on
savepoint.release(). This translates to a RELEASE SAVEPOINT on
postgresql, best described by their docs here:

"""
RELEASE SAVEPOINT destroys a savepoint previously defined in the
current transaction.

Destroying a savepoint makes it unavailable as a rollback point, but
it has no other user visible behavior. It does not undo the effects of
commands executed after the savepoint was established. (To do that,
see ROLLBACK TO SAVEPOINT.) Destroying a savepoint when it is no
longer needed allows the system to reclaim some resources earlier than
transaction end.

RELEASE SAVEPOINT also destroys all savepoints that were established
after the named savepoint was established.
"""
http://developer.postgresql.org/pgdocs/postgres/sql-release-savepoint.html

Laurence