On 11/20/2015 09:56 AM, Jan Wieck wrote:
> Without taking SYNC snapshots in a SERIALIZABLE transaction I believe
> that a Slony-I replica could suffer the same inconsistency dangers that
> a pg_dump without --serializable-deferrable can suffer. Namely a replica
> would not be usable as a source for reporting. From the 9.4 pg_dump docs:

I was wondering if this is actually possible or not.

The remote slon only selects from  sl_event and sl_log_*.  The remote
worker is only going to see rows that are covered by a snapshot range in
the SYNC in sl_event.  Rows in sl_log_* might be visible from a
transaction point of view but they won't be captured by the where
conditions for pulling from sl_log.

The snapshot for the event in sl_log is done by the local sync
connection which is a read-write connection not by a slon remote
read-only connection.

(I'm ignoring copy_set from the above analysis).


> 
> "This option is not beneficial for a dump which is intended only for
> disaster recovery. It could be useful for a dump used to load a copy of
> the database for reporting or other read-only load sharing while the
> original database continues to be updated. Without it the dump may
> reflect a state which is not consistent with any serial execution of the
> transactions eventually committed. For example, if batch processing
> techniques are used, a batch may show as closed in the dump without all
> of the items which are in the batch appearing."
> 
> Changing the default isolation levels(s) may therefore change, what a
> replica can safely be used for and I believe that creating reports is
> one of the major use cases. Using options with big, bold, red, flashing
> warnings in the documentation would be the only way to go.
> 
> 
> Regards, Jan
> 
> 
> 
>>
>>>
>>> 	Tom    :-)
>>>
>>>
>>> On 11/18/15, 10:35 AM, "Greg Sabino Mullane" <greg at endpoint.com> wrote:
>>>
>>>> On Wed, Nov 18, 2015 at 02:26:15PM +0000, Tom Tignor wrote:
>>>> ...
>>>>> Sorry for the delay getting back. Inspired by your questions, I¹ve been
>>>>> reading up on SSI, the Cahill paper and slony1 and postgres code.
>>>> ...
>>>>
>>>> It should be pointed out that 9.1 goes EOL (End of Life) in less than
>>>> a year (Sep 2016), and transaction handling has changed a *lot* since
>>>> then,
>>>> so any changes that core Slony makes may not even work for you.
>>>>
>>>> (FWIW, I think dropping the isolation level in this particular
>>>> instance seems safe, however.)
>>>>
>>>> --
>>>> Greg Sabino Mullane greg at endpoint.com
>>>> End Point Corporation
>>>> PGP Key: 0x14964AC8
>>>
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>>
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> 
>