Thread: [HACKERS] Use of snapshot in logical replication

On 7 June 2017 at 18:16, sanyam jain <sanyamjain22@live.in> wrote:
> Hi,
>
> Can someone explain the usage of exporting snapshot when a logical
> replication slot is created?

It's used to pg_dump the schema at a consistent point in history where
all xacts are known to be in the snapshot (and thus dumped) or known
not to be (so they'll be streamed out on the slot).

See snapbuild.c etc.


-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services

On 08/06/17 03:08, Craig Ringer wrote:
> On 7 June 2017 at 18:16, sanyam jain <sanyamjain22@live.in> wrote:
>> Hi,
>>
>> Can someone explain the usage of exporting snapshot when a logical
>> replication slot is created?
> 
> It's used to pg_dump the schema at a consistent point in history where
> all xacts are known to be in the snapshot (and thus dumped) or known
> not to be (so they'll be streamed out on the slot).
> 

Not just schema, it can also be used to get existing data at consistent
point in history so that changes will follow without gaps or duplicates.

That being said, the built-in logical replication isn't using the
exported snapshots at all.

--  Petr Jelinek                  http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training &
Services

On 2017-06-08 03:14:55 +0200, Petr Jelinek wrote:
> On 08/06/17 03:08, Craig Ringer wrote:
> > On 7 June 2017 at 18:16, sanyam jain <sanyamjain22@live.in> wrote:
> >> Hi,
> >>
> >> Can someone explain the usage of exporting snapshot when a logical
> >> replication slot is created?
> > 
> > It's used to pg_dump the schema at a consistent point in history where
> > all xacts are known to be in the snapshot (and thus dumped) or known
> > not to be (so they'll be streamed out on the slot).
> > 
> 
> Not just schema, it can also be used to get existing data at consistent
> point in history so that changes will follow without gaps or duplicates.
> 
> That being said, the built-in logical replication isn't using the
> exported snapshots at all.

Just because it uses them in the same session doesn't really change the
picture, that's more or less just an optimization.  It's essentially
just a cheaper version of importing a snapshot that'd have to be
exported in same session.

- Andres