Re: Issues in Replication Progress Tracking

On Fri, May 22, 2015 at 11:20 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> On 2015-05-21 09:40:58 +0530, Amit Kapila wrote:
> > On Thu, May 21, 2015 at 12:42 AM, Andres Freund <andres(at)anarazel(dot)de>
wrote:
> > >
> > > On 2015-05-20 19:27:05 +0530, Amit Kapila wrote:
> > >
> > > > 13.
> > > > In function replorigin_session_setup() and or
> > > > replorigin_session_advance(), don't we need to WAL log the
> > > > use of Replication state?
> > >
> > > No, the point is that the replication progress is persisted via an
extra
> > > data block in the commit record. That's important for both performance
> > > and correctness, because otherwise it gets hard to tie a transaction
> > > made during replay with the update to the progress. Unless you use 2PC
> > > which isn't really an alternative.
> > >
> >
> > Okay, but what triggered this question was the difference of those
functions
> > as compare to when user call function pg_replication_origin_advance().
> > pg_replication_origin_advance() will WAL log the information during that
> > function call itself (via replorigin_advance()). So even if the
transaction
> > issuing pg_replication_origin_advance() function will abort, it will
still
> > update
> > the Replication State, why so?
>
> I don't see a problem here. pg_replication_origin_advance() is for
> setting up the initial position/update the position upon configuration
> changes.

Okay, I am not aware how exactly these API's will be used for replication
but let me try to clarify what I have in mind related to this API usage.

Can we use pg_replication_origin_advance() for node where Replay has
to happen, if Yes, then Let us say user of pg_replication_origin_advance()
API set the lsn position to X for the node N1 on which replay has to
happen, so now replay will proceed from X + 1 even though the
information related to X is not persisted, so now it could so happen
X will get written after the replay of X + 1 which might lead to problem
after crash recovery?

> It'd be a fair amount of infrastructure to make it tie into
> transactions - without a point to it afaics?
>

Agreed, that if there is no valid use case then we should keep it
as it is.

> (Just to be clear, I plan to address all the points I've not commented
> upon)
>

Thanks.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com