Re: Minimal logical decoding on standbys

On Tue, 4 Jun 2019 at 21:28, Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2019-06-04 15:51:01 +0530, Amit Khandekar wrote:
> > After giving more thought on this, I think it might make sense to
> > arrange for the xl_running_xact record to be sent from master to the
> > standby, when a logical slot is to be created on standby. How about
> > standby sending a new message type to the master, requesting for
> > xl_running_xact record ? Then on master, ProcessStandbyMessage() will
> > process this new message type and call LogStandbySnapshot().
>
> I think that should be a secondary feature. You don't necessarily know
> the upstream master, as the setup could be cascading one.
Oh yeah, cascading setup makes it more complicated.

> I think for
> now just having to wait, perhaps with a comment to manually start a
> checkpoint, ought to suffice?

Ok.

Since this requires the test to handle the
fire-create-slot-and-then-fire-checkpoint-from-master actions, I was
modifying the test file to do this. After doing that, I found that the
slave gets an assertion failure in XLogReadRecord()=>XRecOffIsValid().
This happens only when the restart_lsn is set to ReplayRecPtr.
Somehow, this does not happen when I manually create the logical slot.
It happens only while running testcase. Working on it ...



-- 
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company