Re: Minimal logical decoding on standbys

On Thu, 14 Mar 2019 at 15:00, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
> I managed to get a recovery conflict by :
> 1. Setting hot_standby_feedback to off
> 2. Creating a logical replication slot on standby
> 3. Creating a table on master, and insert some data.
> 2. Running : VACUUM FULL;
>
> This gives WARNING messages in the standby log file.
> 2019-03-14 14:57:56.833 IST [40076] WARNING: slot decoding_standby w/
> catalog xmin 474 conflicts with removed xid 477
> 2019-03-14 14:57:56.833 IST [40076] CONTEXT: WAL redo at 0/3069E98
> for Heap2/CLEAN: remxid 477
>
> But I did not add such a testcase into the test file, because with the
> current patch, it does not do anything with the slot; it just keeps on
> emitting WARNING in the log file; so we can't test this scenario as of
> now using the tap test.

I am going ahead with drop-the-slot way of handling the recovery
conflict. I am trying out using ReplicationSlotDropPtr() to drop the
slot. It seems the required locks are already in place inside the for
loop of ResolveRecoveryConflictWithSlots(), so we can directly call
ReplicationSlotDropPtr() when the slot xmin conflict is found.

As explained above, the only way I could reproduce the conflict is by
turning hot_standby_feedback off on slave, creating and inserting into
a table on master and then running VACUUM FULL. But after doing this,
I am not able to verify whether the slot is dropped, because on slave,
any simple psql command thereon, waits on a lock acquired on sys
catache, e.g. pg_authid. Working on it.

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