Re: Minimal logical decoding on standbys

Hi,

On 4/7/23 5:47 PM, Andres Freund wrote:
> Hi,
> 
>>> - write a test that invalidated logical slots do not lead to retaining WAL
>>
>> I'm not sure how to do that since pg_switch_wal() and friends can't be executed on
>> a standby.
> 
> You can do it on the primary and wait for the records to have been applied.
> 

Thanks, will give it a try in a couple of hours.

> 
>>> - Further evolve the API of InvalidateObsoleteReplicationSlots()
>>>     - pass in the ReplicationSlotInvalidationCause we're trying to conflict on?
>>>     - rename xid to snapshotConflictHorizon, that'd be more in line with the
>>>       ResolveRecoveryConflictWithSnapshot and easier to understand, I think
>>>
>>
>> Done. The new API can be found in v65-66-InvalidateObsoleteReplicationSlots_API.patch
>> attached. It propagates the cause to InvalidatePossiblyObsoleteSlot() where a switch/case
>> can now be used.
> 
> Integrated. I moved the cause to the first argument, makes more sense to me
> that way.

thanks!

> 
> I made it an error - it's a programming error, not some data level
> inconsistency if that ever happens.

okay, makes sense.
> 
> Integrated all of these.

Thanks!

> 
> 
> I think pg_log_standby_snapshot() should be added in "Allow logical decoding
> on standby", not the commit adding the tests.

Yeah, that's a good point, I do agree.

> 
> Is this patchset sufficient to subscribe to a publication on a physical
> standby, assuming the publication is created on the primary? If so, we should
> have at least a minimal test. If not, we should note that restriction
> explicitly.

I gave it a try and it does work.

"
node3 subscribes to node2 (standby).
Insert done in node1 (primary) where the publication is created => node3 see the changes.
"

I started to create the TAP test but currently stuck as the "create subscription" waits for a
checkpoint/pg_log_standby_snapshot()on the primary.
 

So, trying to make use of things like:

"my %psql_subscriber = ('stdin' => '', 'stdout' => '');
$psql_subscriber{run} =
   $node_subscriber->background_psql('postgres', \$psql_subscriber{stdin},
     \$psql_subscriber{stdout},
     $psql_timeout);
$psql_subscriber{stdout} = '';
"

But in vain so far...

Will resume working on it in a couple of hours.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com