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Date: Mon, 19 Jun 2023 18:26:52 +0200
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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Subject: Re: Synchronizing slots from primary to standby
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Peter Eisentraut <peter.eisentraut@enterprisedb.com>,
 Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>,
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Hi,

On 6/19/23 12:03 PM, Amit Kapila wrote:
> On Mon, Jun 19, 2023 at 11:34 AM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>>
>> Also I think we need to handle the case of invalidated replication slot(s): should
>> we drop/recreate it/them? (as the main goal is to have sync slot(s) on the standby).
>>
> 
> Do you intend to ask what happens to logical slots invalidated (due to
> say max_slot_wal_keep_size) on publisher? I think those should be
> invalidated on standby too. 

Agree that it should behave that way.

> Another thought whether there is chance
> that the slot on standby gets invalidated due to conflict (say
> required rows removed on primary)?

That's the scenario I had in mind when asking the question above.

> I think in such cases the slot on
> primary/publisher should have been dropped/invalidated by that time.

I don't think so.

For example, such a scenario could occur:

- there is no physical slot between the standby and the primary
- the standby is shut down
- logical decoding on the primary is moving forward and now there is vacuum
operations that will conflict on the standby
- the standby starts and reports the logical slot being invalidated (while it is
not on the primary)

In such a case (slot valid on the primary but invalidated on the standby) then I think we
could drop and recreate the invalidated slot on the standby.

> BTW, does the patch handles drop of logical slots on standby when the
> same slot is dropped on publisher/primary?
> 

from what I've seen, yes it looks like it behaves that way (will look closer).

Regards,

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