Re: Synchronizing slots from primary to standby

Hi,

On 9/29/23 1:33 PM, Amit Kapila wrote:
> On Thu, Sep 28, 2023 at 6:31 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>>
>>
>> I think that standby_slot_names could be used to do some filtering (means
>> for which standby(s) we don't want the logical replication on the primary to go
>> ahead and for which standby(s) one would allow it).
>>
> 
> Isn't it implicit that the physical standby that has requested
> 'synchronize_slot_names' should be ahead of their corresponding
> logical walsenders? Otherwise, after the switchover to the new
> physical standby, the logical subscriptions won't work.

Right, but the idea was to let the flexibility to bypass this constraint. Use
case was to avoid a physical standby being down preventing the decoding
on the primary.

> 
>> I think that removing the GUC would:
>>
>> - remove this flexibility
>>
> 
> I think if required we can add such a GUC later as well. Asking users
> to set more parameters also makes the feature less attractive, so I am
> trying to see if we can avoid this GUC.

Agree but I think we have to address the standby being down case.
> 
>> - probably open corner cases like: what if a standby is down? would that mean
>> that synchronize_slot_names not being send to the primary would allow the decoding
>> on the primary to go ahead?
>>
> 
> Good question. BTW, irrespective of whether we have
> 'standby_slot_names' parameters or not, how should we behave if
> standby is down? Say, if 'synchronize_slot_names' is only specified on
> standby then in such a situation primary won't be even aware that some
> of the logical walsenders need to wait.

Exactly, that's why I was thinking keeping standby_slot_names to address
this scenario. In such a case one could simply decide to keep or remove
the associated physical replication slot from standby_slot_names. Keep would
mean "wait" and removing would mean allow to decode on the primary.

> OTOH, one can say that users
> should configure 'synchronize_slot_names' on both primary and standby
> but note that this value could be different for different standby's,
> so we can't configure it on primary.
> 

Yeah, I think that's a good use case for standby_slot_names, what do you think?

Regards,

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