Re: Synchronizing slots from primary to standby

Hi,

On 6/26/23 12:34 PM, Amit Kapila wrote:
> On Mon, Jun 26, 2023 at 11:15 AM Drouvot, Bertrand
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>>
>> On 6/20/23 12:22 PM, Amit Kapila wrote:
>>> On Mon, Jun 19, 2023 at 9:56 PM Drouvot, Bertrand
>>> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>>
>>>> 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.
>>>>
>>>
>>> Will it be safe? Because after recreating the slot, it will reserve
>>> the new WAL location and build the snapshot based on that which might
>>> miss some important information in the snapshot. For example, to
>>> update the slot's position with new information from the primary, the
>>> patch uses pg_logical_replication_slot_advance() which means it will
>>> process all records and update the snapshot via
>>> DecodeCommit->SnapBuildCommitTxn().
>>
>> Your concern is that the slot could have been consumed on the standby?
>>
>> I mean, if we suppose the "synchronized" slot can't be consumed on the standby then
>> drop/recreate such an invalidated slot would be ok?
>>
>
> That also may not be sufficient because as soon as the slot is
> invalidated/dropped, the required WAL could be removed on standby.
>

Yeah, I think once the slot is dropped we just have to wait for the slot to
be re-created on the standby according to the new synchronize_slot_names GUC.

Assuming the initial slot "creation" on the standby (coming from the synchronize_slot_names usage)
is working "correctly" then it should also work "correctly" once the slot is dropped.

If we agree that a synchronized slot can not/should not be consumed (will implement this behavior) then
I think the proposed scenario above should make sense, do you agree?

Regards,

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