Re: Synchronizing slots from primary to standby

Hi,

On 12/5/23 12:32 PM, Amit Kapila wrote:
> On Tue, Dec 5, 2023 at 10:38 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>>
>> On Mon, Dec 4, 2023 at 10:07 PM Drouvot, Bertrand
>> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>>>>
>>>
>>> Maybe another option could be to have the walreceiver a way to let the slot sync
>>> worker knows that it (the walreceiver) was not able to start due to non existing
>>> replication slot on the primary? (that way we'd avoid the slot sync worker having
>>> to talk to the primary).
>>
>> Few points:
>> 1) I think if we do it, we should do it in generic way i.e. slotsync
>> worker should go to no-op if walreceiver is not able to start due to
>> any reason and not only due to invalid primary_slot_name.
>> 2) Secondly, slotsync worker needs to make sure it has synced the
>> slots so far i.e. worker should not go to no-op immediately on seeing
>> missing WalRcv process if there are pending slots to be synced.
>>
>
> Won't it be better to just ping and check the validity of
> 'primary_slot_name' at the start of slot-sync and if it is changed
> anytime? I think it would be better to avoid adding dependency on
> walreciever state as that sounds like needless complexity.

I think the overall extra complexity is linked to the fact that we first
want to ensure that the slots are in sync before shutting down the
sync slot worker.

I think than talking to the primary or relying on the walreceiver state
is "just" what would trigger the decision to shutdown the sync slot worker.

Relying on the walreceiver state looks better to me (as it avoids possibly
useless round trips with the primary).

Also the walreceiver could be down for multiple reasons, and I think there
is no point of having a sync slot worker running if the slots are in sync and
there is no walreceiver running (even if primary_slot_name is a valid one).

That said, I'm also ok with the "ping primary" approach if others have another
point of view and find it better.

Regards,

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