Re: Synchronizing slots from primary to standby

Hi,

On 10/4/23 7:00 AM, shveta malik wrote:
> On Wed, Oct 4, 2023 at 9:56 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:

> The most simplistic approach would be:
>
> 1) maintain standby_slot_names GUC on primary
> 2) maintain synchronize_slot_names GUC on physical standby alone.
>
> On primary, let all logical-walsenders wait on physical-standbys
> configured in standby_slot_names GUC. This will work and will avoid
> all the complexity involved in designs discussed above. But this
> simplistic approach comes with disadvantages like below:
>
> 1) Even if the associated slot of logical-walsender is not part of
> synchronize_slot_names of any of the physical-standbys, it is still
> waiting for all the configured standbys to finish.

That's right. Currently (with walsender waiting an arbitrary amount of time)
that sounds like a -1. But if we're going with a new CV approach (like proposed
in [1], that might not be so terrible). Though I don't feel comfortable with
waiting for no reasons (even if this is for a short amount of time possible).

> 2) If associated slot of logical walsender is part of
> synchronize_slot_names of standby1, it is still waiting on standby2,3
> etc to finish i.e. waiting on rest of the standbys configured in
> standby_slot_names which have not even marked that logical slot in
> their synchronize_slot_names.
>

Same thoughts as above for 1)

> So we need to weigh our options here.
>

With the simplistic approach, if a standby goes down that would impact non related
walsenders on the primary until the standby's associated physical slot is removed
from standby_slot_names and I don't feel comfortable wit this behavior.

So, I'm +1 for the ReplicationSlotPersistentData approach proposed by Amit.

[1]: https://www.postgresql.org/message-id/CAA4eK1LNjgL6Lghgu1PcDfuoOfa8Ug4J7Uv-H%3DBPP8Wgf1%2BpOw%40mail.gmail.com

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
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