Re: Synchronizing slots from primary to standby

Hi,

On 9/25/23 6:10 AM, shveta malik wrote:
> On Fri, Sep 22, 2023 at 3:48 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>
>> On Thu, Sep 21, 2023 at 9:16 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>>>
>>> On Tue, Sep 19, 2023 at 10:29 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>>>
>>> Currently in patch001, synchronize_slot_names is a GUC on both primary
>>> and physical standby. This GUC tells which all logical slots need to
>>> be synced on physical standbys from the primary. Ideally it should be
>>> a GUC on physical standby alone and each physical standby should be
>>> able to communicate the value to the primary (considering the value
>>> may vary for different physical replicas of the same primary). The
>>> primary on the other hand should be able to take UNION of these values
>>> and let the logical walsenders (belonging to the slots in UNION
>>> synchronize_slots_names) wait for physical standbys for confirmation
>>> before sending those changes to logical subscribers. The intent is
>>> logical subscribers should never be ahead of physical standbys.
>>>
>>
>> Before getting into the details of 'synchronize_slot_names', I would
>> like to know whether we really need the second GUC
>> 'standby_slot_names'. Can't we simply allow all the logical wal
>> senders corresponding to 'synchronize_slot_names' to wait for just the
>> physical standby(s) (physical slot corresponding to such physical
>> standby) that have sent ' synchronize_slot_names'list? We should have
>> one physical standby slot corresponding to one physical standby.
>>
>
> yes, with the new approach (to be implemented next) where we plan to
> send synchronize_slot_names from each physical standby to primary, the
> standby_slot_names GUC should no longer be needed on primary. The
> physical standbys sending requests should automatically become the
> ones to be waited for confirmation on the primary.
>

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).

I think that removing the GUC would:

- remove this flexibility
- 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?

So, I'm not sure we should remove this GUC.

Regards,

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