Re: Synchronizing slots from primary to standby - Mailing list pgsql-hackers

Hi,

On 10/25/23 5:00 AM, shveta malik wrote:
> On Tue, Oct 24, 2023 at 11:54 AM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>>
>> Hi,
>>
>> On 10/23/23 2:56 PM, shveta malik wrote:
>>> On Mon, Oct 23, 2023 at 5:52 PM Drouvot, Bertrand
>>> <bertranddrouvot.pg@gmail.com> wrote:
>>
>>>> We are waiting for DEFAULT_NAPTIME_PER_CYCLE (3 minutes) before checking if there
>>>> is new synced slot(s) to be created on the standby. Do we want to keep this behavior
>>>> for V1?
>>>>
>>>
>>> I think for the slotsync workers case, we should reduce the naptime in
>>> the launcher to say 30sec and retain the default one of 3mins for
>>> subscription apply workers. Thoughts?
>>>
>>
>> Another option could be to keep DEFAULT_NAPTIME_PER_CYCLE and create a new
>> API on the standby that would refresh the list of sync slot at wish, thoughts?
>>
> 
> Do you mean API to refresh list of DBIDs rather than sync-slots?
> As per current design, launcher gets DBID lists for all the failover
> slots from the primary at intervals of DEFAULT_NAPTIME_PER_CYCLE.

I mean an API to get a newly created slot on the primary being created/synced on
the standby at wish.

Also let's imagine this scenario:

- create logical_slot1 on the primary (and don't start using it)

Then on the standby we'll get things like:

2023-10-25 08:33:36.897 UTC [740298] LOG:  waiting for remote slot "logical_slot1" LSN (0/C00316A0) and catalog xmin
(752)to pass local slot LSN (0/C0049530) and and catalog xmin (754)
 

That's expected and due to the fact that ReplicationSlotReserveWal() does set the slot
restart_lsn to a value < at the corresponding restart_lsn slot on the primary.

- create logical_slot2 on the primary (and start using it)

Then logical_slot2 won't be created/synced on the standby until there is activity on logical_slot1 on the primary
that would produce things like:

2023-10-25 08:41:35.508 UTC [740298] LOG:  wait over for remote slot "logical_slot1" as its LSN (0/C005FFD8) and
catalogxmin (756) has now passed local slot LSN (0/C0049530) and catalog xmin (754)
 

With this new dedicated API, it will be:

- clear that the API call is "hanging" until there is some activity on the newly created slot
(currently there is "waiting for remote slot " message in the logfile as mentioned above but
I'm not sure that's enough)

- be possible to create/sync logical_slot2 in the example above without waiting for activity
on logical_slot1.

Maybe we should change our current algorithm during slot creation so that a newly created inactive
slot on the primary does not block other newly created "active" slots on the primary to be created
on the standby? Depending on how we implement that, the new API may not be needed at all.

Thoughts?

Regards,

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