Re: Synchronizing slots from primary to standby

On 11/20/23 11:59 AM, Amit Kapila wrote:
> On Mon, Nov 20, 2023 at 3:17 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>>
>> On 11/18/23 11:45 AM, Amit Kapila wrote:
>>> On Fri, Nov 17, 2023 at 5:18 PM Drouvot, Bertrand
>>> <bertranddrouvot.pg@gmail.com> wrote:
>>>>
>>>> On 11/17/23 2:46 AM, Zhijie Hou (Fujitsu) wrote:
>>>>> On Tuesday, November 14, 2023 10:27 PM Drouvot, Bertrand <bertranddrouvot.pg@gmail.com> wrote:
>>>>>
>>>>> I feel the WaitForWALToBecomeAvailable may not be the best place to shutdown
>>>>> slotsync worker and drop slots. There could be other reasons(other than
>>>>> promotion) as mentioned in comments in case XLOG_FROM_STREAM to reach the code
>>>>> there. I thought if the intention is to stop slotsync workers on promotion,
>>>>> maybe FinishWalRecovery() is a better place to do it as it's indicating the end
>>>>> of recovery and XLogShutdownWalRcv is also called in it.
>>>>
>>>> I can see that slotsync_drop_initiated_slots() has been moved in FinishWalRecovery()
>>>> in v35. That looks ok.
>>>>>
>>>
>>> I was thinking what if we just ignore creating such slots (which
>>> require init state) in the first place? I think that can be
>>> time-consuming in some cases but it will reduce the complexity and we
>>> can always improve such cases later if we really encounter them in the
>>> real world. I am not very sure that added complexity is worth
>>> addressing this particular case, so I would like to know your and
>>> others' opinions.
>>>
>>
>> I'm not sure I understand your point. Are you saying that we should not create
>> slots on the standby that are "currently" reported in a 'i' state? (so just keep
>> the 'r' and 'n' states?)
>>
> 
> Yes.
> 

As far the 'i' state here, from what I see, it is currently useful for:

1. Cascading standby to not sync slots with state = 'i' from
the first standby.
2. Easily report Slots that did not catch up on the primary yet.
3. Avoid inactive slots to block "active" ones creation.

So not creating those slots should not be an issue for 1. (sync are
not needed on cascading standby as not created on the first standby yet)
but is an issue for 2. (unless we provide another way to keep track and report
such slots) and 3. (as I think we should still need to reserve WAL).

I've a question: we'd still need to reserve WAL for those slots, no?

If that's the case and if we don't call ReplicationSlotCreate() then ReplicationSlotReserveWal()
would not work as  MyReplicationSlot would be NULL.

Regards,

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