Re: Synchronizing slots from primary to standby

Hi,

On 6/20/23 12:22 PM, Amit Kapila wrote:
> On Mon, Jun 19, 2023 at 9:56 PM Drouvot, Bertrand
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:

>> In such a case (slot valid on the primary but invalidated on the standby) then I think we
>> could drop and recreate the invalidated slot on the standby.
>>
>
> Will it be safe? Because after recreating the slot, it will reserve
> the new WAL location and build the snapshot based on that which might
> miss some important information in the snapshot. For example, to
> update the slot's position with new information from the primary, the
> patch uses pg_logical_replication_slot_advance() which means it will
> process all records and update the snapshot via
> DecodeCommit->SnapBuildCommitTxn().

Your concern is that the slot could have been consumed on the standby?

I mean, if we suppose the "synchronized" slot can't be consumed on the standby then
drop/recreate such an invalidated slot would be ok?

Asking, because I'm not sure we should allow consumption of a "synchronized" slot
until the standby gets promoted.

When the patch has been initially proposed, logical decoding from a standby
was not implemented yet.

> The other related thing is that do we somehow need to ensure that WAL
> is replayed on standby before moving the slot's position to the target
> location received from the primary?

Yeah, will check if this is currently done that way in the patch proposal.

>>> BTW, does the patch handles drop of logical slots on standby when the
>>> same slot is dropped on publisher/primary?
>>>
>>
>> from what I've seen, yes it looks like it behaves that way (will look closer).
>>
>
> Okay, I have asked because I don't see a call to ReplicationSlotDrop()
> in the patch.
>

Right. I'd need to look closer to understand how it works (for the moment
the "only" thing I've done was the re-base shared up-thread).

Regards,

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