Re: Synchronizing slots from primary to standby

On Wed, Oct 4, 2023 at 12:08 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> Hi,
>
> On 10/4/23 7:00 AM, shveta malik wrote:
> > On Wed, Oct 4, 2023 at 9:56 AM shveta malik <shveta.malik@gmail.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).
>

Agreed. Not a good idea to block each logical walsender.

> > 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.

yes, +1 for ReplicationSlotPersistentData approach. Will start
detailed analysis on that approach now.

thanks
Shveta