RE: Synchronizing slots from primary to standby

On Monday, February 26, 2024 1:19 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Feb 23, 2024 at 4:45 PM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >
> > On Fri, Feb 23, 2024 at 09:46:00AM +0000, Zhijie Hou (Fujitsu) wrote:
> > >
> > > Besides, I'd like to clarify and discuss the behavior of standby_slot_names
> once.
> > >
> > > As it stands in the patch, If the slots specified in
> > > standby_slot_names are dropped or invalidated, the logical walsender
> > > will issue a WARNING and continue to replicate the changes. Another
> > > option for this could be to have the walsender pause until the slot
> > > in standby_slot_names is re-created or becomes valid again. Does anyone
> else have an opinion on this matter ?
> >
> > Good point, I'd vote for: the only reasons not to wait are:
> >
> > - slots mentioned in standby_slot_names exist and valid and do catch
> > up or
> > - standby_slot_names is empty
> >
> > The reason is that setting standby_slot_names to a non empty value
> > means that one wants the walsender to wait until the standby catchup.
> > The way to remove this intentional behavior should be by changing the
> > standby_slot_names value (not the existence or the state of the slot(s) it
> points too).
> >
>
> It seems we already do wait for the case when there is an inactive slot as per the
> below code [1] in the patch. So, probably waiting in other cases is also okay and
> also as this parameter is a SIGHUP parameter, users should be easily able to
> change its value if required. Do you think it is a good idea to mention this in
> docs as well?
>
> I think it is important to raise WARNING as the patch is doing in all the cases
> where the slot is not being processed so that users can be notified and they can
> take the required action.

Agreed. Here is the V99 patch which addressed the above.

This version also includes:
1. list_free the slot list when reloading the list due to GUC change.
2. Refactored the validate_standby_slots based on Shveta's suggestion.
3. Added errcode for the warnings as most of existing have errcodes.

Amit's latest comments[1] are pending, we will address that in next version.

[1] https://www.postgresql.org/message-id/CAA4eK1LJdmGATWG%3DxOD1CB9cogukk2cLNBGH8h-n-ZDJuwBdJg%40mail.gmail.com

Best Regards,
Hou zj