Re: Synchronizing slots from primary to standby

On Thu, Dec 7, 2023 at 1:19 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> Hi,
>
> On 12/6/23 11:58 AM, shveta malik wrote:
> > On Wed, Dec 6, 2023 at 3:00 PM Drouvot, Bertrand
> > <bertranddrouvot.pg@gmail.com> wrote:
> >>
> >> Hi,
> >>
> >> On 12/6/23 7:18 AM, shveta malik wrote:
> >>> On Wed, Dec 6, 2023 at 10:56 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >>>>
> >>>> I feel that is indirectly relying on the fact that the primary won't
> >>>> advance logical slots unless physical standby has consumed data.
> >>>
> >>> Yes, that is the basis of this discussion.
> >>
> >> Yes.
> >>
> >>> But now on rethinking, if
> >>> the user has not set 'standby_slot_names' on primary at first pace,
> >>> then even if walreceiver on standby is down, slots on primary will
> >>> keep on advancing
> >>
> >> Oh right, good point.
> >>
> >>> and thus we need to sync.
> >>
> >> Yes and I think our current check "XLogRecPtrIsInvalid(WalRcv->latestWalEnd)"
> >> in synchronize_slots() prevents us to do so (as I think WalRcv->latestWalEnd
> >> would be invalid for a non started walreceiver).
> >>
> >
> > But I think we do not need to deal with the case that walreceiver is
> > not started at all on standby. It is always started. Walreceiver not
> > getting started or down for long is a rare scenario. We have other
> > checks too for 'latestWalEnd' in slotsync worker and I think we should
> > retain those as is.
> >
>
> Agree to not deal with the walreceiver being down for now (we can
> still improve that part later if we encounter the case in the real
> world).
>

yes, agreed.

> Might be worth to add comments in the code (around the WalRcv->latestWalEnd
> checks) that no "lagging" sync are possible if the walreceiver is not started
> though?
>

I am a bit confused. Do you mean as a TODO item? Otherwise the comment
will be opposite of the code we are writing.

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