RE: Synchronizing slots from primary to standby

On Wednesday, November 29, 2023 5:55 PM Drouvot, Bertrand <bertranddrouvot.pg@gmail.com> wrote:

Hi,

> On 11/29/23 6:58 AM, Zhijie Hou (Fujitsu) wrote:
> > On Tuesday, November 28, 2023 8:07 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
> >
> > Hi,
> >
> >> On 11/27/23 9:57 AM, Zhijie Hou (Fujitsu) wrote:
> >>> On Monday, November 27, 2023 4:51 PM shveta malik
> >> <shveta.malik@gmail.com> wrote:
> >>>
> >>> Here is the updated version(v39_2) which include all the changes
> >>> made in
> >> 0002.
> >>> Please use for review, and sorry for the confusion.
> >>>
> >>
> >> Thanks!
> >>
> >> As far v39_2-0001:
> >>
> >> "
> >>       Altering the failover option of the subscription is currently not
> >>       permitted. However, this restriction may be lifted in future versions.
> >> "
> >>
> >> Should we mention that we can alter the related replication slot?
> >
> > Will add.
> >
> >>
> >> +         <para>
> >> +          The implementation of failover requires that replication
> >> +          has successfully finished the initial table synchronization
> >> +          phase. So even when <literal>failover</literal> is enabled for a
> >> +          subscription, the internal failover state remains
> >> +          temporarily <quote>pending</quote> until the
> >> + initialization
> >> phase
> >> +          completes. See column
> >> <structfield>subfailoverstate</structfield>
> >> +          of <link
> >>
> linkend="catalog-pg-subscription"><structname>pg_subscription</struct
> >> na
> >> me></link>
> >> +          to know the actual failover state.
> >> +         </para>
> >>
> >> I think we have a corner case here. If one alter the replication slot
> >> on the primary then "subfailoverstate" is not updated accordingly on the
> subscriber.
> >> Given the 2 remarks above would that make sense to prevent altering a
> >> replication slot associated to a subscription?
> >
> > Thanks for the review!
> >
> > I think we could not distinguish the user created logical slot or
> > subscriber created slot as there is no related info in slot's data.
> 
> Yeah that would need extra work.
> 
> > And user could change
> > the slot on subscription by "alter sub set (slot_name)", so
> > maintaining this info would need some efforts.
> >
> 
> Yes.
> 
> > Besides, I think this case overlaps the previous discussed "alter sub
> > set (slot_name)" issue[1]. Both the cases are because the slot's
> > failover is different from the subscription's failover setting.
> 
> Yeah agree.
> 
> > I think we could handle
> > them similarly that user need to take care of not changing the
> > failover to wrong value. Or do you prefer another approach that
> > mentioned in that thread[1] ? (always alter the slot at the startup of apply
> worker).
> >
> 
> I think I'm fine with documenting the fact that the user should not change the
> failover value. But if he does change it (because at the end nothing prevents it
> to do so) then I think the meaning of subfailoverstate should still make sense.
> 
> One way to achieve this could be to change its meaning? Say rename it to say
> subfailovercreationstate (to reflect the fact that it was the state at the creation
> time) and change messages like:
> 
> "
> ALTER SUBSCRIPTION with refresh and copy_data is not allowed when failover
> is enabled "
> 
> to something like
> 
> "
> ALTER SUBSCRIPTION with refresh and copy_data is not allowed for
> subscription created with failover enabled"
> "
> 
> and change the doc accordingly.
> 
> What do you think?

This idea may work for now, but I think we planned to support ALTER
SUBSCRIPTION (failover) in a later patch, which means the meaning of
subfailovercreationstate may be invalid after that because we will be able to
change this value using ALTER SUBSCRIPTION as well.

I think document the case is OK because:

Currently, user already can create similar inconsistency cases as we don't restrict
user to change the slot on publisher. E.g., User could drop and recreate the
slot used by subscription but with different setting. Or user ALTER
SUBSCRIPTION set (slot_name) to switch to a new slot with different setting.

For example, about two_phase option, user can create a subscription with
two_phase disabled, then later it can set subscription slot_name to a new slot
with two_phase enabled which is the similar case as the failover.

Best Regards,
Hou zj