RE: Synchronizing slots from primary to standby

Dear Ajin,

Thanks for your reply!

> On Thu, Oct 26, 2023 at 6:08 PM Hayato Kuroda (Fujitsu)
> <kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
> >
> > Dear Shveta,
> >
> > > PFA v25 patch set. The changes are:
> >
> > Thanks for making the patch! It seems that there are lots of comments, so
> > I can put some high-level comments for 0001.
> > Sorry if there are duplicated comments.
> >
> > 1.
> > The patch seemed not to consider the case that failover option between
> replication
> > slot and subscription were different. Currently slot option will be overwritten
> > by subscription one.
> >
> > Actually, I'm not sure what specification is better. Regarding the two_phase,
> > 2PC will be decoded only when the both of settings are true. Should we follow?
> >
>
> But this is the intention, we want the Alter subscription to be able
> to change the failover behaviour
> of the slot.

I had not understood how two_phase is enabled. I found that slot->data.two_phase
is overwritten in CreateDecodingContext(), so the failover option now follows two_phase, right?
(I think the overwritten of data.failover should be also done at CreateDecodingContext()).

> > 2.
> > Currently ctx->failover is set only in the pgoutput_startup(), but not sure it is
> OK.
> > Can we change the parameter in CreateDecodingContext() or similar functions?
> >
> > Because IIUC it means that only slots which have pgoutput can wait. Other
> > output plugins must understand the change and set faliover flag as well -
> > I felt it is not good. E.g., you might miss to enable the parameter in
> test_decoding.
> >
> > Regarding the two_phase parameter, setting on plugin layer is good because it
> > quite affects the output. As for the failover, it is not related with the
> > content so that all of slots should be enabled.
> >
> > I think CreateDecodingContext or StartupDecodingContext() is the common
> path.
> > Or, is it the out-of-scope for now?
>
> Currently, the failover field is part of the options list in the
> StartReplicationCmd. This gives some
> level of flexibility such that only plugins that are interested in
> this need to handle it. The options list
> is only deparsed by plugins. If we move it to outside of the options list,
> this sort of changes the protocol for START_REPLICATION and will
> impact all plugins.
> But I agree to your larger point that, we need to do it in such a way that
> other plugins do not unintentionally change the 'failover' behaviour
> of the originally created slot.
> Maybe I can code it in such a way that, only if the failover option is
> specified in the list of options
> passed as part of START_REPLICATION will it change the original slot
> created 'failover' flag by adding
> another flag "failover_opt_given". Plugins that set this, will be able
> to change the failover flag of the slot,
> while plugins that do not support this will not set this and the
> failover flag of the created slot will remain.
> What do you think?

May be OK, but I came up with a corner case that external plugins have a streaming
option 'failover'. What should be? Has the option been reserved?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED