RE: Synchronizing slots from primary to standby

On Tuesday, October 31, 2023 6:45 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Oct 27, 2023 at 4:04 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > On Fri, Oct 27, 2023 at 3:26 PM shveta malik <shveta(dot)malik(at)gmail(dot)com>
> wrote:
> > ==========
> >
> > Open questions regarding change for pt 1 above:
> > a) I think we should restrict the 'alter-sub set failover' when
> > failover-state is currently in 'p' (pending) state i.e. table-sync is
> > going over. Once table-sync is over, then toggle of 'failover' should
> > be allowed using alter-subscription.
> >
>
> Agreed.
>
> > b) Currently I have restricted 'alter subscription.. refresh
> > publication with copy=true' when failover=true (on a similar line of
> > two-phase). The reason being, refresh with copy=true will go for
> > table-sync again and since failover was set in main-slot after
> > table-sync was done, it will need going through the same transition of
> > 'p' to 'e' for main slot making it unsyncable for that time. Should it
> > be allowed?
> >
>
> Yeah, I also think we can't allow refresh with copy=true when 'failover' is
> enabled.
>
> I think the current implementation of this flag seems a bit clumsy because
> 'failover' is a slot property and we are trying to map it to plugin_options. It has
> to be considered similar to the opt_temporary option while creating the slot.
>
> We have create_replication_slot and drop_replication_slot in repl_gram.y. How
> about if introduce alter_replication_slot and handle the 'failover' flag with that?
> The idea is we will either enable 'failover' at the time create_replication_slot by
> providing an optional failover option or execute a separate command
> alter_replication_slot. I think we probably need to perform this command
> before the start of streaming.

Here is an attempt to achieve the same. I added a new replication command
alter_replication_slot and introduced a walreceiver api walrcv_alter_slot to
execute the command. The subscription will call the api to enable/disable
the failover of the slot on publisher.

The patch disallows altering the failover option for the subscription. But we
could release the restriction by using the following approaches in next version:

> I think we will have the following options to allow alter of the 'failover'
> property: (a) we can allow altering 'failover' only for the 'disabled'
> subscription; to achieve that, we need to open a connection during alter
> subscription and change this property of slot; (b) apply worker detects the
> change in 'failover' option; run the alter_replication_slot command; this needs
> more analysis as apply_worker is already doing streaming and changing slot
> property in between could be tricky.

Best Regards,
Hou zj