Re: Synchronizing slots from primary to standby

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.

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.

--
With Regards,
Amit Kapila.