RE: Synchronizing slots from primary to standby

On Tuesday, November 21, 2023 5:33 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
>
> v37 fails to apply to HEAD due to a recent commit e83aa9f92fdd, rebased the
> patches. PFA v37_2 patches.

Thanks for updating the patches.

I'd like to discuss one issue related to the correct handling of failover flag
when executing ALTER SUBSCRIPTION SET (slot_name = 'new_slot')".

Since the command intends to use a new slot on the primary, the new slot needs
to reflect the "failover" state that the subscription currently has. If the
failoverstate of the Subscription is LOGICALREP_FAILOVER_STATE_ENABLED, then I
can reset it to LOGICALREP_FAILOVER_STATE_PENDING and allow the apply worker to
handle it the way it is handled today (just like two_phase handling).

But if the failoverstate is LOGICALREP_FAILOVER_STATE_DISABLED, the original
idea is to call walrcv_alter_slot and alter the slot from the "ALTER
SUBSCRIPTION" handling backend itself. This works if the slot is currently
disabled. But the " ALTER SUBSCRIPTION SET (slot_name = 'new_slot')" command is
supported even if the subscription is enabled. If the subscription is enabled,
then calling walrcv_alter_slot() fails because the slot is still acquired by
apply worker.

So, I am thinking do we need a new mechanism to change the failover flag to
false on an enabled subscription ? For example, we could call walrcv_alter_slot
on startup of apply worker if AllTablesyncsReady(), for both true and false
values of failover flag. This way, every time apply worker is started, it calls
walrcv_alter_slot to set the failover flag on the primary.

Or we could just document that it is user's responsibility to match the failover
property in case it changes the slot_name.

Thoughts ?

Best Regards,
Hou zj