Re: Synchronizing slots from primary to standby

On Fri, Dec 15, 2023 at 5:55 PM Nisha Moond <nisha(dot)moond412(at)gmail(dot)com> wrote:
>
> (1)
> When we try to create a subscription on standby using a synced slot
> that is in 'r' sync_state, the subscription will be created at the
> subscriber, and on standby, two actions will take place -
> (i) As copy_data is true by default, it will switch the failover
> state of the synced-slot to 'false'.
> (ii) As we don't allow to use synced-slots, it will start giving
> the expected error in the log file -
> ERROR: cannot use replication slot "logical_slot" for logical decoding
> DETAIL: This slot is being synced from the primary server.
> HINT: Specify another replication slot.
>
> The first one seems an issue, it toggles the failover to false and
> then it remains false after that. I think it should be fixed.
>

+1. If we don't allow the slot to be used, we shouldn't allow its
state to be changed as well.

> (2)
> With the patch, the 'CREATE SUBSCRIPTION' command with a 'slot_name'
> of an 'active' logical slot fails and errors out -
> ERROR: could not alter replication slot "logical_slot" on
> publisher: ERROR: replication slot "logical_slot1" is active for PID
> xxxx
>
> Without the patch, the create subscription with an 'active' slot_name
> succeeds and the log file shows the error "could not start WAL
> streaming: ERROR: replication slot "logical_slot" is active for PID
> xxxx".
>
> Given that the specified active slot_name has failover set to false
> and the create subscription command also specifies failover=false, the
> expected behavior of the "with-patch" case is anticipated to be the
> same as that of the "without-patch" scenario.
>

Currently, we first acquire the slot to change its state but I guess
if we want the behavior as you mentioned we first need to check the
slot's 'failover' state without acquiring the slot. I am not sure if
that is any better because anyway we are going to fail in the very
next step as the slot is busy.

--
With Regards,
Amit Kapila.