Logical replication from HA cluster

Hi!

I'm working on providing smooth failover to a CDC system in HA cluster.
Currently, we do not replicate logical slots and when we promote a replica. This renders impossible continuation of
changeddata capture (CDC) from new primary after failover. 

We cannot start logical replication from LSN different from LSN of a slot. And cannot create a slot on LSN in the past,
particularlybefore or right after promotion. 

This leads to massive waste of network bandwidth in our installations, due to necessity of initial table sync.

We are considering to use the extension that creates replication slot with LSN in the past [0]. I understand that there
mightbe some caveats with logical replication, but do not see scale of possible implications of this approach. User get
errorif WAL is rotated or waits if LSN is not reached yet, this seems perfectly fine for us. In most of our cases when
CDCagent detects failover and goes to new primary there are plenty of old WALs to restart CDC. 

Are there strong reasons why we do not allow creation of slots with given LSNs, possibly within narrow LSN range (but
widerthat just GetXLogInsertRecPtr())? 

Thanks!

Best regards, Andrey Borodin.


[0] https://github.com/x4m/pg_tm_aux/blob/master/pg_tm_aux.c#L74-L77