RE: Synchronizing slots from primary to standby

On Tuesday, October 31, 2023 6:45 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > 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.

I think for approach b), one challenge is the handling of the error case. E.g.
If the apply worker errored out when executing the alter_replication_slot
command, it may not be able to retry that after restarting, because it won't
know if the value has changed before. (Or we have to execute
alter_replication_slot always at the beginning in apply worker which seems not great).

Best Regards,
Hou zj