Re: Sync Rep v19

Robert Haas <robertmhaas@gmail.com> writes:
> was.  So you could then say things like "is the most recent time at
> which the standby was caught up within the last 30 seconds?", which
> would be a useful thing to monitor, and right now there's no way to do

Well in my experience with replication, that's not what I want to
monitor.  If the standby is synchronous, then it's not catching up, it's
streaming.  If it were not, it would not be a synchronous standby.

When a standby is asynchronous then what I want to monitor is its lag.

So the CATCHUP state is useful to see that a synchronous standby
candidate can not yet be a synchronous standby.  When it just lost its
synchronous status (and hopefully another standby is now the sync one),
then it's just asynchronous and I want to know its lag.

> it.  There's also a BACKUP state, but I'm not sure it makes sense to
> lump that in with the others.  Some day it might be possible to stream
> WAL and take a backup at the same time, over the same connection.
> Maybe that should be a separate column or something.

BACKUP is still meaningful if you stream WAL at the same time, because
you're certainly *not* applying them while doing the base backup, are
you?  So you're not yet a standby, that's what BACKUP means.

Regards,
-- 
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support