Re: Remove Deprecated Exclusive Backup Mode

On Wed, 1 Jul 2020 15:58:57 -0400
Robert Haas <robertmhaas@gmail.com> wrote:

> On Wed, Jul 1, 2020 at 3:50 PM Magnus Hagander <magnus@hagander.net> wrote:
> > As far as I've seen, the one thing that people have problems with in the
> > exclusive mode backups are precisely the fact that they have to keep a
> > persistent conneciton open, and thus it cannot work together with backup
> > software that is limited to only supporting running a pre- and a post
> > script.
> >
> > Something like I have suggested here is to solve *that* problem. I don't
> > think anybody actually explicitly wants "exclusive backups" -- they want a
> > backup solution that plugs into their world of pre/post scripts. And if we
> > can make that one work in a safer way than the current exclusive backups,
> > ohw is that not an improvement?  
> 
> Yeah, I guess that's a pretty fair point. I have to confess to having
> somewhat limited enthusiasm for adding a third mode here, but it might
> be worth it.
> 
> It seems pretty well inevitable to me that people are going to forget
> to end them.

There's so many way an admin could break their backup process and not notice
it. Even with current nonexclusive backup, a bad written script might creates
bad unrecoverable backups. 

In regard with your concern, monitoring in progress backups might be *one*
answer, from server side. This was "easy" with exclusive backup, we just
monitor the backup_label age and warn if it is older than expected [1]. Using
non-exclusive backup...this is not that easy anymore. And pg_is_in_backup() is
quite misleading if the admin found it without reading doc. Maybe an admin
function to list in progress non-exclusive backup and related backend pid might
be a good start?

With such admin function facilities, client side process could monitor backup
and decide to cancel them based on some internal backup policies/process.

Tools like pg_basebackup (and probably pgbackrest/barman to some extends) still
need the backup to abort on disconnection. Maybe it could flag its session
using the replication protocol or call a new admin function or load a hook on
session-shutdown to keep previous API behavior?

Regards,

[1] https://github.com/OPMDG/check_pgactivity/#user-content-backup_label_age-8.1