Re: Create replication slot in pg_basebackup if requested and not yet present

Am Dienstag, den 21.03.2017, 15:34 -0400 schrieb Robert Haas:
> On Sun, Mar 19, 2017 at 12:01 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> > I think maybe we should output a message when the slot is created, at least
> > in verbose mode, to make sure people realize that happened. Does that seem
> > reasonable?
>
> Slots are great until you leave one lying around by accident. I'm
> afraid that no matter what we do, we're going to start getting
> complaints from people who mess that up. For example, somebody
> creates a replica using the new super-easy method, and then blows it
> away without dropping the slot from the master, and then days or weeks
> later pg_wal fills up and takes the server down. The user says, oh,
> these old write-ahead log files should have gotten removed, and
> removes them all. Oops.

Hrm.

Maybe it would be useful to log a warning like "slot XY has not been
active for X minutes", based on some threshold, though possibly the
people not keeping an eye on their replication slots won't spot that in
the logfiles, either.

> So I tend to think that there should always be some explicit user
> action to cause the creation of a slot, like --create-slot-if-needed
> or --create-slot=name. That still won't prevent careless use of that
> option but it's less dangerous than assuming that a user who refers to
> a nonexistent slot intended to create it when, perhaps, they just
> typo'd it.

I guess if we decide (physical) slots should not be created implicitly,
then using the same UI as pg_receivewal makes sense for the sake of
consistency, i.e. "--slot=name --create-slot [--if-not-exists]". That is
rather verbose though and I don't think the --if-not-exists is great UX,
but maybe there is some use-case for erroring out if a slot already
exists that I haven't thought of.

Michael

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