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

From: Michael Banck <michael(dot)banck(at)credativ(dot)de>
To: Magnus Hagander <magnus(at)hagander(dot)net>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Create replication slot in pg_basebackup if requested and not yet present
Date: 2017-04-09 11:22:02
Message-ID: 1491736922.25270.58.camel@credativ.de
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Hi,

Am Freitag, den 24.03.2017, 19:32 +0100 schrieb Michael Banck:
> On Thu, Mar 23, 2017 at 12:41:54PM +0100, Magnus Hagander wrote:
> > On Tue, Mar 21, 2017 at 8:34 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > > 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.
> >
> > Well, the explicit user action would be "--slot". But sure, I can
> > definitely live with a --create-if-not-exists.
>
> Can we just make that option create slot and don't worry if one exists
> already? CreateReplicationSlot() can be told to not mind about already
> existing slots, and I don't see a huge point in erroring out if the slot
> exists already, unless somebody can show that this leads to data loss
> somehow.
>
> > The important thing, to me, is that you can run it as a single
> > command, which makes it a lot easier to work with. (And not like we
> > currently have for pg_receivewal which requires a separate command to
> > create the slot)
>
> Oh, that is how it works with pg_receivewal, I have to admit I've never
> used it so was a bit confused about this when I read its code.
>
> So in that case I think we don't necessarily need to have the same user
> interface at all. I first thought about just adding "-C, --create" (as
> in "--create --slot=foo"), but this on second thought this looked a bit
> shortsighted - who knows what flashy thing pg_basebackup might create in
> 5 years... So I settled on --create-slot, which is only slightly more to
> type (albeit repetive, "--create-slot --slot=foo"), but adding a short
> option "-C" would be fine I thinkg "-C -S foo".
>
> So attached is a patch with adds that option. If people really think it
> should be --create-slot-if-not-exists instead I can update the patch, of
> course.
>
> I again added a second patch with some further refactoring which makes
> it print a message on temporary slot creation in verbose mode.

Rebased, squashed and slighly edited version attached. I've added this
to the 2017-07 commitfest now as well:

https://commitfest.postgresql.org/14/1112/

Michael

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Michael Banck
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Attachment Content-Type Size
0001-Add-option-to-create-a-replication-slot-in-pg_baseba-v3.patch text/x-patch 12.0 KB

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