Re: behave of --create-slot option

2018-06-01 9:00 GMT-03:00 Michael Paquier <michael(at)paquier(dot)xyz>:
> On Thu, May 31, 2018 at 10:59:04PM -0400, Robert Haas wrote:
>> On Wed, May 30, 2018 at 2:00 PM, Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>>> Hm. There could be an argument for improving the user experience here
>>> so as some cleanup is at least attempted except if --no-clean is defined
>>> by the caller when --create-slot is used. Do we want an open item for
>>> this issue?
>>
>> Sounds like new development to me. This isn't a bug.
>
> Still, it seems to me that the user experience is a bit horrible with
> this new interface of pg_basebackup. If --create-slot is used, then a
> slot is created before starting a backup. If the slot already exists,
> then pg_basebackup complains and exits. In order to drop the slot with
> a only user who has replication access rights (because nobody is really
> going to have a user who has SQL access so as the slot is dropped), then
> the only simple way is to use pg_receivewal --drop-slot, making the
> whole flow inconsistent. pg_basebackup is usually in server-side
> packages, and pg_receivewal is on the client side. The server packages
> requiring the client packages, then we are sure that pg_basebackup will
> drag in pg_receivewal.
>
Debian and derivatives put pg_basebackup in the client package
(indeed, it a client program). A possible fix is to drop the slot if
there is an error. However, if the problem is connectivity, we can't
drop it. In this case, we can print a user-friendly error saying that
the user should drop that slot before try again). If we follow this
idea, then I consider it to be a bug fix.

> Still, shouldn't there be a --drop-slot option in pg_basebackup? In
> this case, if --drop-slot is used, then the slot is dropped and
> pg_basebackup exits immediately.
>
I don't like it. You should send an extra command to recover from an
error. It also does not follow the idempotent behavior. I know that
you are trying to mimic pg_receivewal options but I prefer another
option that removes the slot if it exists (say,
--drop-slot-if-exists). In this case, I consider this option as new
development.

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