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Date: Fri, 11 Feb 2022 15:26:03 +0100
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Subject: Re: Synchronizing slots from primary to standby
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To: Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>
Cc: Andres Freund <andres@anarazel.de>,
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From: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
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On 10.02.22 22:47, Bruce Momjian wrote:
> On Tue, Feb  8, 2022 at 08:27:32PM +0530, Ashutosh Sharma wrote:
>>> Which means that if e.g. the standby_slot_names GUC differs from
>>> synchronize_slot_names on the physical replica, the slots synchronized on the
>>> physical replica are not going to be valid.  Or if the primary drops its
>>> logical slots.
>>>
>>>
>>>> Should the redo function for the drop replication slot have the capability
>>>> to drop it on standby and its subscribers (if any) as well?
>>>
>>> Slots are not WAL logged (and shouldn't be).
>>>
>>> I think you pretty much need the recovery conflict handling infrastructure I
>>> referenced upthread, which recognized during replay if a record has a conflict
>>> with a slot on a standby.  And then ontop of that you can build something like
>>> this patch.
>>>
>>
>> OK. Understood, thanks Andres.
> 
> I would love to see this feature in PG 15.  Can someone explain its
> current status?  Thanks.

The way I understand it:

1. This feature (probably) depends on the "Minimal logical decoding on 
standbys" patch.  The details there aren't totally clear (to me).  That 
patch had some activity lately but I don't see it in a state that it's 
nearing readiness.

2. I think the way this (my) patch is currently written needs some 
refactoring about how we launch and manage workers.  Right now, it's all 
mangled together with logical replication, since that is a convenient 
way to launch and manage workers, but it really doesn't need to be tied 
to logical replication, since it can also be used for other logical slots.

3. It's an open question how to configure this.  My patch show a very 
minimal configuration that allows you to keep all logical slots always 
behind one physical slot, which addresses one particular use case.  In 
general, you might have things like, one set of logical slots should 
stay behind one physical slot, another set behind another physical slot, 
another set should not care, etc.  This could turn into something like 
the synchronous replication feature, where it ends up with its own 
configuration language.

Each of these are clearly significant jobs on their own.