Re: Minimal logical decoding on standbys

Hi,

On 4/6/23 7:59 AM, Amit Kapila wrote:
> On Wed, Apr 5, 2023 at 6:14 PM Drouvot, Bertrand
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>>
>> On 4/5/23 12:28 PM, Amit Kapila wrote:
>>> On Wed, Apr 5, 2023 at 2:41 PM Drouvot, Bertrand
>>> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>>>> Maybe we could change the doc with something among those lines instead?
>>>>
>>>> "
>>>> Existing logical slots on standby also get invalidated if wal_level on primary is reduced to
>>>> less than 'logical'. This is done as soon as the standby detects such a change in the WAL stream.
>>>>
>>>> It means, that for walsenders that are lagging (if any), some WAL records up to the parameter change on the
>>>> primary won't be decoded".
>>>>
>>>> I don't know whether this is what one would expect but that should be less of a surprise if documented.
>>>>
>>>> What do you think?
>>>>
>>>
>>> Yeah, I think it is better to document to avoid any surprises if
>>> nobody else sees any problem with it.
>>
>> Ack.
>>
>
> This doesn't seem to be addressed in the latest version.

Right, I was waiting if "nobody else sees any problem with it".

Added it now in V62 posted up-thread.

> And today, I
> think I see one more point about this doc change:
> + <para>
> + A logical replication slot can also be created on a hot standby.
> To prevent
> + <command>VACUUM</command> from removing required rows from the system
> + catalogs, <varname>hot_standby_feedback</varname> should be set on the
> + standby. In spite of that, if any required rows get removed, the slot gets
> + invalidated. It's highly recommended to use a physical slot
> between the primary
> + and the standby. Otherwise, hot_standby_feedback will work, but
> only while the
> + connection is alive (for example a node restart would break it). Existing
> + logical slots on standby also get invalidated if wal_level on
> primary is reduced to
> + less than 'logical'.
>
> If hot_standby_feedback is not set then can logical decoding on
> standby misbehave? If so, that is not very clear from this doc change
> if that is acceptable.

I don't think it would misbehave but that primary may delete system catalog rows
that could be needed by the logical decoding on the standby (as it does not know about the
catalog_xmin on the standby).

Added this remark in V62.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com