Re: Logical replication - initial data synchronization

On Sat, May 18, 2024 at 09:02:11PM +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/16/logical-replication-subscription.html
> Description:
>
> I'm reading up on Logical Replication and have been reading the pages in
> order.
>
> The first 2 pages:
> https://www.postgresql.org/docs/current/logical-replication.html and
> https://www.postgresql.org/docs/current/logical-replication-publication.html
> both speak of the requirement to set up a snapshot and explain that
> publication will then send further updates as they happen to subscribers.
>
> But the 3rd page,
> https://www.postgresql.org/docs/current/logical-replication-subscription.html
> now mentions this: "Additional replication slots may be required for the
> initial data synchronization of pre-existing table data and those will be
> dropped at the end of data synchronization."
>
> For me, reading the first 2 pages implied that I would have to perform some
> manual command that starts the creation of a snapshot of pre-existing table
> data, and unpack this on the subscriber node somehow.
>
> The text on the "Subscription" page sounds to me like this is actually
> something the publisher<-> subscriber model of the postgres software can
> manage on its own. As opposed to a snapshot, which feels more like the
> concept of a basebackup.
>
> Regardless of that being correct or not, my current impression is that the
> description isn't consistent across pages. Maybe the text is obvious for
> people who've performed setup of logical replication before, but I have
> never done this. To me, the description on the first 2 pages seems
> inconsistent with the description I just encountered on the 3rd page. I was
> under the impression there was no such thing as "initial data
> synchronization of pre-existing table data" in terms of postgres doing this
> by itself.
>
> Am I missing something extremely simple, or can the description of the
> involved operations be made more consistent across documentation pages?

Is the attached patch an improvement?

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Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
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