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Date: Sat, 12 Oct 2024 08:33:32 +0200
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Subject: Re: Streaming Replication vs Logical
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On 12.10.24 00:53, Paul A Jungwirth wrote:
> Our docs seem to contrast "streaming replication" to logical, but
> these are not really opposites. Sometimes when they say "streaming"
> they mean "physical".
>
> Probably this is historical: at first physical replication was the
> only kind of streaming we had.
>
> Personally this has caused me a lot of confusion. For example,
> recently when I read "Synchronous replication (see Section 26.2.8) is
> only supported on replication slots used over the streaming
> replication interface," I took it to mean synchronous replication only
> worked for physical replication, not logical. But actually here the
> contrast is between the streaming protocol and SQL commands. I think
> it would help if the docs elsewhere took more care to say "physical"
> not "streaming" when that is the meaning.
>
> This patch fixes a callout note in the Logical Decoding section. It
> was sort of vague before. It dates back to the original commit
> documenting logical decoding, so I'm not sure of the original motive.
> I think the new text is more precise and about the other use for
> replication slots, and also clearly separates streaming replication
> from physical-vs-logical replication. (I used the word "typically"
> because you *can* have a standby with logical replication, although
> elsewhere we warn it may not be as robust.)
>
> I also tried to fix the first three paragraphs of
> https://www.postgresql.org/docs/current/runtime-config-replication.html
> which seem to be setting up a distinction between streaming
> replication and logical. This is tricky, because (1) we are trying to
> introduce how we've organized different config parameters, and some
> affect both physical and logical replication (e.g.
> max_replication_slots and wal_retrieve_retry_interval) (2) you can use
> logical replication without streaming (e.g.
> pg_logical_slot_get_changes).
>
> While I was making edits there, I took out some language about
> "built-in" and "feature", which seem like unnecessary wordiness.
>
> I also added "physical" to a couple places in
> https://www.postgresql.org/docs/current/warm-standby.html as well as a
> Note callout to clarify that the whole page is really about using
> physical replication. Technically you could use logical replication to
> run a standby, but that seems like a separate discussion.
>
> Yours,
>

Adding clear definitions to the glossary would help users and authors alike.

J. Purtz