MIME-Version: 1.0
From: Paul A Jungwirth <pj@illuminatedcomputing.com>
Date: Fri, 11 Oct 2024 15:53:41 -0700
Message-ID: 
 <CA+renyULt3VBS1cRFKUfT2=5dr61xBOZdAZ-CqX3XLGXqY-aTQ@mail.gmail.com>
Subject: Streaming Replication vs Logical
To: pgsql-docs@lists.postgresql.org
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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,

-- 
Paul              ~{:-)
pj@illuminatedcomputing.com

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