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From: Phillip Diffley <[email protected]>
To: Christophe Pettus <[email protected]>
Cc: [email protected]
Subject: Re: Replication slot WAL reservation
Date: Wed, 26 Mar 2025 09:55:06 -0500
Message-ID: <CAGAwPgRoGtUvkX=s4pCQd2Fhy9XqMit6y2eM6=kQyDAN9v71qQ@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <CAGAwPgR=B9VTDwKkHZRHjYTmQzvJ+30mK-uyc7MMw6Tf9s9Fsw@mail.gmail.com>
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	<CAGAwPgQ1MgX2s4UgEspCwfKN_3okudzwh7NzpkoJZSA1HnGqYQ@mail.gmail.com>
	<[email protected]>

> You're looking for Standby Status Update

Awesome! I completely missed that.

> Transactions are always presented to the output plugin in commit order

Ok great. I think that is what I needed to know.

Just to confirm, it sounds like the order messages are sent from the output
plugin is what matters here. When you update confirmed_flush_lsn to LSN
"A", any messages that were sent by the output plugin after the message
with LSN "A" will be replayable. Any messages sent by the output plugin
before the message with LSN "A" will most likely not be replayed, since
their data is freed for deletion. Is that correct?

On Tue, Mar 25, 2025 at 11:32 PM Christophe Pettus <[email protected]> wrote:

>
>
> > On Mar 25, 2025, at 20:56, Phillip Diffley <[email protected]>
> wrote:
> >
> > Is there a message type that is used to confirm what logs have been
> successfully consumed?
>
> You're looking for Standby Status Update:
>
>
> https://www.postgresql.org/docs/current/protocol-replication.html#PROTOCOL-REPLICATION-STANDBY-STATU...
>
> The logical replication protocol is a superset of the binary replication
> protocol, so many of the same messages are used.
>
> > The part I am uncertain about is what "after" means here, since LSNs are
> not presented in order, and the order of data streamed over the replication
> slot does not match the order of the data in the WAL.
>
> I think there's a misunderstanding here (possibly my fault).  Transactions
> are always presented to the output plugin in commit order, and LSNs can be
> reliably used to determine the time ordering of commits.  LSNs are exactly
> what is used to determine how far into the WAL the replication slot has
> gotten.

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