Re: Questions on logical replication

>
> https://www.postgresql.org/docs/current/wal-configuration.html
>
> "Checkpoints are points in the sequence of transactions at which it is
> guaranteed that the heap and index data files have been updated with all
> information written before that checkpoint. At checkpoint time, all
> dirty data pages are flushed to disk and a special checkpoint record is
> written to the WAL file. (The change records were previously flushed to
> the WAL files.) In the event of a crash, the crash recovery procedure
> looks at the latest checkpoint record to determine the point in the WAL
> (known as the redo record) from which it should start the REDO
> operation. Any changes made to data files before that point are
> guaranteed to be already on disk. Hence, after a checkpoint, WAL
> segments preceding the one containing the redo record are no longer
> needed and can be recycled or removed. (When WAL archiving is being
> done, the WAL segments must be archived before being recycled or removed.)"
>

And this is the same for logical replication and physical replication, I
take it.

Thus, if a leader has a standby of the same version, and meanwhile logical
replication is being done to a newer version, both those replications are
taken into account, is that correct?

When you set up logical replication you are 'asking' via the replication

slot that WAL records be kept on the publisher until the subscriber
>
retrieves them.
>

And if it cannot sync them, due to connectivity loss for instance, the WAL
records will not be removed, then?

Regards,
Koen De Groote

On Wed, Jun 5, 2024 at 1:05 AM Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 6/4/24 15:55, Koen De Groote wrote:
> > I recently read the entire documentation on logical replication, but am
> > left with a question on the buildup of WAL
> >
> > On this page:
> >
> https://www.postgresql.org/docs/current/logical-replication-subscription.html#LOGICAL-REPLICATION-SUBSCRIPTION-SLOT
> <
> https://www.postgresql.org/docs/current/logical-replication-subscription.html#LOGICAL-REPLICATION-SUBSCRIPTION-SLOT
> >
> >
> > It is written: " When dropping a subscription, the remote host is not
> > reachable. In that case, disassociate the slot from the subscription
> > using |ALTER SUBSCRIPTION| before attempting to drop the subscription.
> > If the remote database instance no longer exists, no further action is
> > then necessary. If, however, the remote database instance is just
> > unreachable, the replication slot (and any still remaining table
> > synchronization slots) should then be dropped manually; otherwise
> > it/they would continue to reserve WAL and might eventually cause the
> > disk to fill up. Such cases should be carefully investigated."
> >
> >
> > Assuming a situation where I add tables 1 at a time to the publisher,
> > and refresh the subscription every time.
> >
> > What happens if I shut down the subscriber database for a while? The
> > subscription isn't dropped, so am I reading it right that the disk on
> > the publisher will slowly be filling up with WAL? Isn't that always the
> > case if wall is enabled?
>
> https://www.postgresql.org/docs/current/wal-configuration.html
>
> "Checkpoints are points in the sequence of transactions at which it is
> guaranteed that the heap and index data files have been updated with all
> information written before that checkpoint. At checkpoint time, all
> dirty data pages are flushed to disk and a special checkpoint record is
> written to the WAL file. (The change records were previously flushed to
> the WAL files.) In the event of a crash, the crash recovery procedure
> looks at the latest checkpoint record to determine the point in the WAL
> (known as the redo record) from which it should start the REDO
> operation. Any changes made to data files before that point are
> guaranteed to be already on disk. Hence, after a checkpoint, WAL
> segments preceding the one containing the redo record are no longer
> needed and can be recycled or removed. (When WAL archiving is being
> done, the WAL segments must be archived before being recycled or removed.)"
>
> >
> > This "cause disk to fill up" warning is quite concerning, and I'd like
> > to understand what could cause it and how likely it is? I thought
> > logical replication uses WAL by default, so doesn't that mean there has
> > to be a log of changes kept anyhow? Even if the WAL isn't written to
> > disk by an "archive_command"?
>
>
> https://www.postgresql.org/docs/current/warm-standby.html#STREAMING-REPLICATION-SLOTS
>
> "Replication slots provide an automated way to ensure that the primary
> does not remove WAL segments until they have been received by all
> standbys, and that the primary does not remove rows which could cause a
> recovery conflict even when the standby is disconnected."
>
> When you set up logical replication you are 'asking' via the replication
> slot that WAL records be kept on the publisher until the subscriber
> retrieves them.
>
> >
> > Regards,
> > Koen De Groote
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>
>