Horton

Re: Minimal logical decoding on standbys

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Jeff Davis <pgsql@j-davis.com>, Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Amit Khandekar <amitdkhan.pg@gmail.com>, fabriziomello@gmail.com, tushar <tushar.ahuja@enterprisedb.com>, Rahila Syed <rahila.syed@2ndquadrant.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-04-06T12:23:55Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Reduce the log level in 035_standby_logical_decoding.pl.

  2. 035_standby_logical_decoding: Add missing waits for replication

  3. For cascading replication, wake physical and logical walsenders separately

  4. Handle logical slot conflicts on standby

  5. Support invalidating replication slots due to horizon and wal_level

  6. Prevent use of invalidated logical slot in CreateDecodingContext()

  7. Replace replication slot's invalidated_at LSN with an enum

  8. Pass down table relation into more index relation functions

  9. Assert only valid flag bits are passed to visibilitymap_set()

  10. Remove unused _bt_delitems_delete() argument.

  11. Add xl_btree_delete optimization.

On Thu, Apr 6, 2023 at 11:29 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> >
>
> This doesn't seem to be addressed in the latest version. And today, I
> think I see one more point about this doc change:
> +    <para>
> +     A logical replication slot can also be created on a hot standby.
> To prevent
> +     <command>VACUUM</command> from removing required rows from the system
> +     catalogs, <varname>hot_standby_feedback</varname> should be set on the
> +     standby. In spite of that, if any required rows get removed, the slot gets
> +     invalidated. It's highly recommended to use a physical slot
> between the primary
> +     and the standby. Otherwise, hot_standby_feedback will work, but
> only while the
> +     connection is alive (for example a node restart would break it). Existing
> +     logical slots on standby also get invalidated if wal_level on
> primary is reduced to
> +     less than 'logical'.
>
> If hot_standby_feedback is not set then can logical decoding on
> standby misbehave? If so, that is not very clear from this doc change
> if that is acceptable. One scenario where I think it can misbehave is
> if applying WAL records generated after changing wal_level from
> 'logical' to 'replica' physically removes catalog tuples that could be
> referenced by the logical decoding on the standby. Now, as mentioned
> in patch 0003's comment in decode.c that it is possible that some
> slots may creep even after we invalidate the slots on parameter
> change, so while decoding using that slot if some required catalog
> tuple has been removed by physical replication then the decoding can
> misbehave even before reaching XLOG_PARAMETER_CHANGE record.
>

Thinking some more on this, I think such a slot won't decode any other
records. During CreateInitDecodingContext->ReplicationSlotReserveWal,
for standby's, we use lastReplayedEndRecPtr as restart_lsn. This
should be a record before parameter_change record in the above
scenario. So, ideally, the first record to decode by such a walsender
should be parameter_change which will anyway error out. So, this
shouldn't be a problem.

-- 
With Regards,
Amit Kapila.