Horton

RE: Synchronizing slots from primary to standby

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Peter Smith <smithpb2250@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2023-11-06T01:30:58Z
Lists: pgsql-hackers

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On Friday, November 3, 2023 7:32 PM Amit Kapila <amit.kapila16@gmail.com>
> 
> On Thu, Nov 2, 2023 at 2:35 PM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>
> wrote:
> >
> > Here is the new version patch set(V29) which addressed Peter
> > comments[1][2] and fixed one doc compile error.
> >
> 
> Few comments:
> ==============
> 1.
> +       <varlistentry id="sql-createsubscription-params-with-failover">
> +        <term><literal>failover</literal> (<type>boolean</type>)</term>
> +        <listitem>
> +         <para>
> +          Specifies whether the replication slot assocaited with the
> subscription
> +          is enabled to be synced to the physical standbys so that logical
> +          replication can be resumed from the new primary after failover.
> +          The default is <literal>true</literal>.
> 
> Why do you think it is a good idea to keep the default value as true?
> I think the user needs to enable standby for syncing slots which is not a default
> feature, so by default, the failover property should also be false. AFAICS, it is
> false for create_slot SQL API as per the below change; so that way also keeping
> default true for a subscription doesn't make sense.
> @@ -479,6 +479,7 @@ CREATE OR REPLACE FUNCTION
> pg_create_logical_replication_slot(
>      IN slot_name name, IN plugin name,
>      IN temporary boolean DEFAULT false,
>      IN twophase boolean DEFAULT false,
> +    IN failover boolean DEFAULT false,
>      OUT slot_name name, OUT lsn pg_lsn)
> 
> BTW, the below change indicates that the code treats default as false; so, it
> seems to be a documentation error.

I think the document is wrong and fixed it.

> 
> 2.
> -
>  /*
>   * Common option parsing function for CREATE and ALTER SUBSCRIPTION
> commands.
>   *
> 
> Spurious line removal.
> 
> 3.
> + else if (opts.slot_name && failover_enabled) {
> + walrcv_alter_slot(wrconn, opts.slot_name, opts.failover);
> + ereport(NOTICE, (errmsg("altered replication slot \"%s\" on
> + publisher", opts.slot_name))); }
> 
> I think we can add a comment to describe why it makes sense to enable the
> failover property of the slot in this case. Can we change the notice message to:
> "enabled failover for replication slot \"%s\" on publisher"

Added.

> 
> 4.
>  libpqrcv_create_slot(WalReceiverConn *conn, const char *slotname,
> - bool temporary, bool two_phase, CRSSnapshotAction snapshot_action,
> - XLogRecPtr *lsn)
> + bool temporary, bool two_phase, bool failover, CRSSnapshotAction
> + snapshot_action, XLogRecPtr *lsn)
>  {
>   PGresult   *res;
>   StringInfoData cmd;
> @@ -913,7 +917,14 @@ libpqrcv_create_slot(WalReceiverConn *conn, const
> char *slotname,
>   else
>   appendStringInfoChar(&cmd, ' ');
>   }
> -
> + if (failover)
> + {
> + appendStringInfoString(&cmd, "FAILOVER"); if (use_new_options_syntax)
> + appendStringInfoString(&cmd, ", "); else appendStringInfoChar(&cmd, '
> + '); }
> 
> I don't see a corresponding change in repl_gram.y. I think the following part of
> the code needs to be changed:
> /* CREATE_REPLICATION_SLOT slot [TEMPORARY] LOGICAL plugin [options] */
> | K_CREATE_REPLICATION_SLOT IDENT opt_temporary K_LOGICAL IDENT
> create_slot_options
> 

I think after 0266e98, we started to use the new syntax(see the
generic_option_list rule) and we can avoid changing the repl_gram.y when adding
new options. The new failover can be detected when parsing the generic option
list(in parseCreateReplSlotOptions).


> 
> 5.
> @@ -228,6 +230,28 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo
> fcinfo, bool confirm, bool bin
>   NameStr(MyReplicationSlot->data.plugin),
>   format_procedure(fcinfo->flinfo->fn_oid))));
> ..
> + if (XLogRecPtrIsInvalid(upto_lsn))
> + wal_to_wait = end_of_wal;
> + else
> + wal_to_wait = Min(upto_lsn, end_of_wal);
> +
> + /* Initialize standby_slot_names_list */ SlotSyncInitConfig();
> +
> + /*
> + * Wait for specified streaming replication standby servers (if any)
> + * to confirm receipt of WAL upto wal_to_wait.
> + */
> + WalSndWaitForStandbyConfirmation(wal_to_wait);
> +
> + /*
> + * The memory context used to allocate standby_slot_names_list will be
> + * freed at the end of this call. So free and nullify the list in
> + * order to avoid usage of freed list in the next call to this
> + * function.
> + */
> + SlotSyncFreeConfig();
> 
> What if there is an error in WalSndWaitForStandbyConfirmation() before calling
> SlotSyncFreeConfig()? I think the problem you are trying to avoid by freeing it
> here can occur. I think it is better to do this in a logical decoding context and
> free the list along with it as we are doing in commit c7256e6564(see PG15).

I will analyze more about this case and update in next version.

> Also,
> it is better to allocate this list somewhere in
> WalSndWaitForStandbyConfirmation(), probably in WalSndGetStandbySlots,
> that will make the code look neat and also avoid allocating this list when
> failover is not enabled for the slot.

Changed as suggested.


> 
> 6.
> +/* ALTER_REPLICATION_SLOT slot */
> +alter_replication_slot:
> + K_ALTER_REPLICATION_SLOT IDENT '(' generic_option_list ')'
> 
> I think you need to update the docs for this new command. See existing docs
> [1].
> 
> [1] - https://www.postgresql.org/docs/devel/protocol-replication.html

I think the doc for alter_replication_slot was added in V29.

Attach the V30 patch set which addressed above comments and fixed CFbot failures.

Best Regards,
Hou zj

Commits

  1. Doc: Add the new section "Logical Replication Failover".

  2. Fix the review comments and a bug in the slot sync code.

  3. Fix a test in failover slots regression test.

  4. Fix the intermittent buildfarm failures in 040_standby_failover_slots_sync.

  5. Split XLogCtl->LogwrtResult into separate struct members

  6. Ensure that the sync slots reach a consistent state after promotion without losing data.

  7. Introduce a new GUC 'standby_slot_names'.

  8. Fix BF failure introduced by commit b3f6b14cf4.

  9. Fixups for commit 93db6cbda0.

  10. Fix BF failure in commit 93db6cbda0.

  11. Add a new slot sync worker to synchronize logical slots.

  12. Improve ERROR/LOG messages added by commits ddd5f4f54a and 7a424ece48.

  13. Disable autovacuum on primary in 040_standby_failover_slots_sync test.

  14. Fix the incorrect format specifier used in commit 7a424ece48.

  15. Change the LOG level in 040_standby_failover_slots_sync.pl to DEBUG2.

  16. Add more LOG and DEBUG messages for slot synchronization.

  17. Another try to fix BF failure introduced in commit ddd5f4f54a.

  18. Fix BF introduced in commit ddd5f4f54a.

  19. Add a slot synchronization function.

  20. Clean-ups for 776621a5e4 and 7329240437.

  21. Enhance libpqrcv APIs to support slot synchronization.

  22. Add a failover option to subscriptions.

  23. Allow setting failover property in the replication command.

  24. Allow to enable failover property for replication slots via SQL API.

  25. Add macros for looping through a List without a ListCell.

  26. Track conflict_reason in pg_replication_slots.

  27. Allow upgrades to preserve the full subscription's state.

  28. Add support for incremental backup.

  29. Simplify some logic in CreateReplicationSlot()

  30. Fix uninitialized access to InitialRunningXacts during decoding after ERROR.

  31. Flexible options for CREATE_REPLICATION_SLOT.

  32. Empty search_path in logical replication apply worker and walsender.

  33. Fix oldest xmin and LSN computation across repslots after advancing

  34. Require C99 (and thus MSCV 2013 upwards).