Horton

Re: Synchronizing slots from primary to standby

Peter Smith <smithpb2250@gmail.com>

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From: Peter Smith <smithpb2250@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.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>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-03-07T02:04:51Z
Lists: pgsql-hackers 
Here are some review comments for v107-0001

======
src/backend/replication/slot.c

1.
+/*
+ * Struct for the configuration of standby_slot_names.
+ *
+ * Note: this must be a flat representation that can be held in a single chunk
+ * of guc_malloc'd memory, so that it can be stored as the "extra" data for the
+ * standby_slot_names GUC.
+ */
+typedef struct
+{
+ int slot_num;
+
+ /* slot_names contains nmembers consecutive nul-terminated C strings */
+ char slot_names[FLEXIBLE_ARRAY_MEMBER];
+} StandbySlotConfigData;
+

1a.
To avoid any ambiguity this 1st field is somehow a slot ID number, I
felt a better name would be 'nslotnames' or even just 'n' or 'count',

~

1b.
(fix typo)

SUGGESTION for the 2nd field comment
slot_names is a chunk of 'n' X consecutive null-terminated C strings

~

1c.
A more explanatory name for this typedef maybe is 'StandbySlotNamesConfigData' ?

~~~


2.
+/* This is parsed and cached configuration for standby_slot_names */
+static StandbySlotConfigData *standby_slot_config;


2a.
/This is parsed and cached configuration for .../This is the parsed
and cached configuration for .../

~

2b.
Similar to above -- since this only has name information maybe it is
more correct to call it 'standby_slot_names_config'?

~~~

3.
+/*
+ * A helper function to validate slots specified in GUC standby_slot_names.
+ *
+ * The rawname will be parsed, and the parsed result will be saved into
+ * *elemlist.
+ */
+static bool
+validate_standby_slots(char *rawname, List **elemlist)

/and the parsed result/and the result/

~~~

4. check_standby_slot_names

+ /* Need a modifiable copy of string */
+ rawname = pstrdup(*newval);

/copy of string/copy of the GUC string/

~~~

5.
+assign_standby_slot_names(const char *newval, void *extra)
+{
+ /*
+ * The standby slots may have changed, so we must recompute the oldest
+ * LSN.
+ */
+ ss_oldest_flush_lsn = InvalidXLogRecPtr;
+
+ standby_slot_config = (StandbySlotConfigData *) extra;
+}

To avoid leaking don't we need to somewhere take care to free any
memory used by a previous value (if any) of this
'standby_slot_config'?

~~~

6. AcquiredStandbySlot

+/*
+ * Return true if the currently acquired slot is specified in
+ * standby_slot_names GUC; otherwise, return false.
+ */
+bool
+AcquiredStandbySlot(void)
+{
+ const char *name;
+
+ /* Return false if there is no value in standby_slot_names */
+ if (standby_slot_config == NULL)
+ return false;
+
+ name = standby_slot_config->slot_names;
+ for (int i = 0; i < standby_slot_config->slot_num; i++)
+ {
+ if (strcmp(name, NameStr(MyReplicationSlot->data.name)) == 0)
+ return true;
+
+ name += strlen(name) + 1;
+ }
+
+ return false;
+}

6a.
Just checking "(standby_slot_config == NULL)" doesn't seem enough to
me, because IIUIC it is possible when 'standby_slot_names' has no
value then maybe standby_slot_config is not NULL but
standby_slot_config->slot_num is 0.

~

6b.
IMO this function would be tidier written such that the
MyReplicationSlot->data.name is passed as a parameter. Then you can
name the function more naturally like:

IsSlotInStandbySlotNames(const char *slot_name)

~

6c.
IMO the body of the function will be tidier if written so there are
only 2 returns instead of 3 like

SUGGESTION:
if (...)
{
  for (...)
  {
     ...
return true;
  }
}
return false;

~~~

7.
+ /*
+ * Don't need to wait for the standbys to catch up if there is no value in
+ * standby_slot_names.
+ */
+ if (standby_slot_config == NULL)
+ return true;

(similar to a previous review comment)

This check doesn't seem enough because IIUIC it is possible when
'standby_slot_names' has no value then maybe standby_slot_config is
not NULL but standby_slot_config->slot_num is 0.

~~~

8. WaitForStandbyConfirmation

+ /*
+ * Don't need to wait for the standby to catch up if the current acquired
+ * slot is not a logical failover slot, or there is no value in
+ * standby_slot_names.
+ */
+ if (!MyReplicationSlot->data.failover || !standby_slot_config)
+ return;

(similar to a previous review comment)

IIUIC it is possible that when 'standby_slot_names' has no value, then
standby_slot_config is not NULL but standby_slot_config->slot_num is
0. So shouldn't that be checked too?

Perhaps it is convenient to encapsulate this check using some macro:
#define StandbySlotNamesHasNoValue() (standby_slot_config = NULL ||
standby_slot_config->slot_num == 0)

----------
Kind Regards,
Peter Smith.
Fujitsu Australia

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).