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: Peter Smith <smithpb2250@gmail.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: 2024-03-07T06:28:26Z
Lists: pgsql-hackers 
On Thursday, March 7, 2024 10:05 AM Peter Smith <smithpb2250@gmail.com> wrote:
> 
> Here are some review comments for v107-0001

Thanks for the comments.

> 
> ======
> 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',

Changed to 'nslotnames'.

> 
> ~
> 
> 1b.
> (fix typo)
> 
> SUGGESTION for the 2nd field comment
> slot_names is a chunk of 'n' X consecutive null-terminated C strings

Changed.

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

Changed.

> 
> ~~~
> 
> 
> 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 .../

Changed.

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


Changed.

> ~~~
> 
> 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/
> 


Changed.

> ~~~
> 
> 4. check_standby_slot_names
> 
> + /* Need a modifiable copy of string */ rawname = pstrdup(*newval);
> 
> /copy of string/copy of the GUC string/
> 


Changed.

> ~~~
> 
> 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'?
> 

The memory of extra is maintained by the GUC mechanism. It will be
automatically freed when the associated GUC setting is no longer of interest.
See src/backend/utils/misc/README for details.

 
> ~~~
> 
> 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.

The standby_slot_config will always be NULL if there is no value in it.

While checking, I did find a rare case that if there are only some white space
in the standby_slot_names, then slot_num will be 0, and have fixed it so that
standby_slot_config will always be NULL if there is no meaning value in guc.

> 
> ~
> 
> 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)

Changed it to SlotExistsInStandbySlotNames.

> 
> ~
> 
> 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;

I personally prefer the current style.

> 
> ~~~
> 
> 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.

Same as above.

> 
> ~~~
> 
> 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)

Same as above, I think we can avoid checking slot_num.

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).