Horton

Re: Synchronizing slots from primary to standby

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@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>, 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: 2024-01-31T08:32:00Z
Lists: pgsql-hackers 
On Tue, Jan 30, 2024 at 9:53 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Tue, Jan 30, 2024 at 4:06 PM shveta malik <shveta.malik@gmail.com> wrote:
> >
> > PFA v73-0001 which addresses the above comments. Other patches will be
> > rebased and posted after pushing this one.
>
> Since v73-0001 is pushed, PFA  rest of the patches. Changes are:
>
> 1) Rebased the patches.
> 2) Ran pg_indent on all.
> 3) patch001: Updated logicaldecoding.sgml for dbname requirement in
> primary_conninfo for slot-synchronization.
>

Thank you for updating the patches. As for the slotsync worker patch,
is there any reason why 0001, 0002, and 0004 patches are still
separated?

Beside, here are some comments on v74 0001, 0002, and 0004 patches:

---
+static char *
+wait_for_valid_params_and_get_dbname(void)
+{
+   char       *dbname;
+   int         rc;
+
+   /* Sanity check. */
+   Assert(enable_syncslot);
+
+   for (;;)
+   {
+       if (validate_parameters_and_get_dbname(&dbname))
+           break;
+       ereport(LOG, errmsg("skipping slot synchronization"));
+
+       ProcessSlotSyncInterrupts(NULL);

When reading this function, I expected that the slotsync worker would
resume working once the parameters became valid, but it was not
correct. For example, if I changed hot_standby_feedback from off to
on, the slotsync worker reads the config file, exits, and then
restarts. Given that the slotsync worker ends up exiting on parameter
changes anyway, why do we want to have it wait for parameters to
become valid? IIUC even if the slotsync worker exits when a parameter
is not valid, it restarts at some intervals.

---
+bool
+SlotSyncWorkerCanRestart(void)
+{
+#define SLOTSYNC_RESTART_INTERVAL_SEC 10
+

IIUC depending on how busy the postmaster is and the timing, the user
could wait for 1 min to re-launch the slotsync worker. But I think the
user might want to re-launch the slotsync worker more quickly for
example when the slotsync worker restarts due to parameter changes.
IIUC SloSyncWorkerCanRestart() doesn't consider the fact that the
slotsync worker previously exited with 0 or 1.

---
+       /* We are a normal standby */
+       valid = DatumGetBool(slot_getattr(tupslot, 2, &isnull));
+       Assert(!isnull);

What do you mean by "normal standby"?

---
+   appendStringInfo(&cmd,
+                    "SELECT pg_is_in_recovery(), count(*) = 1"
+                    " FROM pg_replication_slots"
+                    " WHERE slot_type='physical' AND slot_name=%s",
+                    quote_literal_cstr(PrimarySlotName));

I think we need to make "pg_replication_slots" schema-qualified.

---
+                   errdetail("The primary server slot \"%s\" specified by"
+                             " \"%s\" is not valid.",
+                             PrimarySlotName, "primary_slot_name"));

and

+               errmsg("slot sync worker will shutdown because"
+                      " %s is disabled", "enable_syncslot"));

It's better to write it in one line for better greppability.

---
When I dropped a database on the primary that has a failover slot, I
got the following logs on the standby:

2024-01-31 17:25:21.750 JST [1103933] FATAL:  replication slot "s" is
active for PID 1103935
2024-01-31 17:25:21.750 JST [1103933] CONTEXT:  WAL redo at 0/3020D20
for Database/DROP: dir 1663/16384
2024-01-31 17:25:21.751 JST [1103930] LOG:  startup process (PID
1103933) exited with exit code 1

It seems that because the slotsync worker created the slot on the
standby, the slot's active_pid is still valid. That is why the startup
process could not drop the slot.

Regards,


--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

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).