Horton

Re: Synchronizing slots from primary to standby

Masahiko Sawada <sawada.mshk@gmail.com>

View thread View on postgr.es
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>, 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>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: 2024-02-20T02:55:08Z
Lists: pgsql-hackers 
On Mon, Feb 19, 2024 at 9:59 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Mon, Feb 19, 2024 at 5:32 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > Few comments on 0001
>
> Thanks for the feedback.
>
> > ====================
> > 1. I think it is better to error out when the valid GUC or option is
> > not set in ensure_valid_slotsync_params() and
> > ensure_valid_remote_info() instead of waiting. And we shouldn't start
> > the worker in the first place if not all required GUCs are set. This
> > will additionally simplify the code a bit.
>
> Sure, removed 'ensure' functions. Moved the validation checks to the
> postmaster before starting the slot sync worker.
>
> > 2.
> > +typedef struct SlotSyncWorkerCtxStruct
> >  {
> > - /* prevents concurrent slot syncs to avoid slot overwrites */
> > + pid_t pid;
> > + bool stopSignaled;
> >   bool syncing;
> > + time_t last_start_time;
> >   slock_t mutex;
> > -} SlotSyncCtxStruct;
> > +} SlotSyncWorkerCtxStruct;
> >
> > I think we don't need to change the name of this struct as this can be
> > used both by the worker and the backend. We can probably add the
> > comment to indicate that all the fields except 'syncing' are used by
> > slotsync worker.
>
> Modified.
>
> > 3. Similar to above, function names like SlotSyncShmemInit() shouldn't
> > be changed to SlotSyncWorkerShmemInit().
>
> Modified.
>
> > 4.
> > +ReplSlotSyncWorkerMain(int argc, char *argv[])
> > {
> > ...
> > + on_shmem_exit(slotsync_worker_onexit, (Datum) 0);
> > ...
> > + before_shmem_exit(slotsync_failure_callback, PointerGetDatum(wrconn));
> > ...
> > }
> >
> > Do we need two separate callbacks? Can't we have just one (say
> > slotsync_failure_callback) that cleans additional things in case of
> > slotsync worker? And, if we need both those callbacks then please add
> > some comments for both and why one is before_shmem_exit() and the
> > other is on_shmem_exit().
>
> I think we can merge these now. Earlier 'on_shmem_exit' was needed to
> avoid race-condition between startup and slot sync worker process to
> drop 'i' slots on promotion.  Now we do not have any such scenario.
> But I need some time to analyze it well. Will do it in the next
> version.
>
> > In addition to the above, I have made a few changes in the comments
> > and code (cosmetic code changes). Please include those in the next
> > version if you find them okay. You need to rename .txt file to remove
> > .txt and apply atop v90-0001*.
>
> Sure, included these.
>
> Please find the patch001 attached.

I've reviewed the v91 patch. Here are random comments:

---
 /*
  * Checks the remote server info.
  *
- * We ensure that the 'primary_slot_name' exists on the remote server and the
- * remote server is not a standby node.
+ * Check whether we are a cascading standby. For non-cascading standbys, it
+ * also ensures that the 'primary_slot_name' exists on the remote server.
  */

IIUC what the validate_remote_info() does doesn't not change by this
patch, so the previous comment seems to be clearer to me.

---
    if (remote_in_recovery)
+   {
+       /*
+        * If we are a cascading standby, no need to check further for
+        * 'primary_slot_name'.
+        */
        ereport(ERROR,
                errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                errmsg("cannot synchronize replication slots from a
standby server"));
+   }
+   else
+   {
+       bool        primary_slot_valid;

-   primary_slot_valid = DatumGetBool(slot_getattr(tupslot, 2, &isnull));
-   Assert(!isnull);
+       primary_slot_valid = DatumGetBool(slot_getattr(tupslot, 2, &isnull));
+       Assert(!isnull);

-   if (!primary_slot_valid)
-       ereport(ERROR,
-               errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-               errmsg("bad configuration for slot synchronization"),
-       /* translator: second %s is a GUC variable name */
-               errdetail("The replication slot \"%s\" specified by
\"%s\" does not exist on the primary server.",
-                         PrimarySlotName, "primary_slot_name"));
+       if (!primary_slot_valid)
+           ereport(ERROR,
+                   errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+                   errmsg("bad configuration for slot synchronization"),
+           /* translator: second %s is a GUC variable name */
+                   errdetail("The replication slot \"%s\" specified
by \"%s\" does not exist on the primary server.",
+                             PrimarySlotName, "primary_slot_name"));
+   }

I think it's a refactoring rather than changes required by the
slotsync worker. We can do that in a separate patch but why do we need
this change in the first place?

---
+        ValidateSlotSyncParams(ERROR);
+
         /* Load the libpq-specific functions */
         load_file("libpqwalreceiver", false);

-        ValidateSlotSyncParams();
+        (void) CheckDbnameInConninfo();

Is there any reason why we move where to check the parameters?

Some comments not related to the patch but to the existing code:

---
It might have already been discussed but is the
src/backend/replication/logical the right place for the slocsync.c? If
it's independent of logical decoding/replication, is under
src/backend/replication could be more appropriate?

---
    /* Construct query to fetch slots with failover enabled. */
    appendStringInfo(&s,
                     "SELECT slot_name, plugin, confirmed_flush_lsn,"
                     " restart_lsn, catalog_xmin, two_phase, failover,"
                     " database, conflict_reason"
                     " FROM pg_catalog.pg_replication_slots"
                     " WHERE failover and NOT temporary");

    /* Execute the query */
    res = walrcv_exec(wrconn, s.data, SLOTSYNC_COLUMN_COUNT, slotRow);
    pfree(s.data);

We don't need 's' as the query is constant.

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).