Horton

RE: Synchronizing slots from primary to standby

Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>

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From: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
To: "'Drouvot, Bertrand'" <bertranddrouvot.pg@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>
Date: 2023-06-29T10:22:06Z
Lists: pgsql-hackers 
Dear Drouvot,

Hi, I'm also interested in the feature. Followings are my high-level comments.
I did not mention some detailed notations because this patch is not at the stage.
And very sorry that I could not follow all of this discussions.

1. I thought that we should not reuse logical replication launcher for another purpose.
   The background worker should have only one task. I wanted to ask opinions some other people...
2. I want to confirm the reason why new replication command is added. IIUC the
   launcher connects to primary by using primary_conninfo connection string, but
   it establishes the physical replication connection so that any SQL cannot be executed.
   Is it right? Another approach not to use is to specify the target database via
   GUC, whereas not smart. How do you think?
3. You chose the per-db worker approach, however, it is difficult to extend the
   feature to support physical slots. This may be problematic. Was there any
   reasons for that? I doubted ReplicationSlotCreate() or advance functions might
   not be used from other databases and these may be reasons, but not sure.
   If these operations can do without connecting to specific database, I think
   the architecture can be changed.
4. Currently the launcher establishes the connection every time. Isn't it better
   to reuse the same one instead?

Following comments are assumed the configuration, maybe the straightfoward:

primary->standby
   |->subscriber

5. After constructing the system, I dropped the subscription on the subscriber.
   In this case the logical slot on primary was removed, but that was not replicated
   to standby server. Did you support the workload or not?

```
$ psql -U postgres -p $port_sub -c "DROP SUBSCRIPTION sub"
NOTICE:  dropped replication slot "sub" on publisher
DROP SUBSCRIPTION

$ psql -U postgres -p $port_primary -c "SELECT * FROM pg_replication_slots"
slot_name |  plugin  | slot_type | datoid | database |...
-----------+----------+-----------+--------+----------+... 
(0 rows)

$ psql -U postgres -p $port_standby -c "SELECT * FROM pg_replication_slots"
 slot_name |  plugin  | slot_type | datoid | database |...
-----------+----------+-----------+--------+----------+...
 sub       | pgoutput | logical   |      5 | postgres |...
(1 row)

```

6. Current approach may delay the startpoint of sync.

Assuming that physical replication system is created first, and then the
subscriber connects to the publisher node. In this case the launcher connects to
primary earlier than the apply worker, and reads the slot. At that time there are
no slots on primary, so launcher disconnects from primary and waits a time period (up to 3min).
Even if the apply worker creates the slot on publisher, but the launcher on standby
cannot notice that. The synchronization may start 3 min later.

I'm not sure how to fix or it could be acceptable. Thought?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED

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).