Horton

Re: Synchronizing slots from primary to standby

shveta malik <shveta.malik@gmail.com>

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From: shveta malik <shveta.malik@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@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>, shveta malik <shveta.malik@gmail.com>
Date: 2023-07-20T11:34:44Z
Lists: pgsql-hackers

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On Fri, Jun 16, 2023 at 3:26 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Apr 17, 2023 at 7:37 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
> >

> 3. As mentioned in the initial email, I think it would be better to
> replace LIST_SLOTS command with a SELECT query.
>

I had a look at this thread. I am interested to work on this and can
spend some time addressing the comments given here.

I tried to replace LIST_SLOTS command with a SELECT query. Attached
rebased patch and PoC patch for LIST_SLOTS removal. For LIST_SLOTs cmd
removal, below are the points where more analysis is needed.

1) I could not use the exposed libpqwalreceiver's functions
walrcv_exec/libpqrcv_exec in LogicalRepLauncher to run select query
instead of LIST_SLOTS cmd. This is because libpqrcv_exec() needs
database connection but since in LogicalReplauncher, we do not have
any (MyDatabseId is not set), so the API gives an error. Thus to make
it work for the time-being, I used 'libpqrcv_PQexec' which is not
dependent upon database connection. But since it is not exposed "yet"
to other layers, I temporarily added the new code to
libpqwalreceiver.c itself. In fact I reused the existing function
wrapper libpqrcv_list_slots and changed the functionality to get info
using select query rather than list_slots.

2) While using connect API walrcv_connect/libpqrcv_connect(), we need
to tell it whether it is for logical or physical replication. In the
existing patch, where we were using LIST_SLOTS cmd, we have this
connection made with logical=false. But now since we need to run
select query to get the same info, using connection with logical=false
gives error on primary while executing select query. "ERROR:  cannot
execute SQL commands in WAL sender for physical replication".
And thus in ApplyLauncherStartSlotSync(), I have changed connect API
to use logical=true for the time being.  I noticed that in the
existing patch, it was using logical=false in
ApplyLauncherStartSlotSync() while logical=true in
synchronize_slots(). Possibly due to the same fact that logical=false
connection will not allow synchronize_slots() to run select query on
primary while it worked for ApplyLauncherStartSlotSync() as it was
running list_slots cmd instead of select query.

I am exploring further on these points to figure out which one is the
correct way to deal with these. Meanwhile posting this WIP patch for
early feedback. I will try addressing other comments as well in next
versions.

thanks
Shveta

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).