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: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, "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: 2023-08-01T11:22:13Z
Lists: pgsql-hackers

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On Thu, Jul 27, 2023 at 12:13 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Thu, Jul 27, 2023 at 10:55 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Wed, Jul 26, 2023 at 10:31 AM shveta malik <shveta.malik@gmail.com> wrote:
> > >
> > > On Mon, Jul 24, 2023 at 9:00 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > On Mon, Jul 24, 2023 at 8:03 AM Bharath Rupireddy
> > > > <bharath.rupireddyforpostgres@gmail.com> wrote:
> > > > >
> > > > > Is having one (or a few more - not
> > > > > necessarily one for each logical slot) worker for all logical slots
> > > > > enough?
> > > > >
> > > >
> > > > I guess for a large number of slots the is a possibility of a large
> > > > gap in syncing the slots which probably means we need to retain
> > > > corresponding WAL for a much longer time on the primary. If we can
> > > > prove that the gap won't be large enough to matter then this would be
> > > > probably worth considering otherwise, I think we should find a way to
> > > > scale the number of workers to avoid the large gap.
> > > >
> > >
> > > How about this:
> > >
> > > 1) On standby, spawn 1 worker per database in the start (as it is
> > > doing currently).
> > >
> > > 2) Maintain statistics on activity against each primary's database on
> > > standby by any means. Could be by maintaining 'last_synced_time' and
> > > 'last_activity_seen time'.  The last_synced_time is updated every time
> > > we sync/recheck slots for that particular database. The
> > > 'last_activity_seen_time' changes only if we get any slot on that
> > > database where actually confirmed_flush or say restart_lsn has changed
> > > from what was maintained already.
> > >
> > > 3) If at any moment, we find that 'last_synced_time' -
> > > 'last_activity_seen' goes beyond a threshold, that means that DB is
> > > not active currently. Add it to list of inactive DB
> > >
> >
> > I think we should also increase the next_sync_time if in current sync,
> > there is no update.
>
> +1
>
> >
> > > 4) Launcher on the other hand is always checking if it needs to spawn
> > > any other extra worker for any new DB. It will additionally check if
> > > number of inactive databases (maintained on standby) has gone higher
> > > (> some threshold), then it brings down the workers for those and
> > > starts a common worker which takes care of all such inactive databases
> > > (or merge all in 1), while workers for active databases remain as such
> > > (i.e. one per db). Each worker maintains the list of DBs which it is
> > > responsible for.
> > >
> > > 5) If in the list of these inactive databases, we again find any
> > > active database using the above logic, then the launcher will spawn a
> > > separate worker for that.
> > >
> >
> > I wonder if we anyway some sort of design like this because we
> > shouldn't allow to spawn as many workers as the number of databases.
> > There has to be some existing or new GUC like max_sync_slot_workers
> > which decided the number of workers.
> >
>
> Currently it does not have any such GUC for sync-slot workers. It
> mainly uses the logical-rep-worker framework for the sync-slot worker
> part and thus it relies on 'max_logical_replication_workers' GUC. Also
> it errors out if 'max_replication_slots' is set to zero. I think it is
> not the correct way of doing things for sync-slot. We can have a new
> GUC (max_sync_slot_workers) as you suggested and if the number of
> databases < max_sync_slot_workers, then we can start 1 worker per
> dbid, else divide the work equally among the max sync-workers
> possible. And for inactive database cases, we can increase the
> next_sync_time rather than starting a special worker to handle all the
> inactive databases.  Thoughts?
>

Attaching the PoC patch (0003) where attempts to implement the basic
infrastructure for the suggested design. Rebased the existing patches
(0001 and 0002) as well.

This patch adds a new GUC max_slot_sync_workers; the default and max
value is kept at 2 and 50 respectively for this PoC patch. Now the
replication launcher divides the work equally among these many
slot-sync workers. Let us say there are multiple slots on primary
belonging to 10 DBs and say new GUC on standby is set at default value
of 2, then each worker on standby will manage 5 dbs individually and
will keep on synching the slots for them. If a new DB is found by
replication launcher, it will assign this new db to the worker
handling the minimum number of dbs currently (or first worker in case
of equal count) and that worker will pick up the new db the next time
it tries to sync the slots.
I have kept the changes in separate patches (003) for ease of review.
Since this is just a PoC patch, many things are yet to be done
appropriately, will cover those 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).