Horton

Re: Synchronizing slots from primary to standby

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>

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From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2021-11-29T06:49:00Z
Lists: pgsql-hackers 
On Mon, Nov 29, 2021 at 11:14 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Mon, Nov 29, 2021 at 9:40 AM Bharath Rupireddy
> <bharath.rupireddyforpostgres@gmail.com> wrote:
> >
> > On Mon, Nov 29, 2021 at 1:48 AM SATYANARAYANA NARLAPURAM
> > <satyanarlapuram@gmail.com> wrote:
> > >
> > >> 3) Instead of the subscriber pulling the slot info, why can't the
> > >> publisher (via the walsender or a new bg worker maybe?) push the
> > >> latest slot info? I'm not sure we want to add more functionality to
> > >> the walsender, if yes, isn't it going to be much simpler?
> > >
> > > Standby pulling the information or at least making a first attempt to connect to the  primary is a better design as primary doesn't need to spend its cycles repeatedly connecting to an unreachable standby. In fact, primary wouldn't even need to know the followers, for example followers / log shipping standbys
> >
> > My idea was to let the existing walsender from the primary/publisher
> > to send the slot info (both logical and physical replication slots) to
> > the standby/subscriber, probably by piggybacking the slot info with
> > the WAL currently it sends. Having said that, I don't know the
> > feasibility of it. Anyways, I'm not in favour of having a new bg
> > worker to just ship the slot info. The standby/subscriber, while
> > making connection to primary/publisher, can choose to get the
> > replication slot info.
>
> I think it is possible that the standby is restoring the WAL directly
> from the archive location and there might not be any wal sender at
> time. So I think the idea of standby pulling the WAL looks better to
> me.

My point was that why can't we let the walreceiver (of course users
can configure it on the standby/subscriber) to choose whether or not
to receive the replication (both physical and logical) slot info from
the primary/publisher and if yes, the walsender(on the
primary/publisher) sending it probably as a new WAL record or just
piggybacking the replication slot info with any of the existing WAL
records.

Or simply a common bg worker (as opposed to the bg worker proposed
originally in this thread which, IIUC, works for logical replication)
running on standby/subscriber for getting both the physical and
logical replication slots info.

> > As I said upthread, the problem I see with standby/subscriber pulling
> > the info is that: how frequently the standby/subscriber is going to
> > sync the slot info from primary/publisher? How can it ensure that the
> > latest information exists say when the subscriber is down/crashed
> > before it picks up the latest slot information?
>
> Yeah that is a good question that how frequently the subscriber should
> fetch the slot information, I think that should be configurable
> values.  And the time delay is more, the chances of losing the latest
> slot is more.

I agree that it should be configurable. Even if the primary/publisher
is down/crashed, one can still compare the latest slot info from both
the primary/publisher and standby/subscriber using a new tool
pg_replslotdata proposed at [1] and see how far and which slots missed
the latest replication slot info and probably drop those alone to
recreate and retain other slots as is.

[1] - https://www.postgresql.org/message-id/CALj2ACW0rV5gWK8A3m6_X62qH%2BVfaq5hznC%3Di0R5Wojt5%2Byhyw%40mail.gmail.com

Regards,
Bharath Rupireddy.

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).