Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

View thread View on postgr.es
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@gmail.com>, Peter Smith <smithpb2250@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>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-01-22T07:41:19Z
Lists: pgsql-hackers 
Hi,

On Fri, Jan 19, 2024 at 05:23:53PM +0530, Amit Kapila wrote:
> On Wed, Jan 17, 2024 at 4:00 PM shveta malik <shveta.malik@gmail.com> wrote:
> >
> 
> Now, the concerns related to this could be that users would probably
> need to change existing mechanisms/tools to update priamry_conninfo

Yeah, for the ones that want the sync slot feature.

> and one of the alternatives proposed is to have an additional GUC like
> slot_sync_dbname. Users won't be able to drop the database this worker
> is connected to aka whatever is specified in slot_sync_dbname but as
> the user herself sets up the configuration it shouldn't be a big deal.

Same point of view here.

> Then we also discussed whether extending libpqwalreceiver's connect
> API is a good idea and whether we need to further extend it in the
> future. As far as I can see, slotsync worker's primary requirement is
> to execute SQL queries which the current API is sufficient, and don't
> see something that needs any drastic change in this API. Note that
> tablesync worker that executes SQL also uses these APIs, so we may
> need something in the future for either of those. Then finally we need
> a slotsync worker to also connect to a database to use SQL and fetch
> results.
>

On my side the nits concerns about using the libpqrcv_connect / walrcv_connect are:

- cosmetic: the "rcv" do not really align with the sync slot worker
- we're using a WalReceiverConn, while a PGconn should suffice. From what I can
see the "overhead" is (1 byte + 7 bytes hole + 8 bytes). I don't think that's a
big deal even if we switch to a multi sync slot worker design later on.

Those have already been discussed in [1] and I'm fine with them.

> Now, let us consider if we extend the replication commands like
> READ_REPLICATION_SLOT and or introduce a new set of replication
> commands to fetch the required information then we don't need a DB
> connection with primary or a connection in slotsync worker. As per my
> current understanding, it is quite doable but I think we will slowly
> go in the direction of making replication commands something like SQL
> because today we need to extend it to fetch all slots info that have
> failover marked as true, the existence of a particular replication,
> etc. Then tomorrow, if we want to extend this work to have multiple
> slotsync workers say workers perdb then we have to extend the
> replication command to fetch per-database failover marked slots. To
> me, it sounds more like we are slowly adding SQL-like features to
> replication commands.

Agree. Also it seems to me that extending the replication commands is more like
a one-way door change.

> Apart from this when we are reading per-db replication slots without
> connecting to a database, we probably need some additional protection
> mechanism so that the database won't get dropped.
> 
> Considering all this it seems that for now probably extending
> replication commands can simplify a few things like mentioned above
> but using SQL's with db-connection is more extendable.

I'd vote for using a SQL db-connection (like we are doing currently).
It seems more extendable and more a two-way door (as compared to extending the
replication commands): I think it still gives us the flexibility to switch to
extending the replication commands if we want to in the future.

[1]: https://www.postgresql.org/message-id/ZZe6sok7IWmhKReU%40ip-10-97-1-34.eu-west-3.compute.internal

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
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).