Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2024-01-20T10:44:16Z
Lists: pgsql-hackers 
On Sat, Jan 20, 2024 at 10:52 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Fri, Jan 19, 2024 at 5:24 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Wed, Jan 17, 2024 at 4:00 PM shveta malik <shveta.malik@gmail.com> wrote:
> > >
> >
> > I had some off-list discussions with Sawada-San, Hou-San, and Shveta
> > on the topic of extending replication commands instead of using the
> > current model where we fetch the required slot information via SQL
> > using a database connection. I would like to summarize the discussion
> > and would like to know the thoughts of others on this topic.
> >
> > In the current patch, we launch the slotsync worker on physical
> > standby which connects to the specified database (currently we let
> > users specify the required dbname in primary_conninfo) on the primary.
> > It then fetches the required information for failover marked slots
> > from the primary and also does some primitive checks on the upstream
> > node via SQL (the additional checks are like whether the upstream node
> > has a specified physical slot or whether the upstream node is a
> > primary node or a standby node). To fetch the required information it
> > uses a libpqwalreciever API which is mostly apt for this purpose as it
> > supports SQL execution but for this patch, we don't need a replication
> > connection, so we extend the libpqwalreciever connect API.
>
> What sort of extension we have done to 'libpqwalreciever'? Is it
> something like by default this supports replication connections so we
> have done an extension to the API so that we can provide an option
> whether to create a replication connection or a normal connection?
>

Yeah and in the future there could be more as well. The other function
added walrcv_get_dbname_from_conninfo doesn't appear to be a problem
either for now.

> > Now, the concerns related to this could be that users would probably
> > need to change existing mechanisms/tools to update priamry_conninfo
> > 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.
>
> Yeah for this purpose users may use template1 or so which they
> generally don't plan to drop.
>

Using template1 has other problems like users won't be able to create
a new database. See [2] (point number 2.2)

>
>  So in case the user wants to drop that
> database user needs to turn off the slot syncing option and then it
> can be done?
>

Right.

> > 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.
>
> While looking into the patch v64-0002 I could not exactly point out
> what sort of extensions are there in libpqwalreceiver.c, I just saw
> one extra API for fetching the dbname from connection info?
>

Right, the worry was that we may need it in the future.

> > 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.
> >
> > 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.
>
> Something like locking the database only while fetching the slots?
>

Possible, but can we lock the database from an auxiliary process?

> > 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.
>
> Even I have similar thoughts.
>

Thanks.

[1] - https://www.postgresql.org/message-id/CAJpy0uBhPx1MDHh903XpFAhpBH23KzVXyg_4VjH2zXk81oGi1w%40mail.gmail.com

-- 
With Regards,
Amit Kapila.

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).