Horton

Re: [HACKERS] logical decoding of two-phase transactions

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: "tanghy.fnst@fujitsu.com" <tanghy.fnst@fujitsu.com>, vignesh C <vignesh21@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Ajin Cherian <itsajin@gmail.com>, Greg Nancarrow <gregn4422@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2022-01-04T03:30:06Z
Lists: pgsql-hackers 
Hi,

On Mon, Aug 9, 2021 at 12:00 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Wed, Aug 4, 2021 at 4:14 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > I have pushed this last patch in the series.
> >
>
> I have closed this CF entry. Thanks to everyone involved in this work!
>

I have a questoin about two_phase column of pg_replication_slots view:
with this feature, pg_replication_slots has a new column two_phase:

             View "pg_catalog.pg_replication_slots"
       Column        |  Type   | Collation | Nullable | Default
---------------------+---------+-----------+----------+---------
 slot_name           | name    |           |          |
 plugin              | name    |           |          |
 slot_type           | text    |           |          |
 datoid              | oid     |           |          |
 database            | name    |           |          |
 temporary           | boolean |           |          |
 active              | boolean |           |          |
 active_pid          | integer |           |          |
 xmin                | xid     |           |          |
 catalog_xmin        | xid     |           |          |
 restart_lsn         | pg_lsn  |           |          |
 confirmed_flush_lsn | pg_lsn  |           |          |
 wal_status          | text    |           |          |
 safe_wal_size       | bigint  |           |          |
 two_phase           | boolean |           |          |

According to the doc, the two_phase field has:

True if the slot is enabled for decoding prepared transactions. Always
false for physical slots.

It's unnatural a bit to me that replication slots have such a property
since the replication slots have been used to protect WAL and tuples
that are required for logical decoding, physical replication, and
backup, etc from removal. Also, it seems that even if a replication
slot is created with two_phase = off, it's overwritten to on if the
plugin enables two-phase option. Is there any reason why we can turn
on and off this value on the replication slot side and is there any
use case where the replication slot’s two_phase is false and the
plugin’s two-phase option is on and vice versa? I think that we can
have replication slots always have two_phase_at value and remove the
two_phase field from the view.

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/

Commits

  1. Add prepare API support for streaming transactions in logical replication.

  2. Doc: minor improvements for logical replication protocol documentation.

  3. Fix test failure in 021_twophase.pl.

  4. Refactor to make common functions in proto.c and worker.c.

  5. Unify PostgresNode's new() and get_new_node() methods

  6. Fix potential buffer overruns in proto.c.

  7. Add support for prepared transactions to built-in logical replication.

  8. Refactor function parse_subscription_options.

  9. Allow enabling two-phase option via replication protocol.

  10. Don't use Asserts to check for violations of replication protocol.

  11. Improve psql tab completion for options of subcriptions and publications

  12. Rearrange logrep worker's snapshot handling some more.

  13. doc: Update information of new messages for logical replication.

  14. ALTER SUBSCRIPTION ... ADD/DROP PUBLICATION

  15. Allow pgoutput to send logical decoding messages.

  16. Refactor function parse_output_parameters.

  17. Avoid repeated decoding of prepared transactions after a restart.

  18. Fix an oversight in ReorderBufferFinishPrepared.

  19. Allow multiple xacts during table sync in logical replication.

  20. Fix replication of in-progress transactions in tablesync worker.

  21. Fix 'skip-empty-xacts' option in test_decoding for streaming mode.

  22. Use Enum for top level logical replication message types.

  23. Add support for streaming to built-in logical replication.

  24. Fix the logical streaming test.

  25. Implement streaming mode in ReorderBuffer.

  26. Extend the logical decoding output plugin API with stream methods.

  27. Store 2PC GID in commit/abort WAL recs for logical decoding