Horton

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

Arseny Sher <a.sher@postgrespro.ru>

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From: Arseny Sher <a.sher@postgrespro.ru>
To: Andres Freund <andres@anarazel.de>
Cc: Nikhil Sontakke <nikhils@2ndquadrant.com>, Petr Jelinek <petr.jelinek@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, David Steele <david@pgmasters.net>, Andrew Dunstan <andrew.dunstan@2ndquadrant.com>, Simon Riggs <simon@2ndquadrant.com>, Craig Ringer <craig@2ndquadrant.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Sokolov Yura <y.sokolov@postgrespro.ru>, Stas Kelvich <s.kelvich@postgrespro.ru>, Dmitry Dolgov <9erthalion6@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Masahiko Sawada <sawada.mshk@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2018-08-07T09:37:14Z
Lists: pgsql-hackers 
Andres Freund <andres@anarazel.de> writes:

>> - On decoding of aborted xacts. The idea to throw an error once we
>>   detect the abort is appealing, however I think you will have problems
>>   with subxacts in the current implementation. What if subxact issues
>>   DDL and then aborted, but main transaction successfully committed?
>
> I don't see a fundamental issue here. I've not reviewed the current
> patchset meaningfully, however. Do you see a fundamental issue here?

Hmm, yes, this is not an issue for this patch because after reading
PREPARE record we know all aborted subxacts and won't try to decode
their changes. However, this will be raised once we decide to decode
in-progress transactions. Checking for all subxids is expensive;
moreover, WAL doesn't provide all of them until commit... it might be
easier to prevent vacuuming of aborted stuff while decoding needs it.
Matter for another patch, anyway.

>> - Currently we will call abort_prepared cb even if we failed to actually
>>   prepare xact due to concurrent abort. I think it is confusing for
>>   users. We should either handle this by remembering not to invoke
>>   abort_prepared in these cases or at least document this behaviour,
>>   leaving this problem to the receiver side.
>
> What precisely do you mean by "concurrent abort"?

With current patch, the following is possible:
* We start decoding of some prepared xact;
* Xact aborts (ABORT PREPARED) for any reason;
* Decoding processs notices this on catalog scan and calls abort()
  callback;
* Later decoding process reads abort record and calls abort_prepared
  callback.

--
Arseny Sher
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

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