Horton

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

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Ajin Cherian <itsajin@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Masahiko Sawada <sawada.mshk@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-09-17T12:34:52Z
Lists: pgsql-hackers 
On Thu, Sep 17, 2020 at 2:02 PM Ajin Cherian <itsajin@gmail.com> wrote:
>
> On Tue, Sep 15, 2020 at 10:43 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>>
>>
>> Few other comments:
>> ===================
>> 1.
>> ReorderBufferProcessTXN()
>> {
>> ..
>> if (streaming)
>> {
>> ReorderBufferTruncateTXN(rb, txn);
>>
>> /* Reset the CheckXidAlive */
>> CheckXidAlive = InvalidTransactionId;
>> }
>> else
>> ReorderBufferCleanupTXN(rb, txn);
>> ..
>> }
>>
>> I don't think we can perform ReorderBufferCleanupTXN for the prepared
>> transactions because if we have removed the ReorderBufferTxn before
>> commit, the later code might not consider such a transaction in the
>> system and compute the wrong value of restart_lsn for a slot.
>> Basically, in SnapBuildProcessRunningXacts() when we call
>> ReorderBufferGetOldestTXN(), it should show the ReorderBufferTxn of
>> the prepared transaction which is not yet committed but because we
>> have removed it after prepare, it won't get that TXN and then that
>> leads to wrong computation of restart_lsn. Once we start from a wrong
>> point in WAL, the snapshot built was incorrect which will lead to the
>> wrong result. This is the same reason why the patch is not doing
>> ReorderBufferForget in DecodePrepare when we decide to skip the
>> transaction. Also, here, we need to set CheckXidAlive =
>> InvalidTransactionId; for prepared xact as well.
>>
>>
>
> Just to confirm what you are expecting here. so after we send out the prepare transaction to the plugin, you are suggesting to NOT do a   ReorderBufferCleanupTXN, but what to do instead?. Are you suggesting to do what you suggested
> as part of concurrent abort handling?
>

Yes.

> Something equivalent to ReorderBufferTruncateTXN()? remove all changes of the transaction but keep the invalidations and tuplecids etc?
>

I don't think you don't need tuplecids. I have checked
ReorderBufferFinishPrepared() and that seems to require only
invalidations, check if anything else is required.

> Do you think we should have a new flag in txn to indicate that this transaction has already been decoded? (prepare_decoded?)
>

Yeah, I think that would be better. How about if name the new variable
as cleanup_prepared?

-- 
With Regards,
Amit Kapila.

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