Horton

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

Ajin Cherian <itsajin@gmail.com>

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From: Ajin Cherian <itsajin@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-11-27T13:05:04Z
Lists: pgsql-hackers 
On Thu, Nov 26, 2020 at 10:43 PM Amit Kapila <amit.kapila16@gmail.com> wrote:

> I think what you need to do to reproduce this is to follow the
> snapshot machinery in SnapBuildFindSnapshot. Basically, first, start a
> transaction  (say transaction-id is 500) and do some operations but
> don't commit. Here, if you create a slot (via subscription or
> otherwise), it will wait for 500 to complete and make the state as
> SNAPBUILD_BUILDING_SNAPSHOT. Here, you can commit 500 and then having
> debugger in that state, start another transaction (say 501), do some
> operations but don't commit. Next time when you reach this function,
> it will change the state to SNAPBUILD_FULL_SNAPSHOT and wait for 501,
> now you can start another transaction (say 502) which you can prepare
> but don't commit. Again start one more transaction 503, do some ops,
> commit both 501 and 503. At this stage somehow we need to ensure that
> XLOG_RUNNING_XACTS record. Then commit prepared 502. Now, I think you
> should notice that the consistent point is reached after 502's prepare
> and before its commit. Now, this is just a theoretical scenario, you
> need something on these lines and probably a way to force
> XLOG_RUNNING_XACTS WAL (probably via debugger or some other way) at
> the right times to reproduce it.
>
> Thanks for trying to build a test case for this, it is really helpful.

I tried the above steps, I was able to get the builder state to
SNAPBUILD_BUILDING_SNAPSHOT but was not able to get into the
SNAPBUILD_FULL_SNAPSHOT state.
Instead the code moves straight from SNAPBUILD_BUILDING_SNAPSHOT  to
SNAPBUILD_CONSISTENT state.

In the function SnapBuildFindSnapshot, either the following check fails:

1327: TransactionIdPrecedesOrEquals(SnapBuildNextPhaseAt(builder),
                                           running->oldestRunningXid))

because the SnapBuildNextPhaseAt (which is same as running->nextXid)
is higher than oldestRunningXid, or when the both are the same, then
it falls through into the below condition higher in the code

1247: if (running->oldestRunningXid == running->nextXid)

and then the builder moves straight into the SNAPBUILD_CONSISTENT
state. At no point will the nextXid be less than oldestRunningXid. In
my sessions, I commit multiple txns, hoping to bump
up oldestRunningXid, I do checkpoints,  have made sure the
XLOG_RUNNING_XACTS are being inserted.,
 but while iterating into SnapBuildFindSnapshot with a ,new
XLOG_RUNNING_XACTS:record,  the oldestRunningXid is being incremented
at one xid at a time, which will eventually make it catch up
running->nextXid and reach a
SNAPBUILD_CONSISTENT state without entering the SNAPBUILD_FULL_SNAPSHOT state.



regards,
Ajin Cherian
Fujitsu Australia

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