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Re: Perform streaming logical transactions by background workers and parallel apply

Tomas Vondra <tomas.vondra@enterprisedb.com>

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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Amit Kapila <amit.kapila16@gmail.com>, "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Dilip Kumar <dilipbalaut@gmail.com>
Date: 2023-01-15T17:09:00Z
Lists: pgsql-hackers 
Hi,

I think there's a bug in how get_transaction_apply_action() interacts
with handle_streamed_transaction() to decide whether the transaction is
streamed or not. Originally, the code was simply:

    /* not in streaming mode */
    if (!in_streamed_transaction)
        return false;

But now this decision was moved to get_transaction_apply_action(), which
does this:

    if (am_parallel_apply_worker())
    {
        return TRANS_PARALLEL_APPLY;
    }
    else if (in_remote_transaction)
    {
        return TRANS_LEADER_APPLY;
    }

and handle_streamed_transaction() then uses the result like this:

    /* not in streaming mode */
    if (apply_action == TRANS_LEADER_APPLY)
        return false;

Notice this is not equal to the original behavior, because the two flags
(in_remote_transaction and in_streamed_transaction) are not inverse.
That is,

   in_remote_transaction=false

does not imply we're processing streamed transaction. It's allowed both
flags are false, i.e. a change may be "non-transactional" and not
streamed, though the only example of such thing in the protocol are
logical messages. Which are however ignored in the apply worker, so I'm
not surprised no existing test failed on this.

So I think get_transaction_apply_action() should do this:

    if (am_parallel_apply_worker())
    {
        return TRANS_PARALLEL_APPLY;
    }
    else if (!in_streamed_transaction)
    {
        return TRANS_LEADER_APPLY;
    }

FWIW I've noticed this after rebasing the sequence decoding patch, which
adds another type of protocol message with the transactional vs.
non-transactional behavior, similar to "logical messages" except that in
this case the worker does not ignore that.

Also, I think get_transaction_apply_action() would deserve better
comments explaining how/why it makes the decisions.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix invalid memory access during the shutdown of the parallel apply worker.

  2. Fix assertion failure in apply worker.

  3. Use elog to report unexpected action in handle_streamed_transaction().

  4. Use appropriate wait event when sending data in the apply worker.

  5. Allow the logical_replication_mode to be used on the subscriber.

  6. Rename GUC logical_decoding_mode to logical_replication_mode.

  7. Display the leader apply worker's PID for parallel apply workers.

  8. Improve the code to decide and process the apply action.

  9. Document the newly added wait events added by commit 216a784829.

  10. Perform apply of large transactions by parallel workers.

  11. Wake up a subscription's replication worker processes after DDL.

  12. Add copyright notices to meson files

  13. Better document logical replication parameters

  14. Add a common function to generate the origin name.

  15. Harmonize parameter names in storage and AM code.

  16. Avoid using list_length() to test for empty list.

  17. Improve two comments related to a boolean DefElem's value

  18. Fix partition table's REPLICA IDENTITY checking on the subscriber.

  19. Fix data inconsistency between publisher and subscriber.

  20. Fix cache look-up failures while applying changes in logical replication.