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

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "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>, Dilip Kumar <dilipbalaut@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-12-06T07:50:24Z
Lists: pgsql-hackers 
On Mon, Dec 5, 2022 at 9:59 AM houzj.fnst@fujitsu.com
<houzj.fnst@fujitsu.com> wrote:
>
> Attach a new version patch set which fixed a testcase failure on CFbot.
>

Few comments:
============
1.
+ /*
+ * Break the loop if the parallel apply worker has finished applying
+ * the transaction. The parallel apply worker should have closed the
+ * file before committing.
+ */
+ if (am_parallel_apply_worker() &&
+ MyParallelShared->xact_state == PARALLEL_TRANS_FINISHED)
+ goto done;

This looks hackish to me because ideally, this API should exit after
reading and applying all the messages in the spool file. This check is
primarily based on the knowledge that once we reach some state, the
file won't have more data. I think it would be better to explicitly
ensure the same.

2.
+ /*
+ * No need to output the DEBUG message here in the parallel apply
+ * worker as similar messages will be output when handling STREAM_STOP
+ * message.
+ */
+ if (!am_parallel_apply_worker() && nchanges % 1000 == 0)
  elog(DEBUG1, "replayed %d changes from file \"%s\"",
  nchanges, path);
  }

I think this check appeared a bit ugly to me. I think it is okay to
get a similar DEBUG message at another place (on stream_stop) because
(a) this is logged every 1000 messages whereas stream_stop can be
after many more messages, so there doesn't appear to be a direct
correlation; (b) due to this, we can identify whether it is due to
spooled messages or due to direct apply; ideally we can use another
DEBUG message to differentiate but this doesn't appear bad to me.

3. The function names for serialize_stream_start(),
serialize_stream_stop(), and serialize_stream_abort() don't seem to
match the functionality they provide because none of these
write/serialize changes to the file. Can we rename these? Some
possible options could be stream_start_internal or stream_start_guts.

-- 
With Regards,
Amit Kapila.

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.