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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: 2022-12-19T12:47:25Z
Lists: pgsql-hackers

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On Sat, Dec 17, 2022 at 7:34 PM houzj.fnst@fujitsu.com
<houzj.fnst@fujitsu.com> wrote:
>
> Agreed. I have addressed all the comments and did some cosmetic changes.
> Attach the new version patch set.
>

Few comments:
============
1.
+ if (fileset_state == FS_SERIALIZE_IN_PROGRESS)
+ {
+ pa_lock_stream(MyParallelShared->xid, AccessShareLock);
+ pa_unlock_stream(MyParallelShared->xid, AccessShareLock);
+ }
+
+ /*
+ * We cannot read the file immediately after the leader has serialized all
+ * changes to the file because there may still be messages in the memory
+ * queue. We will apply all spooled messages the next time we call this
+ * function, which should ensure that there are no messages left in the
+ * memory queue.
+ */
+ else if (fileset_state == FS_SERIALIZE_DONE)
+ {

Once we have waited in the FS_SERIALIZE_IN_PROGRESS, the file state
can be FS_SERIALIZE_DONE immediately after that. So, won't it be
better to have a separate if block for FS_SERIALIZE_DONE state? If you
agree to do so then we can probably remove the comment: "* XXX It is
possible that immediately after we have waited for a lock in ...".

2.
+void
+pa_decr_and_wait_stream_block(void)
+{
+ Assert(am_parallel_apply_worker());
+
+ if (pg_atomic_sub_fetch_u32(&MyParallelShared->pending_stream_count, 1) == 0)

I think here the count can go negative when we are in serialize mode
because we don't increase it for serialize mode. I can't see any
problem due to that but OTOH, this doesn't seem to be intended because
in the future if we decide to implement the functionality of switching
back to non-serialize mode, this could be a problem. Also, I guess we
don't even need to try locking/unlocking the stream lock in that case.
One idea to avoid this is to check if the pending count is zero then
if file_set in not available raise an error (elog ERROR), otherwise,
simply return from here.

3. In apply_handle_stream_stop(), we are setting backendstate as idle
for cases TRANS_LEADER_SEND_TO_PARALLEL and TRANS_PARALLEL_APPLY. For
other cases, it is set by stream_stop_internal. I think it would be
better to set the state explicitly for all cases to make the code look
consistent and remove it from stream_stop_internal(). The other reason
to remove setting the state from stream_stop_internal() is that when
that function is invoked from other places like
apply_handle_stream_commit(), it seems to be setting the idle before
actually we reach the idle state.

4. Apart from the above, I have made a few changes in the comments,
see attached.

-- 
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.