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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: 2022-12-01T12:39:54Z
Lists: pgsql-hackers

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On Wed, Nov 30, 2022 at 4:23 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> 2.
> + /*
> + * The stream lock is released when processing changes in a
> + * streaming block, so the leader needs to acquire the lock here
> + * before entering PARTIAL_SERIALIZE mode to ensure that the
> + * parallel apply worker will wait for the leader to release the
> + * stream lock.
> + */
> + if (in_streamed_transaction &&
> + action != LOGICAL_REP_MSG_STREAM_STOP)
> + {
> + pa_lock_stream(winfo->shared->xid, AccessExclusiveLock);
>
> This comment is not completely correct because we can even acquire the
> lock for the very streaming chunk. This check will work but doesn't
> appear future-proof or at least not very easy to understand though I
> don't have a better suggestion at this stage. Can we think of a better
> check here?
>

One idea is that we acquire this lock every time and callers like
stream_commit are responsible to release it. Also, we can handle the
close of stream file in the respective callers. I think that will make
this part of the patch easier to follow.

Some other comments:
=====================
1. The handling of buffile inside pa_stream_abort() looks bit ugly to
me. I think you primarily required it because the buffile opened by
parallel apply worker is in CurrentResourceOwner. Can we think of
having a new resource owner to apply spooled messages? I think that
will avoid the need to have a special purpose code to handle buffiles
in parallel apply worker.

2.
@@ -564,6 +571,7 @@ handle_streamed_transaction(LogicalRepMsgType
action, StringInfo s)
  TransactionId current_xid;
  ParallelApplyWorkerInfo *winfo;
  TransApplyAction apply_action;
+ StringInfoData original_msg;

  apply_action = get_transaction_apply_action(stream_xid, &winfo);

@@ -573,6 +581,8 @@ handle_streamed_transaction(LogicalRepMsgType
action, StringInfo s)

  Assert(TransactionIdIsValid(stream_xid));

+ original_msg = *s;
+
  /*
  * We should have received XID of the subxact as the first part of the
  * message, so extract it.
@@ -596,10 +606,14 @@ handle_streamed_transaction(LogicalRepMsgType
action, StringInfo s)
  stream_write_change(action, s);
  return true;

+ case TRANS_LEADER_PARTIAL_SERIALIZE:
  case TRANS_LEADER_SEND_TO_PARALLEL:
  Assert(winfo);

- pa_send_data(winfo, s->len, s->data);
+ if (apply_action == TRANS_LEADER_SEND_TO_PARALLEL)
+ pa_send_data(winfo, s->len, s->data);
+ else
+ stream_write_change(action, &original_msg);

Please add the comment to specify the reason to remember the original string.

3.
@@ -1797,8 +1907,8 @@ apply_spooled_messages(TransactionId xid, XLogRecPtr lsn)
  changes_filename(path, MyLogicalRepWorker->subid, xid);
  elog(DEBUG1, "replaying changes from file \"%s\"", path);

- fd = BufFileOpenFileSet(MyLogicalRepWorker->stream_fileset, path, O_RDONLY,
- false);
+ stream_fd = BufFileOpenFileSet(stream_fileset, path, O_RDONLY, false);
+ stream_xid = xid;

Why do we need stream_xid here? I think we can avoid having global
stream_fd if the comment #1 is feasible.

4.
+ * TRANS_LEADER_APPLY:
+ * The action means that we

/The/This. Please make a similar change for other actions.

5. Apart from the above, please find a few changes to the comments for
0001 and 0002 patches in the attached patches.


-- 
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.