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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: "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, Peter Smith <smithpb2250@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-06-15T12:12:44Z
Lists: pgsql-hackers 
On Tue, Jun 14, 2022 at 9:07 AM wangw.fnst@fujitsu.com
<wangw.fnst@fujitsu.com> wrote:
>
>
> Attach the new patches.
> Only changed patches 0001, 0004 and added new separate patch 0005.
>

Few questions/comments on 0001
===========================
1.
In the commit message, I see: "We also need to allow stream_stop to
complete by the apply background worker to avoid deadlocks because
T-1's current stream of changes can update rows in conflicting order
with T-2's next stream of changes."

Thinking about this, won't the T-1 and T-2 deadlock on the publisher
node as well if the above statement is true?

2.
+       <para>
+        The apply background workers are taken from the pool defined by
+        <varname>max_logical_replication_workers</varname>.
+       </para>
+       <para>
+        The default value is 3. This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command
+        line.
+       </para>

Is there a reason to choose this number as 3? Why not 2 similar to
max_sync_workers_per_subscription?

3.
+
+  <para>
+   Setting streaming mode to <literal>apply</literal> could export invalid LSN
+   as finish LSN of failed transaction. Changing the streaming mode and making
+   the same conflict writes the finish LSN of the failed transaction in the
+   server log if required.
+  </para>

How will the user identify that this is an invalid LSN value and she
shouldn't use it to SKIP the transaction? Can we change the second
sentence to: "User should change the streaming mode to 'on' if they
would instead wish to see the finish LSN on error. Users can use
finish LSN to SKIP applying the transaction." I think we can give
reference to docs where the SKIP feature is explained.

4.
+ * This file contains routines that are intended to support setting up, using,
+ * and tearing down a ApplyBgworkerState.
+ * Refer to the comments in file header of logical/worker.c to see more
+ * informations about apply background worker.

Typo. /informations/information.

Consider having an empty line between the above two lines.

5.
+ApplyBgworkerState *
+apply_bgworker_find_or_start(TransactionId xid, bool start)
{
...
...
+ if (!TransactionIdIsValid(xid))
+ return NULL;
+
+ /*
+ * We don't start new background worker if we are not in streaming apply
+ * mode.
+ */
+ if (MySubscription->stream != SUBSTREAM_APPLY)
+ return NULL;
+
+ /*
+ * We don't start new background worker if user has set skiplsn as it's
+ * possible that user want to skip the streaming transaction. For
+ * streaming transaction, we need to spill the transaction to disk so that
+ * we can get the last LSN of the transaction to judge whether to skip
+ * before starting to apply the change.
+ */
+ if (start && !XLogRecPtrIsInvalid(MySubscription->skiplsn))
+ return NULL;
+
+ /*
+ * For streaming transactions that are being applied in apply background
+ * worker, we cannot decide whether to apply the change for a relation
+ * that is not in the READY state (see should_apply_changes_for_rel) as we
+ * won't know remote_final_lsn by that time. So, we don't start new apply
+ * background worker in this case.
+ */
+ if (start && !AllTablesyncsReady())
+ return NULL;
...
...
}

Can we move some of these starting checks to a separate function like
canstartapplybgworker()?

-- 
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.