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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: 2022-11-23T13:40:17Z
Lists: pgsql-hackers 
On Tue, Nov 22, 2022 at 7:30 AM houzj.fnst@fujitsu.com
<houzj.fnst@fujitsu.com> wrote:
>

Few minor comments and questions:
============================
1.
+static void
+LogicalParallelApplyLoop(shm_mq_handle *mqh)
{
+ for (;;)
+ {
+ void    *data;
+ Size len;
+
+ ProcessParallelApplyInterrupts();
...
...
+ if (rc & WL_LATCH_SET)
+ {
+ ResetLatch(MyLatch);
+ ProcessParallelApplyInterrupts();
+ }
...
}

Why ProcessParallelApplyInterrupts() is called twice in
LogicalParallelApplyLoop()?

2.
+ * This scenario is similar to the first case but TX-1 and TX-2 are executed by
+ * two parallel apply workers (PA-1 and PA-2 respectively). In this scenario,
+ * PA-2 is waiting for PA-1 to complete its transaction while PA-1 is waiting
+ * for subsequent input from LA. Also, LA is waiting for PA-2 to complete its
+ * transaction in order to preserve the commit order. There is a deadlock among
+ * three processes.
+ *
...
...
+ *
+ * LA (waiting to acquire the local transaction lock) -> PA-1 (waiting to
+ * acquire the lock on the unique index) -> PA-2 (waiting to acquire the lock
+ * on the remote transaction) -> LA
+ *

Isn't the order of PA-1 and PA-2 different in the second paragraph as
compared to the first one.

3.
+ * Deadlock-detection
+ * ------------------

It may be better to keep the title of this section as Locking Considerations.

4. In the section mentioned in Point 3, it would be better to
separately explain why we need session-level locks instead of
transaction level.

5. Add the below comments in the code:
diff --git a/src/backend/replication/logical/applyparallelworker.c
b/src/backend/replication/logical/applyparallelworker.c
index 9385afb6d2..56f00defcf 100644
--- a/src/backend/replication/logical/applyparallelworker.c
+++ b/src/backend/replication/logical/applyparallelworker.c
@@ -431,6 +431,9 @@ pa_free_worker_info(ParallelApplyWorkerInfo *winfo)
        if (winfo->dsm_seg != NULL)
                dsm_detach(winfo->dsm_seg);

+       /*
+        * Ensure this worker information won't be reused during
worker allocation.
+        */
        ParallelApplyWorkersList = list_delete_ptr(ParallelApplyWorkersList,

                    winfo);

@@ -762,6 +765,10 @@
HandleParallelApplyMessage(ParallelApplyWorkerInfo *winfo, StringInfo
msg)
                                 */
                                error_context_stack = apply_error_context_stack;

+                               /*
+                                * The actual error must be already
reported by parallel apply
+                                * worker.
+                                */
                                ereport(ERROR,

(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                                 errmsg("parallel
apply worker exited abnormally"),




-- 
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.