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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: 2022-11-18T02:26:45Z
Lists: pgsql-hackers 
On Wed, Nov 16, 2022 at 1:50 PM houzj.fnst@fujitsu.com
<houzj.fnst@fujitsu.com> wrote:
>
> On Tuesday, November 15, 2022 7:58 PM houzj.fnst@fujitsu.com <houzj.fnst@fujitsu.com> wrote:
>
> I noticed that I didn't add CHECK_FOR_INTERRUPTS while retrying send message.
> So, attach the new version which adds that. Also attach the 0004 patch that
> restarts logical replication with temporarily disabling the parallel apply if
> failed to apply a transaction in parallel apply worker.
>

Few comments on v48-0001
======================
1. The variable name pending_message_count seems to indicate a number
of pending messages but normally it is pending start/stop streams
except for probably rollback to savepoint case. Shall we name it
pending_stream_count and change the comments accordingly?

2. The variable name abort_toplevel_transaction seems unnecessarily
long. Shall we rename it to toplevel_xact or something like that?

3.
+ /*
+ * Increment the number of messages waiting to be processed by
+ * parallel apply worker.
+ */
+ if (!abort_toplevel_transaction)
+ pg_atomic_add_fetch_u32(&(winfo->shared->pending_message_count), 1);
+ else
+ pa_unlock_stream(xid, AccessExclusiveLock);

It is better to explain here why different actions are required for
subtransaction and transaction rather than the current comment.

4.
+
+ if (abort_toplevel_transaction)
+ {
+ (void) pa_free_worker(winfo, xid);
+ }

{} is not required here.

5.
/*
+ * Although the lock can be automatically released during transaction
+ * rollback, but we still release the lock here as we may not in a
+ * transaction.
+ */
+ pa_unlock_transaction(xid, AccessShareLock);
+

It is better to explain for which case (I think it is for empty xacts)
it will be useful to release it explicitly.

6.
+ *
+ * XXX We can avoid sending pairs of the START/STOP messages to the parallel
+ * worker because unlike apply worker it will process only one transaction at a
+ * time. However, it is not clear whether any optimization is worthwhile
+ * because these messages are sent only when the logical_decoding_work_mem
+ * threshold is exceeded.
  */
 static void
 apply_handle_stream_start(StringInfo s)

I think this comment is no longer valid as now we need to wait for the
next stream at stream_stop message and also need to acquire the lock
in stream_start message. So, I think it is better to remove it unless
I am missing something.

7. I am able to compile applyparallelworker.c by commenting few of the
header includes. Please check if those are really required.
#include "libpq/pqformat.h"
#include "libpq/pqmq.h"
//#include "mb/pg_wchar.h"
#include "pgstat.h"
#include "postmaster/interrupt.h"
#include "replication/logicallauncher.h"
//#include "replication/logicalworker.h"
#include "replication/origin.h"
//#include "replication/walreceiver.h"
#include "replication/worker_internal.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
//#include "storage/procarray.h"
#include "tcop/tcopprot.h"
#include "utils/inval.h"
#include "utils/memutils.h"
//#include "utils/resowner.h"
#include "utils/syscache.h"

8.
+/*
+ * Is there a message sent by parallel apply worker which we need to receive?
+ */
+volatile sig_atomic_t ParallelApplyMessagePending = false;

This comment and variable are placed in applyparallelworker.c, so 'we'
in the above sentence is not clear. I think you need to use leader
apply worker instead.

9.
+static ParallelApplyWorkerInfo *pa_get_free_worker(void);

Will it be better if we name this function pa_get_available_worker()?

-- 
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.