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RE: Perform streaming logical transactions by background workers and parallel apply

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>
To: "kuroda.hayato@fujitsu.com" <kuroda.hayato@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-10-21T09:31:42Z
Lists: pgsql-hackers 
On Wednesday, October 19, 2022 8:50 PM Kuroda, Hayato/黒田 隼人 <kuroda.hayato@fujitsu.com> wrote:
> 
> ===
> 01. applyparallelworker.c - SIZE_STATS_MESSAGE
> 
> ```
> /*
>  * There are three fields in each message received by the parallel apply
>  * worker: start_lsn, end_lsn and send_time. Because we have updated these
>  * statistics in the leader apply worker, we can ignore these fields in the
>  * parallel apply worker (see function LogicalRepApplyLoop).
>  */
> #define SIZE_STATS_MESSAGE (2 * sizeof(XLogRecPtr) + sizeof(TimestampTz))
> ```
> 
> According to other comment styles, it seems that the first sentence of the
> comment should represent the datatype and usage, not the detailed reason.
> For example, about ParallelApplyWorkersList, you said "A list ...". How about
> adding like following message:
> The message size that can be skipped by parallel apply worker

Thanks for the comments, but the current description seems enough to me.

> ~~~
> 02. applyparallelworker.c - parallel_apply_start_subtrans
> 
> ```
> 	if (current_xid != top_xid &&
> 		!list_member_xid(subxactlist, current_xid)) ```
> 
> A macro TransactionIdEquals is defined in access/transam.h. Should we use it,
> or is it too trivial?

I checked the existing codes, it seems both style are being used.
Maybe we can post a separate patch to replace them later.

> ~~~
> 08. worker.c - apply_handle_prepare_internal
> 
> Same as above.
> 
> 
> ~~~
> 09. worker.c - maybe_reread_subscription
> 
> ```
> 	/*
> 	 * Exit if any parameter that affects the remote connection was
> changed.
> 	 * The launcher will start a new worker.
> 	 */
> 	if (strcmp(newsub->conninfo, MySubscription->conninfo) != 0 ||
> 		strcmp(newsub->name, MySubscription->name) != 0 ||
> 		strcmp(newsub->slotname, MySubscription->slotname) != 0 ||
> 		newsub->binary != MySubscription->binary ||
> 		newsub->stream != MySubscription->stream ||
> 		strcmp(newsub->origin, MySubscription->origin) != 0 ||
> 		newsub->owner != MySubscription->owner ||
> 		!equal(newsub->publications, MySubscription->publications))
> 	{
> 		ereport(LOG,
> 				(errmsg("logical replication apply worker for
> subscription \"%s\" will restart because of a parameter change",
> 						MySubscription->name)));
> 
> 		proc_exit(0);
> 	}
> ```
> 
> When the parallel apply worker has been launched and then the subscription
> option has been modified, the same message will appear twice.
> But if the option "streaming" is changed from "parallel" to "on", one of them
> will not restart again.
> Should we modify message?

Thanks, it seems a timing problem, if the leader catch the change first and stop
the parallel workers, the message will only appear once. But I agree we'd
better make the message clear. I changed the message in parallel apply worker.
While on it, I also adjusted some other message to include "parallel apply
worker" if they are in parallel apply worker.

Best regards,
Hou zj

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.