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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: 2022-06-23T08:43:54Z
Lists: pgsql-hackers 
On Thu, Jun 23, 2022 at 12:51 PM wangw.fnst@fujitsu.com
<wangw.fnst@fujitsu.com> wrote:
>
> On Mon, Jun 20, 2022 at 11:00 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > I have improved the comments in this and other related sections of the
> > patch. See attached.
> Thanks for your comments and patch!
> Improved the comments as you suggested.
>
> > > > 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.
> > > Improved the sentence as suggested.
> > >
> >
> > You haven't answered first part of the comment: "How will the user
> > identify that this is an invalid LSN value and she shouldn't use it to
> > SKIP the transaction?". Have you checked what value it displays? For
> > example, in one of the case in apply_error_callback as shown in below
> > code, we don't even display finish LSN if it is invalid.
> > else if (XLogRecPtrIsInvalid(errarg->finish_lsn))
> > errcontext("processing remote data for replication origin \"%s\"
> > during \"%s\" in transaction %u",
> >    errarg->origin_name,
> >    logicalrep_message_type(errarg->command),
> >    errarg->remote_xid);
> I am sorry that I missed something in my previous reply.
> The invalid LSN value here is to say InvalidXLogRecPtr (0/0).
> Here is an example :
> ```
> 2022-06-23 14:30:11.343 CST [822333] logical replication worker CONTEXT:  processing remote data for replication origin "pg_16389" during "INSERT" for replication target relation "public.tab" in transaction 727 finished at 0/0
> ```
>

I don't think it is a good idea to display invalid values. We can mask
this as we are doing in other cases in function apply_error_callback.
The ideal way is that we provide a view/system table for users to
check these errors but that is a matter of another patch. So users
probably need to check Logs to see if the error is from a background
apply worker to decide whether or not to switch streaming mode.

-- 
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.