Horton
Menu

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

Amit Kapila <amit.kapila16@gmail.com>

View thread
From: Amit Kapila <amit.kapila16@gmail.com>
To: Peter Smith <smithpb2250@gmail.com>
Cc: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-08-18T10:20:37Z
Lists: pgsql-hackers 
On Thu, Aug 18, 2022 at 3:40 PM Peter Smith <smithpb2250@gmail.com> wrote:
>
> On Thu, Aug 18, 2022 at 6:57 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > > 47. src/include/replication/logicalproto.h
> > >
> > > @@ -32,12 +32,17 @@
> > >   *
> > >   * LOGICALREP_PROTO_TWOPHASE_VERSION_NUM is the minimum protocol version with
> > >   * support for two-phase commit decoding (at prepare time). Introduced in PG15.
> > > + *
> > > + * LOGICALREP_PROTO_STREAM_PARALLEL_VERSION_NUM is the minimum protocol version
> > > + * with support for streaming large transactions using apply background
> > > + * workers. Introduced in PG16.
> > >   */
> > >  #define LOGICALREP_PROTO_MIN_VERSION_NUM 1
> > >  #define LOGICALREP_PROTO_VERSION_NUM 1
> > >  #define LOGICALREP_PROTO_STREAM_VERSION_NUM 2
> > >  #define LOGICALREP_PROTO_TWOPHASE_VERSION_NUM 3
> > > -#define LOGICALREP_PROTO_MAX_VERSION_NUM LOGICALREP_PROTO_TWOPHASE_VERSION_NUM
> > > +#define LOGICALREP_PROTO_STREAM_PARALLEL_VERSION_NUM 4
> > > +#define LOGICALREP_PROTO_MAX_VERSION_NUM
> > > LOGICALREP_PROTO_STREAM_PARALLEL_VERSION_NUM
> > >
> > > 47a.
> > > I don't think that comment is strictly true. IIUC the new protocol
> > > version 4 is currently only affecting the *extra* STREAM_ABORT members
> > > – but in fact streaming=parallel is still functional without using
> > > those extra members, isn't it? So maybe this description needed to be
> > > modified a bit to be more accurate?
> > >
> >
> > The reason for sending this extra abort members is to ensure that
> > after aborting the transaction, if the subscriber/apply worker
> > restarts, it doesn't need to request the transaction again. Do you
> > have suggestions for improving this comment?
> >
>
> I gave three review comments for v21-0001 that were all related to
> this same point:
> i- #4b (commit message)
> ii- #7 (protocol pgdocs)
> iii- #47a (code comment)
>
> The point was:
> AFAIK protocol 4 is only to let the parallel streaming logic behave
> *better* in how it can handle restarts after aborts. But that does not
> mean that protocol 4 is a *pre-requisite* for "allowing"
> streaming=parallel to work in the first place. I thought that a PG15
> publisher and PG16 subscriber can still work using streaming=parallel
> even with protocol 3, but it just won't be quite as clever for
> handling restarts after abort as protocol 4 (PG16 -> PG16) would be.
>

It is not only that it makes it better but one can say that it is a
break of a replication protocol that after the client (subscriber) has
applied some transaction, it requests the same transaction again. So,
I think it is better to make the parallelism work only when the server
version is also 16.

-- 
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.