Horton

Re: [HACKERS] logical decoding of two-phase transactions

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Peter Smith <smithpb2250@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2021-03-08T08:30:25Z
Lists: pgsql-hackers 
On Mon, Mar 8, 2021 at 1:26 PM Peter Smith <smithpb2250@gmail.com> wrote:
>
> On Mon, Mar 8, 2021 at 4:19 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Mon, Mar 8, 2021 at 10:04 AM Peter Smith <smithpb2250@gmail.com> wrote:
> > >
> > > On Sun, Mar 7, 2021 at 3:00 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > On Sun, Mar 7, 2021 at 7:35 AM Peter Smith <smithpb2250@gmail.com> wrote:
> > > > >
> > > > > Please find attached the latest patch set v51*
> > > > >
> > > >
> > > > Few more comments on v51-0006-Fix-apply-worker-empty-prepare:
> > > > ======================================================
> > > > 1.
> > > > +/*
> > > > + * A Prepare spoolfile hash entry. We create this entry in the
> > > > psf_hash. This is
> > > > + * for maintaining a mapping between the name of the prepared
> > > > spoolfile, and the
> > > > + * corresponding fileset handles of same.
> > > > + */
> > > > +typedef struct PsfHashEntry
> > > > +{
> > > > + char name[MAXPGPATH]; /* Hash key --- must be first */
> > > > + bool allow_delete; /* ok to delete? */
> > > > +} PsfHashEntry;
> > > > +
> > > >
> > > > IIUC, this has table is used for two purposes in the patch (a) to
> > > > check for existence of prepare spool file where we anyway to check it
> > > > on disk if not found in the hash table. (b) to allow the prepare spool
> > > > file to be removed on proc_exit.
> > > >
> > > > I think we don't need the optimization provided by (a) because it will
> > > > be too rare a case to deserve any optimization, we might write a
> > > > comment in prepare_spoolfile_exists to indicate such an optimization.
> > > > For (b), we can use a simple list to track files to be removed on
> > > > proc_exit something like we do in CreateLockFile. I think avoiding
> > > > hash table usage will reduce the code and chances of bugs in this
> > > > area. It won't be easy to write a lot of automated tests to test this
> > > > functionality so it is better to avoid minor optimizations at this
> > > > stage.
> > >
> > > Our data structure psf_hash also needs to be able to discover the
> > > entry for a specific spool file and remove it. e.g. anything marked as
> > > "allow_delete = false" (during prepare) must be able to be re-found
> > > and removed from that structure at commit_prepared or
> > > rollback_prepared time.
> > >
> >
> > But, I think that is not reliable because after restart the entry
> > might not be present and we anyway need to check the presence of the
> > file on disk. Actually, you don't need any manipulation with list or
> > hash at commit_prepared or rollback_prepared, we should just remove
> > the entry for it at the prepare time and there should be an assert if
> > we find that entry in the in-memory structure.
> >
> > > Looking at CreateLockFile code, I don't see that it is ever deleting
> > > entries from its "lock_files" list on-the-fly, so it's not really a
> > > fair comparison to say just use a List like CreateLockFile.
> > >
> >
> > Sure, but you can additionally traverse the list and find the required entry.
> >
> > > So, even though we (currently) only have a single data member
> > > "allow_delete", I think the requirement to do a key lookup/delete
> > > makes a HTAB a more appropriate data structure than a List.
> > >
> >
> > Actually, that member is also not required at all because you just
> > need it till the time of prepare and then remove it.
> >
>
> OK, I plan to change like this.
> - Now the whole hash simply means "delete-on-exit". If the key (aka
> filename) exists, delete that file on exit. If not don't
> - Remove the "allow_delete" member (as you say it can be redundant
> using the new interpretation above)
> - the *only* code that CREATES a key will be when
> prepare_spoolfile_create is called from begin_prepare.
> - at apply_handle_prepare time the key is REMOVED (so that file will
> not be deleted in case of a restart / error before commit/rollback)
>

So, the only real place where you need to perform any search is at the
prepare time and I think it should always be the first element if we
use the list here. Am I missing something? If not, I don't see why you
want to prefer HTAB over a simple list? You can remove the first
element and probably have an assert to confirm it is the correct
element (by checking the path) you are removing.

-- 
With Regards,
Amit Kapila.

Commits

  1. Add prepare API support for streaming transactions in logical replication.

  2. Doc: minor improvements for logical replication protocol documentation.

  3. Fix test failure in 021_twophase.pl.

  4. Refactor to make common functions in proto.c and worker.c.

  5. Unify PostgresNode's new() and get_new_node() methods

  6. Fix potential buffer overruns in proto.c.

  7. Add support for prepared transactions to built-in logical replication.

  8. Refactor function parse_subscription_options.

  9. Allow enabling two-phase option via replication protocol.

  10. Don't use Asserts to check for violations of replication protocol.

  11. Improve psql tab completion for options of subcriptions and publications

  12. Rearrange logrep worker's snapshot handling some more.

  13. doc: Update information of new messages for logical replication.

  14. ALTER SUBSCRIPTION ... ADD/DROP PUBLICATION

  15. Allow pgoutput to send logical decoding messages.

  16. Refactor function parse_output_parameters.

  17. Avoid repeated decoding of prepared transactions after a restart.

  18. Fix an oversight in ReorderBufferFinishPrepared.

  19. Allow multiple xacts during table sync in logical replication.

  20. Fix replication of in-progress transactions in tablesync worker.

  21. Fix 'skip-empty-xacts' option in test_decoding for streaming mode.

  22. Use Enum for top level logical replication message types.

  23. Add support for streaming to built-in logical replication.

  24. Fix the logical streaming test.

  25. Implement streaming mode in ReorderBuffer.

  26. Extend the logical decoding output plugin API with stream methods.

  27. Store 2PC GID in commit/abort WAL recs for logical decoding