Re: Skipping logical replication transactions on subscriber side

On Tue, Mar 15, 2022 at 7:18 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Mar 15, 2022 at 11:43 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > I've attached an updated version patch.
> >
>
> Review:
> =======

Thank you for the comments.

> 1.
> +++ b/doc/src/sgml/logical-replication.sgml
> @@ -366,15 +366,19 @@ CONTEXT: processing remote data for replication
> origin "pg_16395" during "INSER
> transaction, the subscription needs to be disabled temporarily by
> <command>ALTER SUBSCRIPTION ... DISABLE</command> first or
> alternatively, the
> subscription can be used with the
> <literal>disable_on_error</literal> option.
> - Then, the transaction can be skipped by calling the
> + Then, the transaction can be skipped by using
> + <command>ALTER SUBSCRITPION ... SKIP</command> with the finish LSN
> + (i.e., LSN 0/14C0378). After that the replication
> + can be resumed by <command>ALTER SUBSCRIPTION ... ENABLE</command>.
> + Alternatively, the transaction can also be skipped by calling the
>
> Do we really need to disable the subscription for the skip feature? I
> think that is required for origin_advance. Also, probably, we can say
> Finish LSN could be Prepare LSN, Commit LSN, etc.

Not necessary to disable the subscription for skip feature. Fixed.

>
> 2.
> + /*
> + * Quick return if it's not requested to skip this transaction. This
> + * function is called every start of applying changes and we assume that
> + * skipping the transaction is not used in many cases.
> + */
> + if (likely(XLogRecPtrIsInvalid(MySubscription->skiplsn) ||
>
> The second part of this comment (especially ".. every start of
> applying changes ..") sounds slightly odd to me. How about changing it
> to: "This function is called for every remote transaction and we
> assume that skipping the transaction is not used in many cases."
>

Fixed.

> 3.
> +
> + ereport(LOG,
> + errmsg("start skipping logical replication transaction which
> finished at %X/%X",
> ...
> + ereport(LOG,
> + (errmsg("done skipping logical replication transaction which
> finished at %X/%X",
>
> No need of 'which' in above LOG messages. I think the message will be
> clear without the use of which in above message.

Removed.

>
> 4.
> + ereport(LOG,
> + (errmsg("done skipping logical replication transaction which
> finished at %X/%X",
> + LSN_FORMAT_ARGS(skip_xact_finish_lsn))));
> +
> + /* Stop skipping changes */
> + skip_xact_finish_lsn = InvalidXLogRecPtr;
>
> Let's reverse the order of these statements to make them consistent
> with the corresponding maybe_start_* function.

But we cannot simply rever the order since skip_xact_finish_lsn is
used in the log message. Do we want to use a variable for it?

>
> 5.
> +
> + if (myskiplsn != finish_lsn)
> + ereport(WARNING,
> + errmsg("skip-LSN of logical replication subscription \"%s\"
> cleared", MySubscription->name),
>
> Shouldn't this be a LOG instead of a WARNING as this will be displayed
> only in server logs and by background apply worker?

WARNINGs are used also by other auxiliary processes such as archiver,
autovacuum workers, and launcher. So I think we can use it here.

>
> 6.
> @@ -1583,7 +1649,8 @@ apply_handle_insert(StringInfo s)
> TupleTableSlot *remoteslot;
> MemoryContext oldctx;
>
> - if (handle_streamed_transaction(LOGICAL_REP_MSG_INSERT, s))
> + if (is_skipping_changes() ||
>
> Is there a reason to keep the skip_changes check here and in other DML
> operations instead of at one central place in apply_dispatch?

I'd leave it as is as I mentioned in another email. But I've added
some comments as you suggested.

>
> 7.
> + /*
> + * Start a new transaction to clear the subskipxid, if not started
> + * yet. The transaction is committed below.
> + */
> + if (!IsTransactionState())
>
> I think the second part of the comment: "The transaction is committed
> below." is not required.

Removed.

>
> 8.
> + XLogRecPtr subskiplsn; /* All changes which finished at this LSN are
> + * skipped */
> +
> #ifdef CATALOG_VARLEN /* variable-length fields start here */
> /* Connection string to the publisher */
> text subconninfo BKI_FORCE_NOT_NULL;
> @@ -109,6 +112,8 @@ typedef struct Subscription
> bool disableonerr; /* Indicates if the subscription should be
> * automatically disabled if a worker error
> * occurs */
> + XLogRecPtr skiplsn; /* All changes which finished at this LSN are
> + * skipped */
>
> No need for 'which' in the above comments.

Removed.

>
> 9.
> Can we merge 029_disable_on_error in 030_skip_xact and name it as
> 029_on_error (or 029_on_error_skip_disable or some variant of it)?
> Both seem to be related features. I am slightly worried at the pace at
> which the number of test files are growing in subscription test.

Yes, we can merge them.

I'll submit an updated version patch after incorporating all comments I got.

Regards,

--
Masahiko Sawada
EDB: https://www.enterprisedb.com/