Re: Skipping logical replication transactions on subscriber side

On Fri, Dec 10, 2021 at 11:14 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Thu, Dec 9, 2021 at 6:16 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Thu, Dec 9, 2021 at 2:24 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> > > On Thu, Dec 9, 2021 at 11:47 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > I am thinking that we can start a transaction, update the catalog,
> > > > commit that transaction. Then start a new one to update
> > > > origin_lsn/timestamp, finishprepared, and commit it. Now, if it
> > > > crashes after the first transaction, only commit prepared will be
> > > > resent again and this time we don't need to update the catalog as that
> > > > entry would be already cleared.
> > >
> > > Sounds good. In the crash case, it should be fine since we will just
> > > commit an empty transaction. The same is true for the case where
> > > skip_xid has been changed after skipping and preparing the transaction
> > > and before handling commit_prepared.
> > >
> > > Regarding the case where the user specifies XID of the transaction
> > > after it is prepared on the subscriber (i.g., the transaction is not
> > > empty), we won’t skip committing the prepared transaction. But I think
> > > that we don't need to support skipping already-prepared transaction
> > > since such transaction doesn't conflict with anything regardless of
> > > having changed or not.
> > >
> >
> > Yeah, this makes sense to me.
> >
>
> I've attached an updated patch. The new syntax is like "ALTER
> SUBSCRIPTION testsub SKIP (xid = '123')".
>
> I’ve been thinking we can do something safeguard for the case where
> the user specified the wrong xid. For example, can we somewhat use the
> stats in pg_stat_subscription_workers? An idea is that logical
> replication worker fetches the xid from the stats when reading the
> subscription and skips the transaction if the xid matches to
> subskipxid. That is, the worker checks the error reported by the
> worker previously working on the same subscription. The error could
> not be a conflict error (e.g., connection error etc.) or might have
> been cleared by the reset function, But given the worker is in an
> error loop, the worker can eventually get xid in question. We can
> prevent an unrelated transaction from being skipped unexpectedly. It
> seems not a stable solution though. Or it might be enough to warn
> users when they specified an XID that doesn’t match to last_error_xid.
>

I think the idea is good but because it is not predictable as pointed
by you so we might want to just issue a LOG/WARNING. If not already
mentioned, then please do mention in docs the possibility of skipping
non-errored transactions.

Few comments/questions:
=====================
1.
+          Specifies the ID of the transaction whose application is to
be skipped
+          by the logical replication worker. Setting -1 means to reset the
+          transaction ID.

Can we change it to something like: "Specifies the ID of the
transaction whose changes are to be skipped by the logical replication
worker. ...."

2.
@@ -104,6 +104,16 @@ GetSubscription(Oid subid, bool missing_ok)
  Assert(!isnull);
  sub->publications = textarray_to_stringlist(DatumGetArrayTypeP(datum));

+ /* Get skip XID */
+ datum = SysCacheGetAttr(SUBSCRIPTIONOID,
+ tup,
+ Anum_pg_subscription_subskipxid,
+ &isnull);
+ if (!isnull)
+ sub->skipxid = DatumGetTransactionId(datum);
+ else
+ sub->skipxid = InvalidTransactionId;

Can't we assign it as we do for other fixed columns like subdbid,
subowner, etc.?

3.
+ * Also, we don't skip receiving the changes in streaming cases,
since we decide
+ * whether or not to skip applying the changes when starting to apply changes.

But why so? Can't we even skip streaming (and writing to file all such
messages)? If we can do this then we can avoid even collecting all
messages in a file.

4.
+ * Also, one might think that we can skip preparing the skipped transaction.
+ * But if we do that, PREPARE WAL record won’t be sent to its physical
+ * standbys, resulting in that users won’t be able to find the prepared
+ * transaction entry after a fail-over.
+ *
..
+ */
+ if (skipping_changes)
+ stop_skipping_changes(false);

Why do we need such a Prepare's entry either at current subscriber or
on its physical standby? I think it is to allow Commit-prepared. If
so, how about if we skip even commit prepared as well? Even on
physical standby, we would be having the value of skip_xid which can
help us to skip there as well after failover.

-- 
With Regards,
Amit Kapila.