Re: Skipping logical replication transactions on subscriber side

On Wed, Jan 12, 2022 at 11:10 PM vignesh C <vignesh21@gmail.com> wrote:
>
> On Wed, Jan 12, 2022 at 11:32 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Wed, Jan 12, 2022 at 12:21 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > On Wed, Jan 12, 2022 at 5:49 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > >
> > > > On Tue, Jan 11, 2022 at 7:08 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > > >
> > > > > On Tue, Jan 11, 2022 at 1:51 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > > > >
> > > > > > On second thought, the same is true for other cases, for example,
> > > > > > preparing the transaction and clearing skip_xid while handling a
> > > > > > prepare message. That is, currently we don't clear skip_xid while
> > > > > > handling a prepare message but do that while handling commit/rollback
> > > > > > prepared message, in order to avoid the worst case. If we do both
> > > > > > while handling a prepare message and the server crashes between them,
> > > > > > it ends up that skip_xid is cleared and the transaction will be
> > > > > > resent, which is identical to the worst-case above.
> > > > > >
> > > > >
> > > > > How are you thinking to update the skip xid before prepare? If we do
> > > > > it in the same transaction then the changes in the catalog will be
> > > > > part of the prepared xact but won't be committed. Now, say if we do it
> > > > > after prepare, then the situation won't be the same because after
> > > > > restart the same xact won't appear again.
> > > >
> > > > I was thinking to commit the catalog change first in a separate
> > > > transaction while not updating origin LSN and then prepare an empty
> > > > transaction while updating origin LSN.
> > > >
> > >
> > > But, won't it complicate the handling if in the future we try to
> > > enhance this API such that it skips partial changes like skipping only
> > > for particular relation(s) or particular operations as discussed
> > > previously in this thread?
> >
> > Right. I was thinking that if we accept the situation that the user
> > has to set skip_xid again in case of the server crashes, we might be
> > able to accept also the situation that the user has to clear skip_xid
> > in a case of the server crashes. But it seems the former is less
> > problematic.
> >
> > I've attached an updated patch that incorporated all comments I got so far.
>
> Thanks for the updated patch, few comments:

Thank you for the comments!

> 1) Currently skip xid is not displayed in describe subscriptions, can
> we include it too:
> \dRs+  sub1
>                                                         List of subscriptions
>  Name |  Owner  | Enabled | Publication | Binary | Streaming | Two
> phase commit | Synchronous commit |            Conninfo
>
------+---------+---------+-------------+--------+-----------+------------------+--------------------+--------------------------------
>  sub1 | vignesh | t       | {pub1}      | f      | f         | e
>          | off                | dbname=postgres host=localhost
> (1 row)
>
> 2) This import "use PostgreSQL::Test::Utils;" is not required:
> +# Tests for skipping logical replication transactions.
> +use strict;
> +use warnings;
> +use PostgreSQL::Test::Cluster;
> +use PostgreSQL::Test::Utils;
> +use Test::More tests => 6;
>
> 3) Some of the comments uses a punctuation mark and some of them does
> not use, Should we keep it consistent:
> +    # Wait for worker error
> +    $node_subscriber->poll_query_until(
> +       'postgres',
>
> +    # Set skip xid
> +    $node_subscriber->safe_psql(
> +       'postgres',
>
> +# Create publisher node.
> +my $node_publisher = PostgreSQL::Test::Cluster->new('publisher');
> +$node_publisher->init(allows_streaming => 'logical');
>
>
> +# Create subscriber node.
> +my $node_subscriber = PostgreSQL::Test::Cluster->new('subscriber');
>
> 4) Should this be changed:
> + * True if we are skipping all data modification changes (INSERT,
> UPDATE, etc.) of
> + * the specified transaction at MySubscription->skipxid.  Once we
> start skipping
> + * changes, we don't stop it until the we skip all changes of the
> transaction even
> + * if pg_subscription is updated that and MySubscription->skipxid
> gets changed or
> to:
> + * True if we are skipping all data modification changes (INSERT,
> UPDATE, etc.) of
> + * the specified transaction at MySubscription->skipxid.  Once we
> start skipping
> + * changes, we don't stop it until we skip all changes of the transaction even
> + * if pg_subscription is updated that and MySubscription->skipxid
> gets changed or
>
> In "stop it until the we skip all changes", here the is not required.
>

I agree with all the comments above. I've attached an updated patch.

Regards,

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