Allow upgrades to preserve the full subscription's state.

This feature will allow us to replicate the changes on subscriber nodes
after the upgrade.

Previously, only the subscription metadata information was preserved.
Without the list of relations and their state, it's not possible to
re-enable the subscriptions without missing some records as the list of
relations can only be refreshed after enabling the subscription (and
therefore starting the apply worker).  Even if we added a way to refresh
the subscription while enabling a publication, we still wouldn't know
which relations are new on the publication side, and therefore should be
fully synced, and which shouldn't.

To preserve the subscription relations, this patch teaches pg_dump to
restore the content of pg_subscription_rel from the old cluster by using
binary_upgrade_add_sub_rel_state SQL function. This is supported only
in binary upgrade mode.

The subscription's replication origin is needed to ensure that we don't
replicate anything twice.

To preserve the replication origins, this patch teaches pg_dump to update
the replication origin along with creating a subscription by using
binary_upgrade_replorigin_advance SQL function to restore the
underlying replication origin remote LSN. This is supported only in
binary upgrade mode.

pg_upgrade will check that all the subscription relations are in 'i'
(init) or in 'r' (ready) state and will error out if that's not the case,
logging the reason for the failure. This helps to avoid the risk of any
dangling slot or origin after the upgrade.

Author: Vignesh C, Julien Rouhaud, Shlok Kyal
Reviewed-by: Peter Smith, Masahiko Sawada, Michael Paquier, Amit Kapila, Hayato Kuroda
Discussion: https://postgr.es/m/20230217075433.u5mjly4d5cr4hcfe@jrouhaud

1 files changed, 50 insertions, 0 deletions

diff --git a/doc/src/sgml/ref/pgupgrade.sgml b/doc/src/sgml/ref/pgupgrade.sgml
index 2520f6c50dd..87be1fb1c26 100644
--- a/doc/src/sgml/ref/pgupgrade.sgml
+++ b/doc/src/sgml/ref/pgupgrade.sgml
@@ -457,6 +457,56 @@ make prefix=/usr/local/pgsql.new install
    </step>
 
    <step>
+    <title>Prepare for subscriber upgrades</title>
+
+    <para>
+     Setup the <link linkend="logical-replication-config-subscriber">
+     subscriber configurations</link> in the new subscriber.
+     <application>pg_upgrade</application> attempts to migrate subscription
+     dependencies which includes the subscription's table information present in
+     <link linkend="catalog-pg-subscription-rel">pg_subscription_rel</link>
+     system catalog and also the subscription's replication origin. This allows
+     logical replication on the new subscriber to continue from where the
+     old subscriber was up to. Migration of subscription dependencies is only
+     supported when the old cluster is version 17.0 or later. Subscription
+     dependencies on clusters before version 17.0 will silently be ignored.
+    </para>
+
+    <para>
+     There are some prerequisites for <application>pg_upgrade</application> to
+     be able to upgrade the subscriptions. If these are not met an error
+     will be reported.
+    </para>
+
+    <itemizedlist>
+     <listitem>
+      <para>
+       All the subscription tables in the old subscriber should be in state
+       <literal>i</literal> (initialize) or <literal>r</literal> (ready). This
+       can be verified by checking <link linkend="catalog-pg-subscription-rel">pg_subscription_rel</link>.<structfield>srsubstate</structfield>.
+      </para>
+     </listitem>
+     <listitem>
+      <para>
+       The replication origin entry corresponding to each of the subscriptions
+       should exist in the old cluster. This can be found by checking
+       <link linkend="catalog-pg-subscription">pg_subscription</link> and
+       <link linkend="catalog-pg-replication-origin">pg_replication_origin</link>
+       system tables.
+      </para>
+     </listitem>
+     <listitem>
+      <para>
+       The new cluster must have
+       <link linkend="guc-max-replication-slots"><varname>max_replication_slots</varname></link>
+       configured to a value greater than or equal to the number of
+       subscriptions present in the old cluster.
+      </para>
+     </listitem>
+    </itemizedlist>
+   </step>
+
+   <step>
     <title>Stop both servers</title>
 
     <para>