Application cutover
===================

With the PGD cluster fully synchronized, this phase details the final
transition from your Postgres Streaming Replication (PSR) environment to
the Postgres Distributed (PGD) cluster.

Stopping application writes
---------------------------

To prevent data divergence, stop all write traffic to the legacy PSR
cluster at the application level. Ensure no new transactions are being
initiated on the source node.

Performing lag verification
---------------------------

Ensure that the PGD cluster has ingested all outstanding changes from
the source node before shifting traffic. Run the following command on
the seed node to ensure that replication lag on the migration
subscription is zero:

.. code:: sql

   DO $$
   DECLARE
      wait_lsn pg_lsn := pg_current_wal_lsn();
      current_flush_lsn pg_lsn;
   BEGIN
     LOOP
       current_flush_lsn := (SELECT MAX(flush_lsn) FROM pg_stat_replication WHERE application_name IN (migration_seed_sub, migration_add_sub));
       IF wait_lsn <= current_flush_lsn THEN
         EXIT;
       END IF;
       RAISE NOTICE Waiting for replication to catch up, current lag: %,  pg_wal_lsn_diff(wait_lsn, current_flush_lsn);
       PERFORM pg_sleep(1);
     END LOOP;
   END;$$;

Disabling subscriptions
-----------------------

Once the lag is zero, disable the logical subscriptions on all PGD nodes
that connect back to the original PSR source. On all PGD nodes, run:

.. code:: sql

   SET bdr.ddl_replication = off;
   ALTER SUBSCRIPTION migration_seed_sub DISABLE;

Switching the application traffic
---------------------------------

With the PGD cluster confirmed as fully up-to-date and isolated from the
old source, you can now redirect traffic and resume operations.

Update the connection strings to point your application to the
`Connection Manager <https://www.enterprisedb.com/docs/pgd/latest/connection-manager>`_  . The specific method (e.g., updating DNS, Kubernetes
ConfigMaps, or ``.env`` files) depends on your specific application
architecture.

Resume application traffic. The Connection Manager will now route
requests to the correct PGD nodes.

Performing validation
---------------------

After the application is live on the PGD cluster, perform final
verification checks to ensure the PGD mesh is healthy, receiving data
from the application, and propagating across all nodes in the PGD
cluster. This involves re-verifying the replication streams to confirm
that writes are propagating across all nodes.

1. Confirm that all nodes are in an ‘active’ state and that internal
   subscriptions are streaming correctly. On any PGD node, run:

.. code:: sql

       SELECT * FROM bdr.node_summary;
       SELECT * FROM bdr.subscription_summary;

1. Check the replication rates to ensure the cluster is processing the
   new load without building up internal lag. On any PGD node, run:

.. code:: sql

       SELECT * FROM bdr.node_replication_rates;

Next step: :ref:`Performing cleanup <Performing cleanup>`  .