29.3. Logical Replication Failover
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## 29.3. Logical Replication Failover #

To allow subscriber nodes to continue replicating data from the publisher node even when the publisher node goes down, there must be a physical standby corresponding to the publisher node. The logical slots on the primary server corresponding to the subscriptions can be synchronized to the standby server by specifying `failover = true` when creating subscriptions. See Section 47.2.3 for details. Enabling the `failover` parameter ensures a seamless transition of those subscriptions after the standby is promoted. They can continue subscribing to publications on the new primary server.

Because the slot synchronization logic copies asynchronously, it is necessary to confirm that replication slots have been synced to the standby server before the failover happens. To ensure a successful failover, the standby server must be ahead of the subscriber. This can be achieved by configuring `synchronized_standby_slots`.

To confirm that the standby server is indeed ready for failover for a given subscriber, follow these steps to verify that all the logical replication slots required by that subscriber have been synchronized to the standby server:

1. On the subscriber node, use the following SQL to identify which replication slots should be synced to the standby that we plan to promote. This query will return the relevant replication slots associated with the failover-enabled subscriptions.

       test_sub=# SELECT
                      array_agg(quote_literal(s.subslotname)) AS slots
                  FROM  pg_subscription s
                  WHERE s.subfailover AND
                        s.subslotname IS NOT NULL;
        slots
       -------
        {'sub1','sub2','sub3'}
       (1 row)

2. On the subscriber node, use the following SQL to identify which table synchronization slots should be synced to the standby that we plan to promote. This query needs to be run on each database that includes the failover-enabled subscription(s). Note that the table sync slot should be synced to the standby server only if the table copy is finished (See Section 51.55). We don't need to ensure that the table sync slots are synced in other scenarios as they will either be dropped or re-created on the new primary server in those cases.

       test_sub=# SELECT
                      array_agg(quote_literal(slot_name)) AS slots
                  FROM
                  (
                      SELECT CONCAT('pg_', srsubid, '_sync_', srrelid, '_', ctl.system_identifier) AS slot_name
                      FROM pg_control_system() ctl, pg_subscription_rel r, pg_subscription s
                      WHERE r.srsubstate = 'f' AND s.oid = r.srsubid AND s.subfailover
                  );
        slots
       -------
        {'pg_16394_sync_16385_7394666715149055164'}
       (1 row)

3. Check that the logical replication slots identified above exist on the standby server and are ready for failover.

       test_standby=# SELECT slot_name, (synced AND NOT temporary AND invalidation_reason IS NULL) AS failover_ready
                      FROM pg_replication_slots
                      WHERE slot_name IN
                          ('sub1','sub2','sub3', 'pg_16394_sync_16385_7394666715149055164');
         slot_name                                 | failover_ready
       --------------------------------------------+----------------
         sub1                                      | t
         sub2                                      | t
         sub3                                      | t
         pg_16394_sync_16385_7394666715149055164   | t
       (4 rows)

If all the slots are present on the standby server and the result (`failover_ready`) of the above SQL query is true, then existing subscriptions can continue subscribing to publications on the new primary server.

The first two steps in the above procedure are meant for a PostgreSQL subscriber. It is recommended to run these steps on each subscriber node, that will be served by the designated standby after failover, to obtain the complete list of replication slots. This list can then be verified in Step 3 to ensure failover readiness. Non-PostgreSQL subscribers, on the other hand, may use their own methods to identify the replication slots used by their respective subscriptions.

In some cases, such as during a planned failover, it is necessary to confirm that all subscribers, whether PostgreSQL or non-PostgreSQL, will be able to continue replication after failover to a given standby server. In such cases, use the following SQL, instead of performing the first two steps above, to identify which replication slots on the primary need to be synced to the standby that is intended for promotion. This query returns the relevant replication slots associated with all the failover-enabled subscriptions.

    /* primary # */ SELECT array_agg(quote_literal(r.slot_name)) AS slots
                   FROM pg_replication_slots r
                   WHERE r.failover AND NOT r.temporary;
     slots
    -------
     {'sub1','sub2','sub3', 'pg_16394_sync_16385_7394666715149055164'}
    (1 row)

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