failover logical replication slots - Mailing list pgsql-hackers

I'm working with logical replication in a PostgreSQL 17 setup, and I'm exploring the new option to make replication slots failover safe in a highly available environment 

using physical standby nodes managed by Patroni.

After a switchover, I encounter an error message in the PostgreSQL logs and observe unexpected behavior.

Here are the different steps I followed:

1) Setting up a new subscription 

Logical replication is established between two databases on the same PostgreSQL instance.

A logical replication slot is created on the source database:

SELECT pg_create_logical_replication_slot('sub_test', 'pgoutput', false, false, true);

A subscription is then configured on the target database:

CREATE SUBSCRIPTION sub_test CONNECTION 'dbname=test host=localhost port=5432 user=user_test' 

PUBLICATION pub_test WITH (create_slot=false, copy_data=false, failover=true);

The logical replication slot is active and in failover mode.

select * from pg_replication_slots where slot_type = 'logical';

2) Starting the physical standby

A logical replication slot is successfully created on the standby

3) Cluster switchover

The switchover is initiated using the Patroni command:

patronictl switchover

The operation completes successfully, and roles are reversed in the cluster.

4) Issue encountered

After the switchover, an error appears in the PostgreSQL logs:

2025-06-10 16:40:58.996 CEST [739829]: [1-1] user=,db=,client=,application= LOG: slot sync worker started

2025-06-10 16:40:59.011 CEST [739829]: [2-1] user=,db=,client=,application= ERROR: exiting from slot synchronization because same name slot "sub_test" already exists on the standby

the slot on the new standby in not in sync mode.

select * from pg_replication_slots where slot_type = 'logical';

+-[ RECORD 1 ]--------+-------------------------------+

| slot_name           | sub_test                |

| plugin              | pgoutput                      |

| slot_type           | logical                       |

| datoid              | 58458                         |

| database            | test                      |

| temporary           | f                             |

| active              | f                             |

| active_pid          |                               |

| xmin                |                               |

| catalog_xmin        | 1976743                       |

| restart_lsn         | 8/5F000080                    |

| confirmed_flush_lsn | 8/5F000130                    |

| wal_status          | reserved                      |

| safe_wal_size       |                               |

| two_phase           | f                             |

| inactive_since      | 2025-06-10 16:33:49.573016+02 |

| conflicting         | f                             |

| invalidation_reason |                               |

| failover            | t                             |

| synced              | f                             |

+---------------------+-------------------------------+

In the source code (slotsync.c), the check for the synced flag triggers an error:

/* Search for the named slot */

if ((slot = SearchNamedReplicationSlot(remote_slot->name, true))) {

    bool synced;

    SpinLockAcquire(&slot->mutex);

    synced = slot->data.synced;

    SpinLockRelease(&slot->mutex);

    /* A user-created slot with the same name exists → raise ERROR */

    if (!synced) 

        ereport(ERROR,

                errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),

                errmsg("exiting from slot synchronization because same"

                       " name slot \"%s\" already exists on the standby",

                       remote_slot->name));

}

5) Dropping the slot

If the slot on the standby is deleted, it is then recreated with synced = true, and at that point, it successfully resynchronizes with the primary slot. Everything works correctly.

Question:

Why does the synced flag fail to change to true, even though sync_replication_slots is enabled (on)?

Thanks for helping

Fabrice