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Debugging PostgreSQL Logical Replication

Check for errors related to replication in the logs of the primary database

  • Go to the Logs Explorer in the Supabase Dashboard
  • Select Templates / Errors
  • Look for errors related to your replication

Errors related to not having enough replication slots

A common issue that comes up is a lack of available replication slots or available worker processes. By default, Supabase projects are set up with a max of 4 worker processes (max_worker_processes) and 5 replication slots (max_replication_slots).

View current settings for worker processes or replication slots

show max_worker_processes;
show max_replication_slots;

Change settings for worker processes or replication slots

alter system set max_worker_processes to '8';
alter system set max_replication_slots to '8';

These new settings will not take effect until you restart your database. You can do this with the Restart Server button in the Supabase Dashboard.

Refresh the subscription

On the replica:

ALTER SUBSCRIPTION my_subscription REFRESH PUBLICATION;

Check the replication slot status(es) on the primary server

select * from pg_replication_slots;

Drop a subscription (safely)

To drop a subscription, disable it, drop the slot, then drop the subscription.

ALTER SUBSCRIPTION my_subscription REFRESH PUBLICATION;
ALTER SUBSCRIPTION my_subscription DISABLE;
ALTER SUBSCRIPTION my_subscription SET (slot_name=NONE);
DROP SUBSCRIPTION my_subscription;

Dropping a slot on the primary server

When re-recreating the subscription on the replica, you may receive an error saying that you have a duplicate slot. In that case, drop the slot on the primary server:

select * from pg_replication_slots;
select pg_drop_replication_slot('my_subscription');