#Repairing a Broken Streaming Replica

#Problem Description

A standby in a Patroni-managed PostgreSQL cluster is not replicating: it shows a large lag, is stuck, or is otherwise out of sync with the leader. The leader has no active streaming standby for it.

#Diagnosis

#1. Inspect the cluster topology

kubectl exec -n $NAMESPACE $CLUSTER_NAME-0 -c postgres -- patronictl list

A member with a large Lag in MB, a Pending restart, or a non-running state is the broken replica.

#2. Check replication state on the leader

-- On the leader: a healthy standby appears here in state 'streaming'
SELECT application_name, state, sent_lsn, replay_lsn, sync_state
FROM pg_stat_replication;

-- An inactive slot / stale restart_lsn indicates a stuck standby
SELECT slot_name, active, restart_lsn FROM pg_replication_slots;

If pg_stat_replication returns no row for the standby, it is not streaming.

#Resolution

Reinitialize the broken member from the leader. This re-clones the standby's data directory from the current leader.

kubectl exec -n $NAMESPACE $CLUSTER_NAME-0 -c postgres -- \
  patronictl reinit $CLUSTER_NAME $CLUSTER_NAME-1 --force

Replace $CLUSTER_NAME-1 with the name of the broken member. Without --force, patronictl prompts for confirmation.

After the reinit completes, confirm the member is healthy:

kubectl exec -n $NAMESPACE $CLUSTER_NAME-0 -c postgres -- patronictl list

The repaired member should show role Replica, state running/streaming, and Lag in MB of 0. On the leader, pg_stat_replication should now list the member in state streaming.