Replication Lag & Replica Drift Incident Runbook

Identify whether the bottleneck is network transfer latency or replica disk I/O replay starvation.

-- PostgreSQL:
SELECT client_addr, state, sync_state, pg_wal_lsn_diff(pg_current_wal_lsn(), write_lsn) AS write_lag_bytes, pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn) AS replay_lag_bytes FROM pg_stat_replication;

-- MySQL:
SHOW REPLICA STATUS\G

Long analytical queries on read replicas can cause recovery conflicts, stalling WAL replay.

SELECT pid, now() - query_start AS duration, query FROM pg_stat_activity WHERE state != 'idle' AND (now() - query_start) > interval '2 minutes';

An inactive replication slot retains WAL files on Primary. Drop stuck slots if master disk capacity reaches >90%.

SELECT slot_name, active, pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn)) AS retained_wal FROM pg_stat_replication_slots;