UitbreidenOS Replication Monitor
When to activate
When replication lag exceeds acceptable thresholds, during failover testing, when setting up new replicas, or during routine weekly health checks.
When NOT to use
Skip if replication lag is under 1 second and all replicas report healthy. Skip for single-node development databases.
Instructions
- Check replication status:
pg_stat_replicationfor PostgreSQL,SHOW REPLICA STATUSfor MySQL. - Measure lag: write lag (sent vs flush), replay lag (flush vs replay), byte lag.
- Identify lag sources: large transactions, DDL operations, network saturation, disk I/O on replica.
- Test failover: simulate primary failure, measure time-to-promote, verify data consistency.
- Check for replication conflicts: long-running reads blocking WAL apply, hot standby feedback issues.
Example
REPLICATION STATUS: prod-primary → 2 replicas
Replica: prod-replica-1 (us-east-1)
State: streaming ✅
Write lag: 120ms | Replay lag: 180ms
Byte lag: 2.4MB
Conflicts: 0
Replica: prod-replica-2 (us-west-2)
State: streaming ✅
Write lag: 450ms | Replay lag: 620ms ⚠️
Byte lag: 18MB
Conflicts: 3 (long-running analytics queries)
Action: Cancel 3 conflicting queries on replica-2 or increase
max_standby_streaming_delay to 30s.