Replication lag causes stale reads on replicas
warningStreaming replication lag between primary and replica nodes increases, causing replicas to serve stale data. High lag indicates replica cannot keep up with primary write volume, network issues, or resource constraints on replica.
Replication replay_lag exceeds 5 seconds sustained for 2 minutes, or replay_lsn_delay exceeds 16MB. Critical when lag exceeds 30 seconds or 100MB.
Check replica server resources (CPU, I/O, memory). Identify long-running queries on replica blocking WAL replay using pg_stat_activity. Verify network bandwidth between primary and replica. Monitor wal_receiver_connected metric. Consider scaling up replica hardware, optimizing replica query workload, or adjusting max_standby_streaming_delay if queries on replica are essential.