Replication lag causes replica data staleness and read inconsistencies

Replication lag between primary and replica nodes increases, causing replicas to serve stale data. Multiple tools track this through various lag metrics.

Monitor replication lag using postgres_exporter metrics or pgwatch dashboards. Investigate network throughput, replica resource constraints (CPU/disk I/O), and WAL generation rate on primary. Consider increasing wal_sender_timeout or max_wal_senders if network is unstable.