Replication lag causing stale reads on standby

Standby servers fall behind primary as measured by pg_wal_lsn_diff(sent_lsn, replay_lsn) on primary or pg_last_xact_replay_timestamp() on standby, causing applications to read outdated data

Monitor lag bytes on primary using pg_stat_replication with pg_wal_lsn_diff. On standby, check time lag with pg_last_xact_replay_timestamp. Optimize by adjusting max_wal_senders, wal_keep_segments on primary, and max_standby_streaming_delay, hot_standby_feedback on standby using ALTER SYSTEM.