Backend & systems·Consistency and replication
the replica is forty seconds behind the primary and the report you generated from it is missing this morning's orders.
Replication lag
How far behind a follower is from the leader, measured in time or log position. It grows under write bursts, long transactions, and single-threaded replication apply. It is the number to alert on before you send any read traffic to replicas, and the number that decides whether a failover loses data.