Replication lag.
also: replica lagThe delay between a write on the leader and that write appearing on a follower: milliseconds normally, minutes under load.
the two-minute refresher
- Normal: milliseconds to low seconds. Under load: minutes. A disconnected follower: unbounded.
- Anomalies: you don't see your own write; a refresh goes back in time; effects appear before causes.
- Monitor it (seconds behind, bytes behind) and alert before it hurts users.
- Fixes: read your own writes from the leader, pin users to a replica, or wait for replicas to catch up.