What is replication lag and why does it matter?
Replication lag is the delay between a write landing on the leader and that same write becoming visible on a replica. It matters because a client reading from a lagging replica can see stale data — placing an order and then not seeing it on a follow-up read, because that read hit a replica that hasn't caught up yet. This is the direct, user-visible cost of asynchronous replication.
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Database Replication Explained: Single-Leader, Multi-Leader, and LeaderlessWhat replication lag actually breaks, and how single-leader, multi-leader, and leaderless models trade off consistency for availability.
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