Configuration
NTS per keyspace with per-DC RF. Snitch aware of DC + rack. LOCAL_QUORUM for reads + writes on hot path.
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Replication lag
Cross-DC async. Typically ms-seconds. Read at LOCAL_QUORUM shows local view; other DC catches up.
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Failover
Application aware of local DC. Failover: point at another DC. Data available (replicated). Requires coordination.