Why it matters

Global apps need low latency worldwide. Cassandra's multi-DC model handles this without sacrificing consistency locally. Getting the setup right shapes user experience.

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The architecture

Datacenter definition: logical grouping of nodes. Configured per keyspace via NetworkTopologyStrategy with per-DC replication factor.

LOCAL_* consistency: strong within DC. Reads and writes only wait for local replicas.

Multi-DC architectureMulti-DC keyspaceNTS with per-DC RFLOCAL_QUORUMstrong localAsync cross-DCeventual globalCross-DC repair required regularly to catch async replication drift
Multi-DC replication model.
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How it works end to end

Writes: coordinator writes to local replicas synchronously, cross-DC replicas asynchronously.

Reads: LOCAL_QUORUM reads from local DC only. If local DC fails, application must retry with different CL or route to another DC.

Cross-DC repair essential: async replication can drop writes; regular repair catches divergence.