Why it matters

Wrong consistency level either loses consistency or destroys availability. Getting this right per workload is critical.

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The architecture

Consistency formula: R + W > N gives strong consistency (N = replication factor). Common: R=W=QUORUM with RF=3 gives R=W=2, R+W=4>3, strong.

Local variants: LOCAL_QUORUM only requires local-DC quorum. Cross-DC latency avoided.

Consistency spectrumONEfast, weakQUORUMR+W>N strongALLslowest, strongestLOCAL_QUORUM is standard for multi-DC production: strong local, tolerates DC issues
Consistency level ladder.
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How it works end to end

Write consistency: coordinator waits for N replica acks. LOCAL_QUORUM writes in current DC.

Read consistency: coordinator queries N replicas, returns most recent value (using timestamps).

Read repair: on inconsistent reads, coordinator can write back correct value to lagging replicas.