What are Quorum Reads and Writes?

Quorum reads and writes are operations in distributed databases where a majority of replicas (a quorum) must respond before the read or write is considered successful, ensuring consistency even with failures.

When to Use

Use quorum reads and writes when designing distributed systems that must balance consistency and availability. They are common in systems like Dynamo-style databases where multiple replicas handle the same data.

Example

In a 3-node cluster, a write may succeed once 2 replicas acknowledge it, and a read may succeed after querying 2 replicas to confirm the latest value.

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Why Is It Important

Quorums ensure that read and write sets overlap (W + R > N), reducing the chance of stale data. This improves reliability while still tolerating node failures.

Interview Tips

When asked in interviews, explain the W + R > N rule clearly. Mention trade-offs and give a quick example. This shows you understand both theory and real-world impact.

Trade-offs

Quorums strengthen consistency and tolerate failures, but they increase latency and reduce pure availability compared to eventual consistency.

Pitfalls

A common misconception is assuming quorum guarantees the latest data.

Network delays or poorly chosen R/W values can still lead to stale reads or broken consistency.