hardSystem Design

What is a read/write quorum?

Q: What is a read/write quorum?

## Read/write quorums In a replicated store with **N** copies of each key, a **quorum** is the number of replicas that must respond for an operation to succeed: **W** for a write, **R** for a read. The magic rule is: > If **W + R > N**, the read set and write set are guaranteed to **overlap** on at least one replica — so a read always sees the latest acknowledged write. N = 5 replicas of a key n1 n2 n3 <text class='d-sub'

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Explain quorum-based consistency.

quorumconsistency

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Quorum write + read

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N, W, R = 5, 3, 3   # W + R > N guarantees overlap

replicas = [{"value": None, "version": 0} for _ in range(N)]

def write(value, version):
    acks = 0
    for r in replicas:               # send to all, succeed once W ack
        r["value"], r["version"] = value, version
        acks += 1
        if acks == W:
            break
    return acks >= W

def read():
    sample = replicas[:R]            # read from R replicas
    latest = max(sample, key=lambda r: r["version"])  # newest wins
    return latest["value"], latest["version"]

write("v2", 2)
print("read ->", read())   # sees v2 because the read/write sets overlap

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