Quorum-Based Replication

Quorum-Based Replication

When data is replicated across N nodes, a quorum is the minimum number of nodes that must agree for an operation to succeed. Quorums ensure consistency even when some nodes are unavailable.

The Quorum Rule

For N replica nodes:

• Write quorum (W): minimum nodes that must acknowledge a write.
• Read quorum (R): minimum nodes that must respond to a read.

For strong consistency: W + R > N

This guarantees that every read will overlap with at least one write, so reads always see the latest write.

Common Configurations (N = 3)

function hasQuorum(n, w, r) {
  return w + r > n;
}

function canWrite(n, w, availableNodes) {
  return availableNodes >= w;
}

function canRead(n, r, availableNodes) {
  return availableNodes >= r;
}

Dynamo-Style Quorums

Amazon DynamoDB uses quorums with N=3. Default settings are W=2, R=2, providing strong consistency while tolerating 1 node failure for both reads and writes.

If you need lower latency, you can use W=1, R=1 (eventual consistency) or W=3, R=1 (all writes acknowledged, fast reads).

Your Task

Implement three functions:

• hasQuorum(n, w, r) — returns true if W + R > N (strong consistency guaranteed)
• canWrite(n, w, available) — returns true if enough nodes are available to complete a write
• canRead(n, r, available) — returns true if enough nodes are available to complete a read