Read and write quorums

01Count the participants, then ask what they promise

Five machines A, B, C, D, and E keep copies of one value. A client write waits until three machines have safely saved the change. A read asks three machines for their versions. Those required participant groups are quorums.

Define the counts before using the equations:

• N is the number of replicas in the fixed group: five here.
• W is the number of required write acknowledgments: three here.
• R is the number of responses the read collects: three here.
• Say what each acknowledgment proves. Receipt in memory is not the same promise as a saved durable log entry.

Work a particular overlap:

1. The write saves version 8 at A, B, and C.
2. A read collects responses from A, D, and E.
3. Both groups contain A. That shared participant can tell the read about version 8.
4. The two groups contain six places in total, but only five different machines exist. At least one machine must occur in both groups.

The minimum overlap is R + W − N when that number is positive. Here 3 + 3 − 5 = 1. This proves a fact about shared participants. It does not yet define which returned value the read selects or how concurrent writes are resolved.

02Use the equations without claiming too much

Use the count rules for exactly what they prove:

• R + W > N makes every allowed read group intersect every successful-write group within the same fixed replica set.
• W + W > N makes any two write-acknowledgment groups intersect. It does not make their writes automatically ordered or give one writer exclusive permission.
• If R + W <= N, disjoint groups are possible. With five replicas and two participants per operation, write A/B and read D/E share nobody.

Now inspect values, not just counts:

1. A successful write saved version 8 at A/B/C.
2. A read of A/D/E receives versions 8, 7, and 7.
3. Returning the fastest response, perhaps 7, ignores the relevant newer value available within the collected group.

Even selecting the highest version requires more protocol rules. Consider an incomplete write:

1. An attempted version 9 reaches only A, then the client times out without success.
2. A read of A/D/E sees 9 at A and returns 9.
3. A later read of B/C/E sees only 8 and returns 8.
4. With no intervening update, the reads have moved backward. Both collected three responses; overlap alone did not prevent this.

A complete protocol must handle partially stored values and simultaneous writers. Some protocols make a reader copy its selected value to enough replicas before returning, called read write-back. In this five-replica example, a reader could save its selected version 9 to three replicas before completing. Any later three-replica read then shares at least one of those saved copies. Version selection, simultaneous writers and incomplete operations still need the full protocol’s rules. Others use a full agreement protocol to order accepted changes. These are algorithms with additional rules, not consequences of the equation alone.

A timestamp-based last-write-wins rule also needs care. Clock disagreement can make an older real-world edit appear newer, ties need a rule, and a discarded edit does not reappear merely because replicas later agree.

03Keep membership, failure, and repair explicit

The overlap proof assumes reads and writes choose participants from the same fixed replica set. Check that assumption against the actual operation.

A sloppy quorum can use temporary substitutes when normal owners are unavailable:

1. The intended replicas are A through E.
2. A write actually saves on A and substitutes X and Y.
3. A later read asks B, C, and D.
4. That read shares nobody with the actual write, despite the intended count of five owners. Calculating against the intended list cannot prove this operation’s overlap.

Changing the owner list must transfer accepted values and prevent the old and new groups from accepting unrelated histories. Otherwise a read using new owners can miss a write completed under the old list even when each list’s own counts satisfy the equation. Use the store’s membership-change protocol rather than changing the list independently at each client.

Test failure counts and physical placement separately:

• With five replicas and W=3, three communicating eligible replicas can meet the count while two are unavailable, provided every other protocol rule holds.
• A three-versus-two network split can let the larger side proceed while the smaller cannot. This is useful partial service, not completion at every healthy node.
• Three acknowledgments in one zone do not protect against destruction of that zone.
• Larger required groups can add waiting and prevent progress in more failure cases. Smaller groups are safe only if the complete protocol still provides the required result.

In an interview, write actual participant names beside their returned versions. Explain what they saved, how the read chooses a result, what happens to incomplete writes, and how membership changes. Quorum intersection supplies information to a correct protocol; it is not a substitute for that protocol or for permission checks.