R + W > N and It Still Returned Yesterday’s Value
Read what each party saw, commit to a cause, and only then find out which of them was right. The root cause, why the obvious reading was wrong, and the fixes are all held back until you have answered.
The symptom
What was reported, before anyone knew what was happening.
What each party saw, and what each concluded
The evidence, in the form it actually arrived in — several parties, several partial views, several confident conclusions.
| Who | What they could actually see | What they concluded | Verdict |
|---|---|---|---|
| Writer | Write acknowledged by 2 of 3 nodes in 4ms. | W=2 satisfied; the value is durable and any R=2 read will see it. | ✕ wrong |
| Reader | Two responses agreeing on the old value, from 2 of 3 nodes. | R=2 satisfied with agreement; this is the current value. | ✕ wrong |
| Coordinator node | Node C unreachable during the write. Sent the second replica to node D — a healthy neighbour — as a hinted handoff. | Availability preserved; W=2 met. | ✕ wrong |
| Reviewer of the design doc | R + W = 4 > N = 3. | Read-your-writes is guaranteed by the arithmetic. | ✕ wrong |
4 of 4 parties reasoned correctly from what they could see and still reached the wrong conclusion. Nobody in this table is careless. Each one acted on complete-looking local information, and the information was local. That gap — between what a node can observe and what is true — is the whole domain, and one of these readings will usually be yours.
Commit before you read on
The arithmetic is correct: 2 + 2 > 3. Commit before reading on: state the assumption the overlap argument depends on, and identify which component in this incident broke it.
Write it down even if you are unsure. An unwritten guess quietly becomes “that is what I thought” the moment you read the answer.