Quorum (N/R/W) Explorer
Dial the replica count and the read/write quorum sizes and see at a glance whether reads are guaranteed strongly consistent (R + W > N), how many node failures you can survive, and where you land on the latency–availability spectrum. The tunable-consistency intuition, made tactile.
write set (W=3) read set (R=3) overlap
Balanced strong consistency (majority quorums).
N, R, W and the quorum rule
In a replicated store, N is how many copies of each item exist, W is how many replicas must acknowledge a write before it's done, and R is how many must respond to a read. Tuning these three dials the trade-off between consistency, availability and latency — per operation, not once for the whole system.
The one inequality that matters
If R + W > N, the replicas a read touches are guaranteed to overlap the replicas the last write touched — so a read always sees the newest value. That's strong consistency. If R + W ≤ N there may be no overlap and a read can return stale data — eventual consistency, in exchange for lower latency and higher availability.
Common settings
- W = N, R = 1 — fast reads, fragile writes (any replica down blocks writes).
- W = 1, R = N — fast writes, slow reads; durable only once replication catches up.
- R = W = ⌈(N+1)/2⌉ — balanced quorum, the usual default (N=3 → R=W=2).
More required acks means more durability but lower availability, because more nodes must be up. Set N, R and W above and watch which side of the consistency line you land on.
How it works
- N = replicas, W = acks needed to write, R = replies needed to read.
- R + W > N ⇒ read and write sets overlap ⇒ strongly consistent reads.
- Writes tolerate N − W failures; reads tolerate N − R failures.
- Bigger quorums = stronger guarantees but higher latency / lower availability.
Frequently asked questions
What do N, R and W mean in a quorum system?
N is the number of replicas a piece of data is stored on. W is the number of replicas that must acknowledge a write before it is considered successful. R is the number of replicas that must respond to a read. Tuning R and W lets you trade consistency against latency and availability.
Why does R + W > N guarantee strong consistency?
If the read set and the write set must each be a majority such that they overlap, then any read is guaranteed to touch at least one replica that saw the latest write. That overlap is exactly what R + W > N ensures, so reads never miss the most recent value.
What is the trade-off when tuning R and W?
Larger quorums mean stronger guarantees but higher latency and lower availability (more replicas must be reachable). Common presets: W=N, R=1 for fast reads; W=1, R=N for fast writes; and W=R=majority for balanced strong consistency.
How many node failures can a quorum survive?
Writes survive up to N − W replica failures and reads survive up to N − R failures while still completing. This tool shows both numbers as you adjust the sliders so you can see your fault tolerance directly.