Quorum write and Quorum read allow to detect staled value in a leaderless replication system.

For example, we have 3 replicators A, B, C (N=3). C is down during a user update. The update is accepted in both A and B (Write = 2).

When the user reads the value, C comes back. It's possible to read a staled value in C. In order to detect the staled value, the user will also read from B (Read = 2).

When the user received updates from B and C, a version number can be used to determine which value is newer(B have a newer version number).

In this scenario, where Write = 2, Read = 2, N = 3, R + W > 3, we are certain that any staled value can be detected.

For R + W = 3, it's possible to have write in A and B, read from C. We can't detect the staled value.

Quorum write and Quorum read allow to detect stale values in a leaderless replication system.

For example, we have 3 replicators A, B, C (N=3). C is down during a user update. The update is accepted on both A and B (Write = 2).

When the user reads the value, C comes back. It's possible to read a stale value in C. In order to detect the stale value, the user will also read from B (Read = 2).

When the user received updates from B and C, a version number can be used to determine which value is newer(B has a newer version number).

In this scenario, where Write = 2, Read = 2, N = 3, R + W > 3, we are certain that any stale value can be detected.

For R + W = 3, it's possible to have written in A and B, but only read from C. In this case, we can't detect the stale value.