Quorum

Read/Write Quorum

In a system with $n$ replicas, we can ensure consistent

• writes if a write is acknowledged by $w$ replicas (write quorum)
• reads if we request reads from $r$ replicas (read quorum)
  • e.g. send 3 requests, only 2 have to come back. Choose most up to date based on timestamp

Key thing to note is that $r+w>n$, typically, $r=w=\frac{n+1}{2}$. This means that quorum is generally majority. Thus, reads can tolerate $n-r$ unavailable replicas and writes can tolerate $n-w$ unavailable replicas.

Then the read will see the previously written value (as the read and write quorum share $\ge 1$ replica). Client can then ‘repair’ the servers by sending its most up to state to servers that are out of date (with original logical timestamp! this is an idempotent operation, should be fine) — this is called read repair.