A quorum is a minimum number of nodes that must participate in a read or write operation to ensure data consistency.
Consistency
Write Quorum (W): The number of replicas that must acknowledge a write operation before it’s considered successful.
Read Quorum (R): The number of replicas that must be queried for a read operation.
N: The total number of replicas.
For strong consistency, the rule is R + W > N. This guarantees that a read operation will always overlap with the latest write operation, ensuring you get the most recent data.
Sloppy Quorum
A write operation can be considered successful even if a replica in the primary quorum is down. The write is sent to a different, “available” node.
Hinted Handoff
The “available” node that received the write acts as a temporary holder. It is “hinted” to pass the data to the correct, original replica once that replica becomes available again. This ensures eventual consistency and high availability.
Preference List
I.e. Ring, is a list of nodes responsible for storing a particular data item. When a node wants to write data, it sends it to the first available node on the preference list. This helps distribute data and ensures multiple replicas are created.