Quorum Variations

So far we have examined what is called a strict quorum in contrast to a sloppy quorum which we’ll discuss next. Consider a system composed of several dozen nodes. It may not make sense to replicate each data value to every node so we may choose a subset of nodes in the cluster for replicating each value instead of replicating the values on every node in the cluster. Going back to our inequality R + W > N, the N in this scenario is less than the number of nodes in the cluster.

When a network partition takes place, it can happen that the chosen subset of nodes where we want to replicate a given value is not reachable partially. However, other nodes in the cluster are still reachable which aren’t designated to hold the value for the key, we want to write or update. At this juncture, if a write request is received the system can decline it since enough nodes aren’t available to record the write request or the system can accept the write request and temporarily ...