Basically if you want to replicate data across multiple nodes, all of them need to agree on the sequence of data updates. For example if I did [x=6, x=7], then

Basically if you want to replicate data across multiple nodes, all of them need to agree on the sequence of data updates. For example if I did [x=6, x=7], then the outcome of those ops is very different if I did it in the opposite order [x=7, x=6]. Easiest way to do this sequencing is to have a unique leader who will impose the unique order. Strict sequencing is not really serializability, but linearizability. For example, Cassandra doesn't by default have the concept of a leader and hence can't provide linearizability. Of course, there is a problem on how you decide who is the leader and what happens if two nodes think they are the leader at the same time. That's where the genius of Multi-Paxos/Raft comes in. You can read more about sequencing here https://medium.com/swlh/replication-and-linearizability-in-distributed-systems-cd9036ea7b40 https://medium.com/swlh/replication-and-linearizability-in-d...