So far in this chapter we have only considered replication architectures using a single leader. Although that is a common approach, there are interesting alternatives.
Leader-based replication has one major downside: there is only one leader, and all writes must go through it. If you can’t connect to the leader for any reason, for example due to a network interruption between you and the leader, you can’t write to the database.
A natural extension of the leader-based replication model is to allow more than one node to accept writes. Replication still happens in the same way: each node that processes a write must forward that data change to all the other nodes. We call this a multi-leader configuration (also known as master-master or active/active replication). In this setup, each leader simultaneously acts as a follower to the other leaders.
-  If the database is partitioned (see Chapter 6), each partition has one leader. Different partitions may havetheir leaders on different nodes, but each partition must nevertheless have one leader node.