Replica Sets in MongoDB are very useful. They provide multiple copies of data, automated failover and read scalability. A Replica Set can consist of up to 12 nodes, with only one primary node (or master node) able to accept writes. In case of primary node failure, a new primary is auto-elected.
It is advisable to have an odd number of nodes in a Replica Set, so as to avoid vote locking when a new primary node is being elected. Replica Sets require a majority of the remaining nodes present to elect a primary. If you have e.g. 2 nodes in a Replica Set, then one option is to add an arbiter. An arbiter is a mongod instance that is part of a Replica Set, but does not hold any data. Because of the minimal resource requirements, it can be colocated with an application server or the ClusterControl server. The arbiter should not be colocated with any of the members of the Replica Set.
