MongoDB ReplicaSet: Copy and Backup

A MongoDB Replicaset therefore consists of a primary (or master) node and several secondary (or slave) nodes created from exact copies of the master. There should be at least two secondary nodes, but a minimum of three copies is recommended. Data transfer is always done exclusively from the primary node to the secondary nodes. If the master node should fail due to an error, system failure or maintenance work, a slave node will immediately take the place of the master to ensure data availability for the duration of the failure. Once the faulty node is repaired, the slave node becomes a secondary node again.