HDFS Federation improves the existing HDFS architecture through a clear separation of namespace and storage, enabling generic block storage layer. It enables support for multiple namespaces in the cluster to improve scalability and isolation. Federation also opens up the architecture, expanding the applicability of HDFS cluster to new implementations and use cases.
Namenodes are federated, that is, all these NameNodes work independently and don’t require any coordination with each other.
It’s one way to reduce the number of potential single points of failure in a Hadoop environment.