Apache Cassandra

Apache Cassandra is a non-relational, completely distributed Database Management System. Initially developed by Facebook for its internal use, it has been subsequently released as open source project.
The spread of this technology followed the rise of big data environments and distributed computing. It is now becoming a must for IT professionals and analysts operating in the field of data storage, parallel computing, data analysis and data mining.

Available resources: many interesting free courses can be found on the official Datastax Academy, where formal certifications can be issued too.
https://academy.datastax.com/courses

Prerequisites: Background in ICT or Computer Scientist with knowledge of Linux environments and systems administration