Cassandra is a NoSQL database based on Dynamo and developed by Facebook. It uses a key-value data model where each row has a unique row key and columns are organized into column families. To efficiently query real-time metrics data by dimensions like time, location, category etc., bitmaps can be used to track unique user IDs and counts across different groupings in a compressed way within the columnar data structure. The document also mentions that the company Redudata is hiring.