The in-memory, Blob, and OLTP data stores 

The in-memory data store stores data in RAM and is usually mostly used to store key-value types of data for quick lookup. Redis is one of the popular in-memory data stores. Blob stores such as Riak are specialty data stores used to store data such as documents, wave forms, images, and so on. Data can also be stored in HDFS and accessed during batch processing of data using map-reduce as part of the analytics. Finally, some data, such as events, is stored in a relational DB for faster access and processing.