However, it is not only the amount of data collected that has grown dramatically but also the variety of data. Just consider the following list of online data sources: emails, blogs, photos, tweets, likes, shares, web searches, video uploads, online purchases, podcasts. And if we consider the metadata (data describing the structure and properties of the raw data) of these events, we can begin to understand the meaning of the term big data. Big data are often defined in terms of the three Vs: the extreme volume of data, the
