A Data Lake is a central repository from various data sources where the emphasis is put on storing data rapidly and for a low cost, at the expense of a well defined structure. A wide variety of data can be stored in data lakes such as structured data (like columns and rows in classical RDBMS), semi-structured data (CSV, XML and JSON files), and unstructured data (images, videos, emails, web pages…). In a data lake, the data is stored in a raw format, untouched, making it flexible for later usage. In practice, data lakes are often based on the Hadoop framework.