Abstract The success of NoSQL DBMSs has pushed the adoption of polyglot storage systems that take advantage of the best characteristics of different technologies and data models. While operational applications take great benefit from this choice, analytical applications suffer the absence of schema consistency, not only between different DBMSs but within a single NoSQL system as well. In this context, the discipline of data science is steering analysts away from traditional data warehousing and toward a more flexible and lightweight approach to data analysis. The idea is to perform OLAP analyses in a pay-as-you-go manner across heterogeneous schemas and data models, where the integration is progressively carried out by the user as the available data is explored. In this paper, we propose an approach to support data analysis within a high-variety multistore, with heterogeneous schemas and overlapping records. Our approach supports relational, document, wide-column, and key-value data models by automatically handling both data model and schema heterogeneity through a dataspace layer on top of the underlying DBMSs. The expressiveness we enable corresponds to GPSJ queries, which are the most common class of queries in OLAP applications. We rely on nested relational algebra to define a cross-database execution plan. The system has been prototyped on Apache Spark.

Renewable Energy Sources such as wind and solar do not emit CO2but their production vary considerably depending on time and weather. Thus, it is important to use the flexibility in device loads to shift energy consumption to follow the production. For example, an Electrical Vehicle (EV) can be charged very flexibly between arriving home at 5PM and leaving again at 7AM. Utilizing all available energy flexibility requires applying machine learning and AI on massive amounts of Big Data from many different actors and devices, ranging from private consumers, over companies, to energy network operators, and using this to create digital solutions to enable and exploit flexibility. The project Flexible Energy Denmark (FED) is building the foundation for this for the entire Danish society. Specifically, FED collects data from a number of Living Labs (LLs) in representative real-life physical environments. The data is stored in the Danish National Energy Data Lake, called FED Data Lake (FEDDL) to enable efficient and advanced analysis. FEDDL is built using only open source tools which can run both on-premise and in cloud settings. In this paper, we describe the requirements for FEDDL based on a representative LL case study, present its technical architecture, and provide a comparison of relevant tools along with the arguments for which ones we selected.