Consistency and availability of Data in replicated NoSQL databases

Highly distributed NoSQL key-value databases have been designed to meet the needs of various applications in the field of social networks, Web search and e-commerce. High availability, fault tolerance and scalability (i.e. Quality of Service, QoS) of such systems is in conflict with the strong consistency (Quality of Data, QoD). Thus, a new functionality of services, balancing QoS and QoD, is needed. This functionality is referred as Service Level Agreements, SLA) (Terry et al., 2013). We present a system Lorq as a proposal of a solution to this problem. Lorq is a consensus quorum based algorithm for NoSQL data replication. In this paper, we discuss how different consistency levels are guaranteed by Lorq.