future internet Review Consistency Models of NoSQL Databases Miguel Diogo 1, Bruno Cabral 1,2 and Jorge Bernardino 2,3,* 1 Department of Informatics Engineering, University of Coimbra, Coimbra 3030-290, Portugal;
[email protected] (M.D.);
[email protected] (B.C.) 2 Centre of Informatics and Systems of University of Coimbra (CISUC), Coimbra 3030-290, Portugal 3 ISEC—Coimbra Institute of Engineering, Polytechnic of Coimbra, Coimbra 3030-199, Portugal * Correspondence:
[email protected] Received: 30 December 2018; Accepted: 11 February 2019; Published: 14 February 2019 Abstract: Internet has become so widespread that most popular websites are accessed by hundreds of millions of people on a daily basis. Monolithic architectures, which were frequently used in the past, were mostly composed of traditional relational database management systems, but quickly have become incapable of sustaining high data traffic very common these days. Meanwhile, NoSQL databases have emerged to provide some missing properties in relational databases like the schema-less design, horizontal scaling, and eventual consistency. This paper analyzes and compares the consistency model implementation on five popular NoSQL databases: Redis, Cassandra, MongoDB, Neo4j, and OrientDB. All of which offer at least eventual consistency, and some have the option of supporting strong consistency. However, imposing strong consistency will result in less availability when subject to network partition events. Keywords: consistency models; NoSQL databases; redis; cassandra; MongoDB; Neo4j; OrientDB 1. Introduction Consistency can be simply defined by how the copies from the same data may vary within the same replicated database system [1]. When the readings on a given data object are inconsistent with the last update on this data object, this is a consistency anomaly [2].