Measuring Elasticity for Cloud Databases

The rise of the Internet and the multiplication of data sources have multiplied the number of "Bigdata" storage problems. These data sets are not only very big but also tend to grow very fast, sometimes in a short period. Distributed databases that work well for such data sets need to be not only scalable but also elastic to ensure a fast response to growth in demand of computing power or storage. The goal of this article is to present measurement results that characterize the elasticity of three databases. We have chosen Cassandra, HBase, and mongoDB as three representative popular horizontally scalable NoSQL databases that are in production use. We have made measurements under realistic loads up to 48 nodes, using the Wikipedia database to create our dataset and using the Rackspace cloud infrastructure. We define precisely our methodology and we introduce a new dimensionless measure for elasticity to allow uniform comparisons of different databases at different scales. Our results show clearly that the technical choices taken by the databases have a strong impact on the way they react when new nodes are added to the clusters.