An Empirical Study on the Design and Evolution of NoSQL Database Schemas

We study how software engineers design and evolve their domain model when building applications against NoSQL data stores. Specifically, we target Java projects that use object-NoSQL mappers to interface with schema-free NoSQL data stores. This is a popular software stack. Given the source code of ten real-world database applications, we extract the NoSQL database schema thus implied in the application code. We can confirm that schemas are generally denormalized, as is recommended practice in data modeling for NoSQL data stores. Further, we analyze the entire project history, and with it, the evolution history of the NoSQL database schema. We show that NoSQL schemas evolve in all analyzed projects, and observe a comparatively high frequency in schema changes. In doing so, we conduct the so far largest empirical study on NoSQL schema design and evolution.