Towards a Taxonomy of Schema Changes for NoSQL Databases: The Orion Language

The emergence of NoSQL databases and polyglot persistence demands to address classical research topics in the context of new data models and database systems. Schema evolution is a crucial aspect in database management to which limited attention has been paid for NoSQL systems. The definition of a taxonomy of changes is a central issue in the design of any schema evolution approach. Proposed taxonomies of changes for NoSQL databases have considered simple data models, which significantly reduce the set of considered schema change operations. In this paper, we present a unified logical data model that includes aggregation and reference relationships, and takes into account the structural variations that can occur in schemaless NoSQL stores. For this data model, we introduce a new taxonomy of changes with operations not considered in the existing proposed taxonomies for NoSQL. A schema definition language will be used to create schemas that conform to the generic data model, and a database-independent language, created to implement this taxonomy of changes, will be shown. We will show how this language can be used to automatically generate evolution scripts for a set of NoSQL stores, and validated on a case study for a real dataset.