  The World Wide Web infrastructure together with its more than 2 billion users
enables to store information at a rate that has never been achieved before.
This is mainly due to the will of storing almost all end-user interactions
performed on some web applications. In order to reply to scalability and
availability constraints, many web companies involved in this process recently
started to design their own data management systems. Many of them are referred
to as NOSQL databases, standing for 'Not only SQL'. With their wide adoption
emerges new needs and data integration is one of them. In this paper, we
consider that an ontology-based representation of the information stored in a
set of NOSQL sources is highly needed. The main motivation of this approach is
the ability to reason on elements of the ontology and to retrieve information
in an efficient and distributed manner. Our contributions are the following:
(1) we analyze a set of schemaless NOSQL databases to generate local
ontologies, (2) we generate a global ontology based on the discovery of
correspondences between the local ontologies and finally (3) we propose a query
translation solution from SPARQL to query languages of the sources. We are
currently implementing our data integration solution on two popular NOSQL
databases: MongoDB as a document database and Cassandra as a column family
store.
