  In today's large enterprises there is a significant increasing trend in the
amount of data that has to be stored and processed. To complicate this scenario
the complexity of organizing and managing a large collection of data,
structured according to a single, unified schema, makes so that there is almost
never a single place where to look to satisfy an information need.
  The Ontology-Based Data Access (OBDA) paradigm aims at mitigating this
phenomenon by providing to the users of the system a unified and shared
conceptual view of the domain of interest (ontology), while still enabling the
data to be stored in different data sources, which are managed by a relational
database. In an OBDA system the link between the data stored at the sources and
the ontology is provided through a declarative specification given in terms of
a set of mappings.
  In this work we focus on comparing two of the available systems for OBDA,
namely, Mastro and Ontop, by adopting OBDA specifications based on W3C
recommendations. We first show how support for R2RML mappings has been
integrated in Mastro, which was the last feature missing in order to enable the
system to use specifications based solely on W3C recommendations relevant to
OBDA. We then proceed in performing a comparison between these systems over two
OBDA specifications, the NPD Benchmark and the ACI specification.
