Interoperability patterns in digital library systems federations

Service providers willing to offer functionality over an aggregation of information objects are becoming the key actors in the Digital Library world. This aggregation is usually performed by collecting and processing objects from a federation of data providers - e.g., institutional repositories, data archives - through wellknown standard protocols for data-exchange. Higher-level interoperability problems affect these scenarios and determine the quality of the service offered by and the success of these providers initiatives. Interoperability problems are mainly due to the impedance mismatch occurring between data models of the objects to be exported by data providers and the data model of the service provider and can be measured in terms of "structural heterogeneity", "semantic heterogeneity", and "granularity-ofrepresentation heterogeneity". In this paper, we define a basic architecture through which we describe the interoperability patterns arising when constructing such a federations and present the D-NET Software Toolkit as a general-purpose practical solution to these issues.