The main aim an of ongoing research project entitled “Mechanisms of atomic services for distributed digital libraries” is to define a coherent set of open protocols dedicated for digital libraries' atomic services. Each of the defined protocols covers a different area of functionality typical for digital library systems. With such protocols it is possible to set up a digital library on the basis of several independent atomic services. Each of those services may be implemented on a different software platform, and this platform may not even be originally created for digital libraries. It just should be accessible via one of defined protocols. In order to achieve this, we have analyzed the functionality of the most popular digital library systems like DSpace, Greenstone or Fedora and the following models: OAIS, 5S and DELOS Digital Library Reference Model. Results of this analysis were the basis for decomposition of a digital library system into several atomic services. We have defined four such services:

• content storage service, which allows storing any type of digital objects content and to version it;
• metadata storage service, which allows storing metadata describing digital objects;
• composition and relation service, which allows defining various relations between digital objects and defining multiple groups of related objects;
• search service, which allows searching in previously defined set of digital objects.

For each of those services we designed a communication protocol in compliance with the REST architecture. We defined three elements in context of each service: the structure of the URL (reflecting the nature of service resources), the meaning of HTTP request types, headers and error codes and finally the structure of any required additional XML messages. Such RESTful atomic services can be used on diverse layers of distributed systems: from Web 2.0 AJAX interfaces to the lowest middleware components. Another designed element is a registry of atomic services. This registry uses the OWL-S ontology and allows the discovery of atomic services instances. Each instance of an atomic service can submit its semantic description to such registry and other services can query the registry for services they need. The presented set of atomic services, in conjunction with the registry, enables building various applications on top of distributed components. It was used in a Polish network of distributed digital libraries and it is the basis of PIONIER Network Digital Libraries Federation (http://fbc.pionier.net.pl/).

ACKNOWLEDGMENTS: Work under the research project nr 3 T11C 02330, “Mechanisms of atomic services for distributed digital libraries” financed by Polish Ministry of Science and Higher Education.

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