A core requirement for digital libraries is a common infrastructure. From a technical viewpoint, this infrastructure has to support state-of-the-art and promising innovative models and techniques, and frameworks to develop and evaluate digital libraries, and has to be highly customizable, configurable and adaptive. To this end, various activities and developments have to be seamlessly integrated into a coherent whole to develop such a generic and modular digital library infrastructure. This includes the following architectural approaches, processes, and activities. Architectural Approaches Peer-to-Peer Architectures: Information providers within digital libraries are highly autonomous.

Data and documents cannot be integrated into a single source. Hence, mechanisms to retain this autonomy and to loosely couple information providersare needed. This also includes the user clients so as to facilitate some collaborative data sharing among them (e.g., for annotations and recommendations about DL contents). Peer-to-peer (P2P) architectures allow for such loosely coupled integration. Different aspects of peer-to-peer systems (e.g. indexes, and P2P application platforms) have to be combined and integrated into an infrastructure for digital libraries.

Grid Architectures: Certain services within digital libraries are complex and computationally intensive (e.g., calculation of certain features of multimedia documents to support content-based similarity search). Grid computing architectures allow for sophisticated load balancing strategies within a cluster of components. Following the idea of a service grid, and the handling of the control of shared resources, similar concepts have to be integrated into an infrastructure for digital libraries.

Service-oriented Architectures: When access to data and documents is provided by dedicated services, appropriate mechanisms to describe the semantics and usage of such services have to be put in place. In the context of web services, descriptions of services using service description languages, are stored in service registries. These elements have to be integrated as building blocks into a digital library. Moreover, common service interfaces have to be defined based on existing standards to facilitate service composition.

Workflow Management: Applications within digital libraries must consider the autonomy and distribution of information providers. Hence, accessing information means combining existing services into mega-applications, i.e. workflow processes. The same is true for applications aiming at managing and controlling the consistency of a digital library. Different aspects of workflow management have to be integrated: such as self-configuration and flexibility, both at the application and at the systems level, and high availability and scalability.

Publish/Subscribe Techniques, Evolution: Services within a digital library have to be made available to the public and have to be accessed by service repositories. Publish/subscribe techniques are a means to make information within digital libraries available and to refresh derived and replicated information sources. Digital libraries are long-lasting institutions, so they have to anticipate changes in the software as well as in the schemata, the ontologies and similar data. The infrastructure for digital libraries therefore has to provide mechanisms to distribute and co-ordinate updates to these components, and to manage the software configuration in such a dynamic environment.

Mobile Information Components: The combination of wireless and wired connectivity in a pervasive computing environment with increasingly small and powerful mobile devices, such as laptops, personal digital assistants, handheld PCs, and smart phones, enables a wide range of new digital library applications. This additional flexibility has to be supported by the underlying digital library middleware. In particular, profiling and proxy management have to be integrated into the infrastructure. Additionally, mobile devices will require sophisticated visualisation techniques to present digital publications adequately on limited displays.