iServer
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Prof. Dr. Beat Signer
Vrije Universiteit Brussel
Department of Computer Science
Pleinlaan 2, 1050 Brussels
(Belgium)
+32 2 629 1239, bsigner@vub.be
Office: PL9.3.60 (Pleinlaan 9)
VUB
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iServer

The iServer platform developed in the Global Information Systems (GlobIS) research group at ETH Zurich enables cross-media linking based on a general Resource-Selector-Link (RSL) model. iServer not only supports links between different kinds of digital media, but also allows for the integration of physical and digital content. Links managed by the iServer platform are always directed. Every link is bound to at least one source anchor and points to one or more target components (multi-headed links). Furthermore, iServer does not distinguish between components which can be used as link sources and those applicable as target objects. This is achieved by introducing a general concept of entities as link anchors and targets within the underlying RSL hypermedia metamodel.
Resource-Selector-Link (RSL) model
Fig. 1: Resource-Selector-Link (RSL) hypermedia metamodel
The simplest type of an entity is a resource, always representing an entire information unit. Using the resource concept we can, for example, link from an HTML document to a movie clip. To control the granularity of link anchors and targets we introduce selector as a subtype of entity enabling us to address parts of a resource. Further, each selector is located on a specific layer which allows for overlapping links with well-defined semantics (multi-layered links). Access control and link sharing is supported by the user management unit.

The iServer architecture is designed as a platform which can be extended based on a plug-in mechanism. By implementing media-specific instances of the selector and resource components, any new type of media can be integrated. The framework has been used within the European projects Paper++ and PaperWorks for integrating physical paper and digital content and is the main component of the interactive paper infrastructure. Further, we are developing plug-ins for XHTML documents, movie clips and sound files. The sharing of link knowledge is supported by the distributed iServer version based on peer-to-peer concepts.

More recently active components have been added to the iServer platform. An active component is a configurable piece of program code which gets executed on link activation. By using active components, we can not only link to static content but design very interactive information environments.

Resource Plug-ins

A resource plug-in mechanism based on the resource and selector concepts enables the integration of new resource types by providing a resource-specific Java implementation of a resource and its corresponding selector.
Resource plug-ins
Fig. 2: Resource plug-ins
The Selectors representing elements within a resource in combination with the Resources which represent entire resources are the central components for the resource plug-in mechanism. For a particular media type, we can extend the iServer platform by introducing a component that defines selectors and resources for that media type. Figure 2 shows a component for movies (iMovie) where a resource is a movie and a selector may be a time span. Another plug-in is responsible for dealing with web pages (iWeb) where a resource is an XHTML document and a selector is an XPointer expression.

A major advantage of the chosen plug-in mechanism is the tight integration over all types of media based on a common core link model rather than isolated applications for specific kinds of media. As soon as a plug-in for a new resource type has been implemented, entities of that medium can be cross-linked with instances of any existing media type. With each resource plug-in introduced, the iServer platform becomes more powerful and provides a richer cross-media information space. In The following table provides a list of existing iServer resource plug-ins represented by the corresponding resource and selector types for each type of media. Note that the list of plug-ins is far from being complete since arbitrary digital or physical resources could be added.

Medium Resource Selector
paper document page shape
web page XHTML document XPointer
movie mpeg file, avi file etc. time span
movie mpeg file, avi file etc. shape
sound mp3 file, wav file etc. time span
image gif file, jpeg file etc. shape
database database workspace query
physical object RFID space RFID tag


For a given resource type there may be varying types of selectors based on the requirements of specific applications. In the table, we suggest time spans to be used as a candidate for movie selectors. However, a specific application might need to link movies based on spatial information within the movie whereas another application might need to define links based on a combination of temporal and spatial information. The iServer architecture therefore supports the definition of different selectors for a single resource type.

A first iServer plug-in was implemented for interactive paper to integrate paper and digital content. More recently we developed three additional iServer plug-ins to fully integrate XHTML content, movies and arbitrary RFID-tagged physical objects. Of course, XHTML documents and movies already have some basic support in the core iServer implementation as it is possible to link for example from a web page to a movie. However, to fully support these resource types, it should be possible to link to/from elements within a resource and not just associate entire resources. This implies that we have to implement a resource plug-in with the corresponding selector for addressing parts of a resource.

Related Publications

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