Introduction
One very important aspect of making digital objects available online is the ability to creat permanent links to those objects. Such links may be useful, for example, in a scientific work, as an element of a bibliography, or for referring to digital objects in external services, such as library catalogs, bibliographic bases, or even Wikipedia. This section is about the way in which links to digital objects are created in the dLibra system; it is also explained which of them should be used to ensure the maximum durability of the references.
The Data Model of the dLibra System
In the image below, the basic data model in the dLibra system is shown. The model includes:
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- http://www.wbc.poznan.pl/Content/8711/ – the link to the content of the edition with identifier number 8711 in the Digital Library of Wielkopolska redirects the reader to the main file of the publication.
- http://www.wbc.poznan.pl/Content/8711/index.html – the link to the main file of the content of the edition with identifier number 8711 in the Digital Library of Wielkopolska, obtained as a result of a redirection from the http://www.wbc.poznan.pl/Content/8711/ address.
- http://www.wbc.poznan.pl/Content/8711/toml-014.jpg – the link to one of the files comprising the content of the edition with identifier number 8711 in the Digital Library of Wielkopolska, obtained by browsing the publication.
The Types of Links and the Manner of Processing Reader Queries
In table 1, two types of links to particular elements of the data model in the dLibra system are shown. That differentiation results from the assumed manner of processing reader queries, illustrated in figure 2. The key differentiation here is the one between the reader application and the permalink module. The task of the module is to make a part of the links in the dLibra system independent from the technological changes in the reader application which are related to developing it, making its subsequent versions available, and technological modernizations. Examples of such permalinks are shown in the last column of table 1. The task of the module which manages those links is ensuring their unchanged functioning despite the changes introduced in the subsequent versions of the reader application. The penultimate column in table 1 contains links which have been unchanged in the dLibra system since its 0.4 version, published in 2004. Those links, however, may – in theory – be changed in the future, for technical reasons. That is why it is recommended that the permalinks presented in the last column be used for creating references to objects.
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Fig. 2. A simplified schema of the architecture of the dLibra system.
Permalinks and Exceptional Situations
In some specific cases, permalinks cannot be offered solely on the basis of the digital library system. When we look at a sample permalink (for example, http://www.wbc.poznan.pl/publication/290787), we can say that it has two elements which can change. They are:
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- http://rcin.org.pl/igipz/publication/13932 – a reference on the level of the digital library,
- http://fbc.pionier.net.pI/id/oai:rcin.org.pl:2289 – a link which makes use of an OAI identifier, realized on the basis of a service of the Digital Library Federation
- http://fbc.pionier.net.pI/id/10.7163/GPol.2011.Sl.3 – a link which makes use of a DOI identifier, realized on the basis of a service of the Digital Library Federation; and
- http://dx.doi.Org/10.7163/GPol.2011.Sl.3 – a link which makes use of a DOI identifier, realized on the basis of a service of the dx.doi.org portal.
Summary
By way of a summary, the table below contains the advantages and disadvantages of particular kinds of links to objects available in the dLibra system.
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