Robust Hyperlinks and Locations

We suggest that building "permissive, but robust" digital library systems and services is an attractive alternative to the library and computer science tradition of building "strict, but fragile" systems. Strict, but fragile, systems are efforts to engineer complete systems that ensure desired properties, but which often prove impractical in distributed environments without a central authority to coordinate change. In the permissive, but robust, approach, we permit individual components to change in ways that might, in fact, cause a desired property to fail to persist. However, we engineer components to be robust, so that it is likely that desired properties persist even under a great deal of uncoordinated change. We have applied the permissive, but robust, approach to two related problems of reference in distributed information systems. The first application yields robust hyperlinks, and the second, robust locations. Robust hyperlinks address the familiar issue of providing persistent reference to networked resources, such as Web pages, given changing, uncooperating services. Robust locations concern a somewhat less familiar, but, we suspect, soon-to-be just as big a problem, namely, references to changing sub-document resources. Robust locations, we suggest, provide essential grounding for next-generation web functionality, such as annotations that survive document editing. In both cases, robustness is achieved by providing multiple, independent descriptions across boundaries where change is likely to be uncoordinated. If the different descriptions are property selected, then most uncoordinated changes will be unlikely to cause all the descriptions to fail. Thus, while there is no guarantee that references will remain coherent, a single failure is unlikely to be catastrophic. Instead, the failure of one, even a primary, method will generally allow graceful recovery via other methods.