A Vision for Open Hypermedia Systems Nürnberg and Leggett

3.2.3 Semantics of Operations
There has been some controversy in the OHSWG concerning whether or not the hypermedia services should be differentiated semantically from one another in the protocol. The current proposal assigns semantics to some operations (e.g., LaunchDocument), but many services are implemented through the RequestService call. The basic idea is that clients request a list of available services from an OHSWG server. The server returns two parallel arrays of strings. One array contains service names as understood by the server, and one is to be used as the client sees fit, presumably for display on a menu of hypermedia services available.

The heart of the controversy seems to be that this model requires human interaction to effect the hypermedia services from the client, since choosing the correct service to perform an action requires a human to interpret the strings in the second array. This model of service definition is obviously inspired by the Microcosm approach to service definition. At first glance, this seems the "least common denominator" approach to this issue, since systems that normally provide semantically meaningful service definitions can simply remove these semantics from their definitions to comply with the protocol. However, the key problem with this approach is that it still relies on human interaction at the client side. The fact that OHS's other than Microcosm can define their interfaces in this semantics-free way does not address this reliance.

We believe the key point in this issue is whether or not reliance on human interaction and interpretation of service names is reasonable in every case. The automatic indexer scenario summarized in Section 3.1 shows this not to be the case. For the automatic indexer to work, it must work autonomously. It must be able to determine how to "follow a link" without human intervention. That is, it must know how to request this service. Simply allowing that arbitrary services have arbitrary names makes it impossible for a program to know what name is associated with a particular service (e.g. Follow Link). The current protocol's proposed method of semantics-free service definition cannot address this situation. Thus, we feel that the protocol must include semantics.

One objection previously raised to this approach is that assigning semantics to services seemed to close the set of services that could be offered. We do not believe this to be the case. Both our architecture and protocol proposals have layers of open structure abstractions, and by extension necessarily admit open sets of operations. However, with the definition of a structure abstraction to be added to the set served by an OHSWG server, it is necessary that semantically well-defined operations also be defined.


Peter J. Nürnberg, John J. Leggett
HRL, CSDL, Texas A&M
original page URL: http://jodi.ecs.soton.ac.uk/Articles/v01/i02/Nurnberg/scen_impl_seman.html