5.2 Sproc Generic Layer

A Vision for Open Hypermedia Systems

Nürnberg and Leggett

5.2 Sproc Generic Layer

The Sproc Generic (SG) Layer uses the services of the Generic Encoding Layer, and thus may assume reliable ordered structure object transport. This layer is responsible for mapping the abstractions of the Sproc Specific Layer to generic structural objects.

One complication in this layer is that the set of abstractions that the Sproc Specific Layer presents to its clients is open. It cannot be the case that the SG layer be expected to be able to translate arbitrary structural abstractions into generic structure objects. To remedy this, each instance of a Sproc Specific layer protocol is expected to be expressed in terms of generic structure objects when the services of the SG Layer are invoked.

Since the Sproc Generic Layer must already express its abstractions in terms of generic structure objects, the SG Layer need not do any translation. This raises the question of why the SG layer exists at all. This is because there are some aspects of OHP messages that are common to all Sproc Specific Layers. This common functionality is encapsulated in the SG layer. This functionality involves tagging messages with standard "header" type information. This information has been the subject of several posts to the OHS mailing list, including Nürnberg's 8 June post and Reich's 9 June post.

What information should be considered "header" information? Basically, the criterion is that it be applicable to OHP messages independent of the particular Sproc and structural abstractions handled by the messages. Working from the previous OHS list posts, we propose the following header information:

source process identifier: This information uniquely identifies the source of the message. The machine identifier, process identifier, and thread identifier should be included in this process identifier.
destination process identifiers: This information identifies the intended destinations of the message. The number of destination identifiers should be specified. Each destination process identifier should take the same general form as the source identifier.
time sent: This should be an indication of when the source of the message sent the message to the SG Layer.
time received: This should be an indication of when a particular destination of the message received the message at the Generic Encoding Layer. It should not be set by the source process.

Details as to the data types and representations of these types are provided in Appendix B.