The Within Component Layer

The layer is considered as being outside of the hypertext model intrinsically. It is specifically concerned with the contents and the structure in the hypertext document. A mechanism that provides linking in the documents while maintaining a clean separation between the storage and within component layers is called anchor. The within-component layer must deliver the data type that is to be used to identify parts of a component in a way the within-component layer understands.

Anchoring in the Dexter model

The unique identifier (UID) uniquely identified each component. But in order to implement links from/to parts of a component it must be possible to also identify substructures within components. The content/structure within the components need to be separated from the hypertext network, therefore they have to have a boundary in between. In the Dexter model, identifying substructures within components is by an indirect addressing entity, called the anchor.

The anchor value is an arbitrary value that specifies some location, region, item, or substructure within a component. This anchor value can only be interpreted by the applications responsible for handling the content/structure of the component. It is primitive and unrestricted from the viewpoint of the storage layer. Anchors can be uniquely identified across the whole universe by a component UID and an anchor id.

The two part composition of an anchor is designed to provide a fixed point of reference for use by the storage layer, the anchor id, combined with a variable field for use by the within-component layer, the anchor value. As a component changes over time (e.g., when it is edited within the runtime layer), the within-component application will change the anchor value to reflect changes to the internal structure of the component or to reflect a movement within the component of the point, region or items to which the anchor is conceptually attached. The anchor id, however, will remain constant, providing a fixed referent that can be used to specify a given structure within a component.