HyTime Semantic Grove EXPRESS Data Model

Overview

The HyTime property set, published in ISO/IEC 10744:1997 (HyTime), Annex B, defines the abstract data model for the information a HyTime engine needs to maintain in order to implement the hypermedia processing semantics of the HyTime architecture. As published in the HyTime standard, this data model is defined as a "property set", which is a simple form of data model. A property set defines data objects called "nodes" that have properties. Some properties of nodes are atomic values, such as strings or integers. Other properties are lists of other nodes, thus establishing node-to-node relationships. The general property set mechanism is defined in ISO/IEC 10744:1997 (HyTime), Annex A.4.

Nodes are analogous to STEP entities. Properties are analogous to entity attributes. Thus it is possible to redefine the data model defined by any property set using EXPRESS. This document describes our attempt at redefining the HyTime property set in EXPRESS. The value of this is that it provides a ready-made definition, in STEP terms, of the data objects needed to represent and manage arbitrary relationships, e.g., hyperlinks, among STEP entities (and non-STEP data). It provides an addressing model that is not limited to the addressing of entity instances within a single STEP repository.

The model presented here does not reflect the entire HyTime property set. It only reflects those classes and properties used for hyperlinks and location addresses. The scheduling and rendition facilities of the HyTime architecture are not reflected in this model.

EXPRESS-G Model

A larger, clearer version is provided by this PDF file:hytimesg.pdf:

EXPRESS Model

The following is the EXPRESS schema generated from the EXPRESS-G model. It has not been enhanced to reflect those constraints defined by the HyTime property set or the HyTime architecture that would require Where rules.

SCHEMA HyTime_Semantic_Grove;

TYPE general_strnorm = STRING;
END_TYPE;

TYPE hydoc_content = SELECT
  (location_address,
   hyperlink);
END_TYPE;

ENTITY abs_anchor
  ABSTRACT SUPERTYPE OF (ONEOF(list_anchor,anchor));
  anchor_role_name : general_strnorm;
  anchor_role : anchor_role;
END_ENTITY;

ENTITY anchor
  SUBTYPE OF (abs_anchor);
  anchored_object : grovenode;
  return_traversal : BOOLEAN;
  departure : BOOLEAN;
  int_arrival_trav : LIST [0:?] OF anchor;
  ext_arrival_trav : LIST [0:?] OF anchor;
  next_left : OPTIONAL anchor;
  next_right : OPTIONAL anchor;
END_ENTITY;

ENTITY anchor_role;
  list : BOOLEAN;
END_ENTITY;

ENTITY anchored_object;
  object : grovenode;
END_ENTITY;

ENTITY arc_semantics;
  arc_public_id : STRING;
  hyperlink_types : LIST [0:?] OF hyperlink_type;
END_ENTITY;

ENTITY bos_member;
  in_hytime_bos : BOOLEAN;
  memroot : grovenode;
END_ENTITY;

ENTITY grovenode;
END_ENTITY;

ENTITY hyperdocument;
  bos : LIST [1:?] OF bos_member;
  arc_semantics : LIST [0:?] OF arc_semantics;
  hub_document : hytime_document;
  hytime_documents : LIST [0:?] OF hytime_document;
  value_refs : LIST [0:?] OF valueref;
  anchored_objects : LIST [0:?] OF anchored_object;
  hyperlink_types : LIST [0:?] OF hyperlink_type;
END_ENTITY;

ENTITY hyperlink;
  anchors : LIST [2:?] OF abs_anchor;
  definition : grovenode;
  hyperlink_type_name : general_strnorm;
  hyperlink_type : hyperlink_type;
END_ENTITY;

ENTITY hyperlink_type;
  roles : LIST [2:?] OF anchor_role;
END_ENTITY;

ENTITY hytime_document;
  is_hub_document : BOOLEAN;
  content : LIST [0:?] OF hydoc_content;
  sgml_document : grovenode;
  reference_targets : LIST [0:?] OF reference_targets;
END_ENTITY;

ENTITY list_anchor
  SUBTYPE OF (abs_anchor);
  members : LIST [0:?] OF anchor;
END_ENTITY;

ENTITY location_address;
  definition : grovenode;
  location_source : grovenode;
  location_selector : LIST [1:?] OF grovenode;
  located_nodes : LIST [0:?] OF grovenode;
END_ENTITY;

ENTITY reference_targets;
  reference : grovenode;
  targets : LIST [0:?] OF grovenode;
  location_path : OPTIONAL location_address;
END_ENTITY;

ENTITY valueref;
  value_of : grovenode;
  value : LIST [0:?] OF grovenode;
END_ENTITY;

END_SCHEMA;

Overview of the HyTime Semantic Grove Data Model

A set of entity instances conforming to the HyTime property set data model consitute a "HyTime semantic grove", reflecting the results of applying HyTime-defined processing to a set of SGML documents and other data objects (which could include STEP entities). Thus this data model is called the "HyTime semantic grove" data model. A grove is nothing more than a directed graph of nodes. In STEP terms, a grove would be a collection of entity instances related together by their attribute values. Groves can be represented as collections of STEP entities, where each grove node is represented by one or more entities.

The HyTime semantic grove data model is an example of an "early-bound" grove-to-STEP mapping where each node class in the property set is mapped to one entity class in the EXPRESS model.

Node properties that are node lists are represented in the EXPRESS model as list data types. In the grove model, any node list may, potentially, be an empty node list (that is a list of zero members). Thus, most of the lists are defined as [0:?]. However, a few properties require at least one node to be present if the property is exhibited at all. These are defined as [1:?] in the EXPRESS model. Nodal properties that are singleton node lists are represented by an attribute whose value is a single entity, rather than as a list of one member.

One purpose of the HyTime semantic grove is to record pointers to nodes in groves other than the HyTime semantic grove (e.g., element nodes in SGML document groves). The grove model provides the notion of "unrestricted references" from one node to other nodes, in the same or different groves. This is reflected in the EXPRESS model through the "grove node" entity, which represents nodes that could be in any grove and have any node class. The grove node class can be used as the superclass of any entity type as the grove node class has no attributes in this early bound form.

The following sections walk through the different entity types in the model, starting with the hyperdocument entity type.