Context Data Model 2.0
- This page describes version 1.1 of the Higgins Context Data Model
- See Context Data Model 1.0 for the released Higgins 1.0 version
Although the CDM can be used for almost any kind of data, the focus of CDM is to provide a foundation for integrating, unifying, and sharing identity-related data. In particular we are focused on information about a person, a group or an entire organization. This might include contact information, authentication data, preferences, email addresses, interests, employer-related information. An object representing a single person, might have relationships to other objects and other people in the same or different data contexts.
- Higgins Data Model Intro (PPT) for an overview.
- Context Data Model Background for information about motivations and design goals.
CDM Core Semantics
The Context Data Model (CDM) encompasses the core semantics of the W3C's Resource Description Framework (RDF); anything expressible in RDF is expressible in the CDM although the converse isn't true.
Most of the subtle but important differences between CDM and RDF are derived from differences in the choice of identifiers used to identify objects in each model. RDF is based on pure HTTP URIs, whereas CDM is based on a more generalized URI called a UDI. Objects identified by URIs in RDF are called Resources, whereas in CDM they are identified by UDIs and are called Entities.
The CDM includes a kind of object called a Context that has no analog in RDF. Individual Contexts can be thought of as containers of portions of the overall graph of objects. Contexts partition the data space into disjoint sets of objects. Context Provider plug-in implementations map data stored in a various kinds of data stores into objects within Context boundaries in CDM.
The CDM also differs from RDF on a syntactic (semantically lossless) level. In RDF an object may have N properties of type T each of which has a single value whereas in the CDM an object may only have 0..1 property of type T, and if the property exists it has 1..N values. Further, in the CDM these properties are called Attributes.
Top level concepts
- Context - a container of objects called Entities. Contexts are identified by a ContextId
- Entity - instances of objects (as well as the Entity Classes and Attribute Types that define them) within a Context. All are identified within a Context by an EntityId
- Attribute - a property of an Entity. Attributes of an Entity are distinguished from one another by its AttributeId. Attributes have 1..N values. These values may be simple (literals) or complex (other Entities). Complex-valued Attributes are called Entity Relations (think "links")
- Data Range - a definition of a kind of simple, literal Attribute value. Generally a syntax restriction on one of the XML Schema datatypes.
More core concepts
Here are a few more foundational concepts:
- The CDM is self-describing, recursive. We use special kinds of Entities called Entity Classes to describe classes of entities and we use other kinds of Entities called Attribute Types that describe Attributes.
- Speaking of recursive, there is Statement class that allows Attributes to be added to a (single) value of an Attribute of an Entity.
- Getting into the weeds a bit, there is also a utility class called TimeSpan (and related Attributes: validFrom and validTo)
Building on the core described above, the CDM introduces the following slightly more specialized concepts:
- There is an Entity class called Agent along with three subclasses: Group, Organization and Person.
- Entities and Contexts can be correlated using the Entity Correlation and Context Correlation links respectively.
- There are a set of pre-defined Attributes:
- part, and its sub-attribute member
- partOf, and its sub-attribute memberOf
- The following Attributes are defined to describe Attributes: displayOrder, category, authority, lastModified, lastVerifiedFromSource, lastVerifyAttempt
- Contexts can be correlated using the contextCorrelation Attribute
- Starting in 1.1M4 we have define a set of Entity Classes and Attributes that describe access control policies.
- The new Entity classes are Policy, an abstract superclass for many kinds of policy we might want to model in the future, and AccessControl, a subclass of Policy
- A new abstract super-attribute called accessControl is defined with these sub-attributes:
- operation, and its sub-attributes: add, delete, modify and read
- selfOperation, and its sub-attributes: selfAdd, selfDelete, selfModify, selfRead
The CDM model is a set of concepts. From 1.1M4 forward, we divide this set of concepts into two subsets. The first subset contains concepts that can are expressed in OWL1.1 and can be directly imported and used as a base ontology by RDF/OWL data sources and systems. These are contained in the file higgins.owl 1.1 -- a file that has been nicknamed HOWL. This file describes many of the core concepts used in the CDM. higgins.owl 1.1 is an abstract (sometimes called an "upper") ontology for identity information. It is abstract in that it doesn't describe any concrete attributes such as "email address" or "first name". It also doesn't define very specialized classes of objects such as "calendar event" or "student", "movie", "book", etc. These are left to Context Providers to define for themselves.
The second subset contains CDM concepts that, while they can be approximated in OWL, are unique to CDM and are not compatible with existing RDF/OWL data sources. These are described in a file called cdm.owl 1.1.
Building on higgins.owl 1.1
Developers must create specialized ontologies based on HOWL that describe specific concrete domains.
For example, if a developer wanted to describe a CRM database, she would create an OWL ontology that would describe the data objects in the CRM database. This CRM database is called a Context in Higgins. If, for example, the database contained records about customers and those customers had full-names and email addresses, then the developer would define "Customer" as a sub-class of Entity and "full-name" and "email" as kinds of Attributes.