Overview

Archetype Definition Language (ADL) is a formal language for expressing archetypes, which are constraint-based models of domain entities, or what some might call 'structured business rules'. The archetype concept is described by Beale [Beale_2000], [Beale_2002]. The openEHR Archetype Object Model 1.4 describes the definitive semantic model of archetypes, in the form of an object model. The ADL syntax is one possible serialisation of an archetype.

The openEHR archetype framework is described in terms of Archetype Definitions and Principles and an Archetype System. Other semantic formalisms considered in the course of the development of ADL, and some which remain relevant are described in detailed in [Relationship of ADL to Other Formalisms].

ADL uses three other syntaxes, cADL (constraint form of ADL), dADL (data definition form of ADL), and a version of first-order predicate logic (FOPL), to describe constraints on data which are instances of some information model (e.g. expressed in UML). It is most useful when very generic information models are used for describing the data in a system, for example, where the logical concepts PATIENT, DOCTOR and HOSPITAL might all be represented using a small number of classes such as PARTY and ADDRESS. In such cases, archetypes are used to constrain the valid structures of instances of these generic classes to represent the desired domain concepts. In this way future-proof information systems can be built - relatively simple information models and database schemas can be defined, and archetypes supply the semantic modelling, completely outside the software. ADL can thus be used to write archetypes for any domain where formal object model(s) exist which describe data instances.

When archetypes are used at runtime in particular contexts, they are composed into larger constraint structures, with local or specialist constraints added, via the use of templates. The formalism of templates is dADL. Archetypes can be specialised by creating an archetypes that reference existing archetypes as parents; such archetypes can only make certain changes while remaining compatible with the parent.

Another major function of archetypes is to connect information structures to formal terminologies. Archetypes are language-neutral, and can be authored in and translated into any language.

Finally, archetypes are completely path-addressable in a manner similar to XML data, using path expressions that are directly convertible to Xpath expressions.

Archetypes are distinct, structured models of domain content, such as 'data captured for a blood pressure observation'. They sit between lower layers of knowledge resources in a computing environment, such as clinical terminologies and ontologies, and actual data in production systems. Their primary purpose is to provide a reusable, interoperable way of managing generic data so that it conforms to particular structures and semantic constraints. Consequently, they bind terminology and ontology concepts to information model semantics, in order to make statements about what valid data structures look like. ADL provides a solid formalism for expressing, building and using these entities computationally. Every ADL archetype is written with respect to a particular information model, often known as a 'reference model', if it is a shared, public specification.

Archetypes are applied to data via the use of templates, which are defined at a local level. Templates generally correspond closely to screen forms, and may be re-usable at a local or regional level. Templates do not introduce any new semantics to archetypes, they simply specify the use of particular archetypes, further compatible constraints, and default data values.

A third artifact governing the functioning of archetypes and templates at runtime is the local palette, which specifies which natural language(s) and terminologies are in use in the locale. The use of a palette removes irrelevant languages and terminology bindings from archetypes, retaining only those relevant to actual use. The following figure illustrates the overall environment in which archetypes, templates, and a locale palette exist.

With ADL parsing tools it is possible to convert ADL to any number of forms, including various XML formats. XML instance can be generated from the object form of an archetype in memory. An XML-schema corresponding to the ADL Object Model is published at openEHR.org.

For existing users of ADL or archetype development tools, the following provides a guide to the changes in the syntax.