UML ITS Technical

The UML ITS is a technical infrastructure for making V3 wire formats easier to implement.

The XML ITS includes a number of 'secret' transforms (actually, they are not secret, but they are exceedingly poorly documented). The UML ITS proposes to be extremely specific about the wire format, in the recognition that it's the wire format that users actually finally encounter and must master. It's goal is to be able to go to an implementer and offer them a "thing" they can use, an implementable model. The implementable model will be normative and will be published as UML and XML schema.

There may be a number of implementation decisions and transforms made between the RBM and the on-the-wire format. These are made before the UML and schema are built, and are explicit in them. These changes are discussed in UML ITS Policy.

RBM <--Policies--> XUM Implementable Models (UML diagram <-> Schema)

The output of the ITS can be used for code generation. It will be tested with common tools. The XUM works with the commonly used tools, and it correctly specifies the wire format. It doesn't completely describe the validation rules for the message, no schema or UML model will be able to do that, because the HL7 models rely heavily on terminology. A XUM consists of a pair of schemas and UML diagrams that specify the exact same wire format, implementers can code generate from them and exchange data. WYSIWYG: What you see in the schema or in the diagram, is exactly what goes on the wire.

There is existing work outside HL7 on the question of transforming from UML diagrams to an XML instance or a schema. The UML ITS is intended to leverage this work (One of the existing issues is that UML to XSD transform requires sequencing of the associations, which is a "bit of magic", but does need to be represented explicitly).

Normative

XUMs will be normative. We do not claim that these are the only model that can apply, but we assert that the XUM is a reliable correct definition of the wire format. Specifically we allow for things such as schema artifacts in the instance of the message. If the message fails schema validation, it is necessarily incorrect. If the message passes schema validation, the format is correct, but the actual data contents may not.

It expresses a RIM-Based Model (RBM) both in terms of a XML schema as well as a UML model. The UML model is the equivalent (or as closely as possible) of the schema and vice versa.

UML

Each XUM is represented by a single UML class diagram with a single UML package. The UML representation of the XUM may use the following UML constructs:

• Classes with Attributes. Operations are not supported since these are representations of wire formats with no behavior.
• Parameterized classes with one class parameter. All parameterized classes are collections
• Constraints using OCL in notations attached to the class.
• Constraints must include the context <class name>
• Generalization associations
• Named composition associations (represented as by value in XML)
• Named associations (represented as by reference in XML)
• The {xor} notation (? For discussion. this is a neat construct for us, but not very well supported. However it may not matter, we can encode as an {xor} constraint and an OCL constraint)
• The stereotype <<enumeration>> for enumerations
• The stereotype <<io>> for marking entry points.
• The inbuilt types from the OCL 2 kernel, or any types found in other XUMs which must be explicity accessed as UML packages
• Comments in attached Notations.

The UML representation will be published as a GIF image, and SVG picture, and an XMI file.

XML

Each XUM is represented by a single schema. The XML schema representation of the XUM may use the following schema constructs:

• Complex Types
• Element and attribute definitions
• Global elements for entry points
• Simple Types for enumerations
• Sequences
• Choices for choices
• Schematron rules for constraints
• The inbuilt types from the schema standard, or any types found in other XUMs which must be explicity imported as schemas
• Comments in AppInfo annotations

The schema representation will be published as an xsd file. All the schemas will be in the same namespace.

Relationship between XML and UML

The UML diagram and the schema will be as isomorphic as possible. Other than in the constraints, the two representations will be identical. There will be simple rule for describing which UML attributes are represented as XML attributes, and which UML attributes are represented as XML elements

Elements vs Attributes

It's still not decided whether to use XML elements or XML attributes in the new ITS.