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  1. OMG Issue

XMI24 — opaque content

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  • Key: XMI24-14
  • Legacy Issue Number: 8321
  • Status: closed  
  • Source: BAE Systems ( Pete Kirkham)
  • Summary:

    Inside UML and SysML, there are cases where opaque content occurs as strings. In particular, in the body of comments and constraint expressions. In SysML it is suggested that MathML be used for parametric constraints, similarly Z can be used instead of OCL. MathML is an XML language, and Z may be interchanged using ZedML, an XML language. In comments, it would be nice to allow XHTML for formatting rich text. At the moment, these are all possible by escaping the XML in the opaque string which ends up in an attribute value: <Unable to render embedded object: File (XMI SYSTEM "http://schema.omg.org/spec/XMI/2.1" [ <!ENTITY math "http://www.w3.org/1998/Math/MathML"> <!ENTITY modelica "http://www.modelica.org/documents/ModelicaSpec21.pdf">]> <xmi:XMI xmlns:UML="http://schema.omg.org/spec/UML/1.4" xmlns:SysML="http://schema.omg.org/spec/SysML/0.9" xmlns:xmi="http://schema.omg.org/spec/XMI/2.1"> <Documentation> <shortDescription>Some queries on parametric constraints in SysML.</shortDescription> </Documentation> <UML:Model name="BaseLibrary" xmi:id="z0"> <ownedElement xmi:type="UML:Package" xmi:label="BasicPhysics" xmi:id="z1"> <ownedElement xmi:type="SysML:ParamConstraint" xmi:label="NewtonsSecondLawOfMotion"> <ownedElement xmi:type="SysML:Parameter" xmi:label="F" type="Force"/> <ownedElement xmi:type="SysML:Parameter" xmi:label="m" type="Mass"/> <ownedElement xmi:type="SysML:Parameter" xmi:label="a" type="Acceleration"/> <ownedElement xmi:type="UML:Constraint" constrainedElement"> <) not found.-- SysML suggests MathML as a constraint language, but the body of a constraint is defined as a string, so XMI maps it to a xsd:string. This means XML literals such as MathML have to be escaped. This complicates general XML tool based queries of the model. -> <specification xmi:type="UML:OpaqueExpression" language="&math;" body=" <math xmlns="&math;"e;> <apply> <eq/> <ci>F</ci> <apply> <times/> <ci>m</ci> <ci>a</ci> </apply> </apply> </math> </math> "/> </specification> <Unable to render embedded object: File (type="UML:Expression" symbol="&math;#eq"> <operand xmi:type="UML:Expression" symbol="F"/> <operand xmi:type="UML:Expression" symbol="&math;#times"> <operand xmi:type="UML:Expression" symbol="m"/> <operand xmi:type="UML:Expression" symbol="a"/> </operand> </specification> <) not found.- In UML2 specification is multiplicity [0..1]; is there no mechanism for specifications in multiple languages? You /could/ use an 'one-of-n' Expression on these, but that sort of implies the expressions may be evaluated, rather than 'choose the one you can evaluate in your context', which is what you can do with XPath and the language attribute. Should you define multiple constraints in such cases, even though it's the same constraint, expressed in a manner to be processed by different tools? -> <specification xmi:type="UML:OpaqueExpression" language="&modelica;" body="F = m * a"/> </ownedElement> </ownedElement> </ownedElement> </UML:Model> </xmi:XMI> If you are post-processing the XMI using standard XML technologies (XPath, XSLT, XQuery), it is far better to have XML as nested elements rather than escaped strings even using XMI's option for having a <body> element will still require the content to be xsd:string. Can the mapping be changed so that opaque content is string or any element() from a different namespace, allowing MathML and ZedML to be used as opaque constraints and XHTML etc. as comments without breaking the way XML is intended to work? The data value is opaque anyway, and converting XML to escaped XML inside XML is reduces generic tool support and adds no value. This may need to effect the UML core too- changing the type of the opaque string to OpaqueString rather than string; alternatively a tag 'org.omg.xmi.parsedContent' could be added to indicate the string (assumed to be a valid XML document fragement) is serialised as parsed XML content rather than unparsed content. It is understood that Extension elements allow xsd:any content, but in these cases the structured content is the actual value of the attribute as an opaque string, rather than an extension to the model - the body of the contstraint is the MathML, just as much as it would be if it were OCL, rather than the MathML being additional, vendor specific data. Pete
  • Reported: XMI 2.0 — Wed, 23 Feb 2005 05:00 GMT
  • Disposition: Resolved — XMI 2.4
  • Disposition Summary:

    Note: The sample XMI in the issue is not valid in that it shows instances of SysML stereotypes nested within their owning UML elements. Furthermore, as the issue later acknowledges, XMI allows properties to be serialized as XML elements instead of XML attributes: this can be forced by setting org.omg.xmi.attribute to true at either the property or Class or package/profile level.

    The issue asks: “In UML2 specification is multiplicity [0..1]; is there no mechanism for specifications in multiple languages? “
    Though not an XMI question, the answer is ‘yes’: in Finalization of UML 2.0, OpaqueExpression::body and OpaqueExpression::language were made multi-valued.

    The substantive issue is how to allow nesting of other XML in the attribute value.
    This can be achieved by using xsd:anyType as the value of the org.omg.xmi.schemaType property. This validates the following:
    <body>
    <math xmlns="math">
    <apply>
    <eq/>
    <ci>F</ci>
    <apply>
    <times/>
    <ci>m</ci>
    <ci>a</ci>
    </apply>
    </apply>
    </math>
    </body>

    This Tag could even be added to the UML metamodel by the SysML Profile XMI to change the serialization specifically for SysML (though this would risk interoperability with UML tools).

    Revised Text:

    • none -

    Disposition: Closed – No Change
  • Updated: Fri, 6 Mar 2015 20:59 GMT