PyEmofUC_XMI

The XMI specification has been proposed by the OMG in order to standardize the interchange of models among heterogeneous environments and for storage in persistent files.
An XMI document is an XML document formalized through the W3C-Schema XMI.xsd.

In the PyEmofUC environment, the XMI format is used in two cases:

Model persistence in a file.
For transferring models between environments through a communication channel based on a stream of characters.

The XMI textual representation uses tagged XML text and incorporates certain restrictions and extensions to standardize its formulation within UML environments.
In addition, it describes solutions for creating XML documents and Schemas that share objects consistently.

Key aspects specified by the XMI specification are:

The representation of objects in terms of XML elements and attributes.

The standard mechanisms for linking objects within the same file or across files.

The validation of XMI documents using XML Schemas.

Object identity, which allows objects to be referenced from other objects in terms of IDs and UUIDs.

Specific XMI specification in PyEmofUC

Structure: Although XMI imposes strong restrictions on the structure and format for model information codification, many options are left open for accommodating the formats used by different environment designers.
PyEmofUC adopts the following rules for textual encoding of models and meta-models:

The main element is the same for any models or meta-models. It contains global information about the model identity and about the other referenced models. Its attributes URI and date identify the model whereas the mmUrl one describes the locator of its meta-model and the nsPrefix one features its codification. Last, the importedUrls attribute describes the dependency relationships with other models:

The name is emof:METAMODEL if it is a meta-model (M2) and «Metamodel nsPrefix»:MODEL if it is a model (M1).

The defined attributes are:

date (optional) : It represents the model version. For instance, it may be the last update.

uri (mandatory): It represents the model URI.

mmUrl (mandatory): It is the meta-model locator.

If it is a meta-model, the value is always http://unican.es/istr/PyEmofUC/EmofUC.xmi, which is the meta-meta-model locator (by convention).
If it is a model, the value must be a valid locator in the context where it is interpreted.

importUrls (optional): List of URLs of other models referenced from the model. They appear space-separated and if a referenced model is not included in this list, the model will not be loaded in the environment and the references to it will appear as URI textual format. This list does not include the actual meta-model, which is already referenced by mmUrl.

nsPrefix (mandatory): The namespace prefix for the model. It is an optional prefix that represents the model URI in some contexts.
For instance, it may be used for simplifying the references to the model elements from other models.

rootType (mandatory in meta-models): Indicates the type of the root container of the models compliant to it. It is used during the automatic generation of the corresponding schema.

xmlns:«meta-model nsPrefix» (mandatory): Declaration of the model namespace.

xmlns:emof (optional): Declaration of the meta-meta-model namespace. If the model is a meta-model, this and the previous attribute are the same.

xsi:schemaLocation (optional): URI and schema locator against which it is validated. It is only used if edited by smart XML editors.

xmlns:xmi (mandatory): It declares the document as an XML document.

xmlns:xsi (mandatory): It defines W3C-Schema namespace.

Main element of a meta-model

<?xml version="1.0" encoding="UTF-8"?>

<emof:METAMODEL date="2015-03-06"

uri="http://unican.es/istr/PyEmofUC/CountyCity/County"

mmUrl="http://unican.es/istr/PyEmofUC/EmofUC.xmi"

importedUrls="platform:CountyCity/City.xmi"

nsPrefix="cnty"

rootType="cnty.County"

xmlns:xmi="http://www.omg.org/XMI"

xmlns:emof="http://unican.es/istr/PyEmofUC/EmofUC"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://unican.es/istr/PyEmofUC/EmofUC ..\EMOF.xsd">

<rootPackage name="rootPackage" . . . .

. . . . .

</rootPackage>

</emof:METAMODEL>

Main element of a model

<?xml version="1.0" encoding="UTF-8"?>

<cty:MODEL date="2013-03-07T20:34:18"

uri="http://unican.es/istr/PyEmofUC/CountyCity/EagleCity"

mmUrl="platform:CountyCity/City.xmi"

importedUrls="platform:CountyCity/EagleCounty.xmi"

nsPrefix="ecty"

xmlns:xmi="http://www.omg.org/XMI"

xmlns:cty="http://unican.es/istr/PyEmofUC/CountyCity/City"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://unican.es/istr/PyEmofUC/CountyCity/City City.xsd">

<rootPackage . . . .

. . . .

</rootPackage>

</cty:MODEL>

rootPackage (mandatory): It is the model main container, an instance of the type described in its meta-model.

Model elements are organized within the XML document based on containment relationships between them. If an element is contained in another element in the EMOF model, the XML element representing it in the XMI document is a child element of the XML element representing the container element.
All elements in the XML document have two general attributes introduced by the XMI specification:

Attribute “xmi:id” (required if it is referenced from other element) => It represents an identifier of the XML element and it must be unique within the scope of the model. It is optional, but must be assigned if the element is referenced from the same model or from another model.

Attribute “xsi:type” (mandatory) => In XML documents, this attribute references the element of W3C-Schema that defines its content. This element corresponds to the meta-model meta-class and is used to assign value to the metaclass attribute defined in PyEmofObject, as previously described. Although it is an optional attribute for XML documents, it is always required by the PyEmofUC specification.

. . . .

</company>

References formulation: In the XMI formulation, references to model elements from elements of the same model or other models use the generic URI syntax. However, in order to unify these reference with other XML standards like XLink, without duplicating data, different format schemes are used, depending from where it is done:

Attribute xsi:type: In this case the reference has the a twofold function: designating the W3C-Schema element (XML viewpoint) and referencing the meta-model class (EMOF viewpoint). The dominant viewpoint is XML, using the same format used in the xsi:type attribute, i. e. <nsPrefix schema>:<name of refereced schema element> (for instance, xsi:type="emof:Class"). When it is interpreted as a reference to the meta-class, the character "," in the reference should be replaced by the dot (".") character (metaclass = "emof.class").

Reference from one element to another one belonging to the same model: the format “#<xmi_id of referenced element>” is used (for instance, theType="#cty.Company").

Reference to a model element from an element of another model: In this case, two formats are used interchangeably:

Complete format: <URI>#<xmi_id>, where URI is the identifier of the model containing the referenced element and xmi_id is the identifier of the referenced element (e.g. theType="http://unican.es/istr/PyEmofUC/CountyCity/County#cnty.Adult").

Reduced format: <nsPrefix of the model container of the referenced element>#<xmi_id> (e. g. theType="cnty#cnty.Adult").

Schemas

Every XMI file defining models or meta-models within the PyEmofUC environment has an associated W3C-Schema generated from the corresponding meta-model.
It defines the structure, format and content of any model compliant to that meta-model.

There is a schema (EmofUC.xsd) against which every meta-model is validated. This schema is provided by the PyEmofUC environment.
The schema that formalizes a model is automatically generated by the environment from the corresponding meta-model when it is stored (dump() method) with the extension ".xmi".
Its name is the meta-model name but with the extension ".xsd and it is stored along with the meta-model, in the same folder.

The goal of the generated schema is to provide assistance to the operator when he uses a smart XML editor for creating or editing models compliant to it.
Its formulation is lax, using the following criteria:

The names of the elements of the nested elements and attributes are defined for the corresponding element in the metamodel.
Nested elements are unordered.
Derived attributes are not included.
The xml: id and xsi: type attributes are mandatory for every element.
Every attribute is of xsd:string type.
Every cross-reference is of xsd:anyURI type.

The table below shows a sample meta-model (left column) and its corresponding schema (right column), as generated by PyEmofUC.

<?xml version="1.0" encoding="UTF-8"?>

<emof:METAMODEL date="2015-03-06"

uri="http://unican.es/istr/PyEmofUC/CountyCity/City"

mmUrl="http://unican.es/istr/PyEmofUC/EmofUC.xmi"

importedUrls="platform:CountyCity/County.xmi"

nsPrefix="cty"

rootType="#cty.CompanyDirectory"

xmlns:xmi="http://www.omg.org/XMI"

xmlns:emof="http://unican.es/istr/PyEmofUC/EmofUC"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://unican.es/.../EmofUC ..\EMOF.xsd">

<rootPackage name="rootPackage"

xmi:id="cty.rootPacage"

xsi:type="emof:Package">

<ownedType name="CompanyDirectory"

xmi:id="cty.CompanyDirectory"

xsi:type="emof:Class"

superclass="emof#emof.Object">

<ownedAttribute name="company"

xmi:id="cty.CompanyDirectory.company"

xsi:type="emof:Property"

theType="#cty.Company"

isComposite="True"

upper="*"/>

</ownedType>

<ownedType name="Company"

xmi:id="cty.Company"

xsi:type="emof:Class"

superclass="emof#emof.Object">

<ownedAttribute name="name"

xmi:id="cty.Company.name"

xsi:type="emof:Property"

theType="emof#emof.PyString"/>

<ownedAttribute name="employ"

xmi:id="cty.Company.employ"

xsi:type="emof:Property"

theType="#cty.Employ"

isComposite="True"

lower='0'

upper="*"

opposite="#cty.Employ.employer"/>

</ownedType>

<ownedType name="Employ"

xmi:id="cty.Employ"

xsi:type="emof:Class"

superclass="emof#emof.Object">

<ownedAttribute name="name"

xmi:id="cty.Employ.name"

xsi:type="emof:Property"

theType="emof#emof.PyString"/>

<ownedAttribute name="salary"

xmi:id="cty.Employ.salary"

xsi:type="emof:Property"

theType="emof#emof.PyReal"

default="0.0"/>

<ownedAttribute name="employer"

xmi:id="cty.Employ.employer"

xsi:type="emof:Property"

theType="#cty.Company"

opposite="#cty.Company.employ"/>

<ownedAttribute name="worker"

xmi:id="cty.Employ.worker"

xsi:type="emof:Property"

theType="cnty#cnty.Adult"

opposite="cnty#cnty.Adult.job"/>

</ownedType>

</rootPackage>

</emof:METAMODEL>

<?xml version="1.0" encoding="UTF-8"?>

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"

xmlns:cty="http://unican.es/istr/PyEmofUC/CountyCity/City"

xmlns:xmi=http://www.omg.org/XMI

xmlns:emof=http://unican.es/istr/PyEmofUC/EmofUC

targetNamespace="http://unican.es/istr/PyEmofUC/CountyCity/City">

<xsd:import namespace=http://www.omg.org/XMI

schemaLocation="../XMI.xsd"/>

<xsd:import namespace=http://unican.es/istr/PyEmofUC/EmofUC

schemaLocation="../EMOF.xsd"/>

<xsd:complexType name="CompanyDirectory">

<xsd:complexContent>

<xsd:extension base="emof:PyEmofObject">

<xsd:sequence>

<xsd:element name="company"

type="cty:Company"

maxOccurs="unbounded"/>

</xsd:sequence>

</xsd:extension>

</xsd:complexContent>

</xsd:complexType>

<xsd:complexType name="Company">

<xsd:complexContent>

<xsd:extension base="emof:PyEmofObject">

<xsd:sequence>

<xsd:element name="employ"

type="cty:Employ"

minOccurs="0"

maxOccurs="unbounded"/>

</xsd:sequence>

<xsd:attribute name="name"

type="xsd:string"

use="required"/>

</xsd:extension>

</xsd:complexContent>

</xsd:complexType>

<xsd:complexType name="Employ">

<xsd:complexContent>

<xsd:extension base="emof:PyEmofObject">

<xsd:attribute name="name"

type="xsd:string"

use="required"/>

<xsd:attribute name="salary"

type="xsd:float"

use="optional"

default="0.0"/>

<xsd:attribute name="employer"

type="xsd:anyURI"

use="optional"/>

<xsd:attribute name="worker"

type="xsd:anyURI"

use="required"/>

</xsd:extension>

</xsd:complexContent>

</xsd:complexType>

<xsd:element name="MODEL">

<xsd:complexType>

<xsd:complexContent>

<xsd:extension base="emof:Model">

<xsd:sequence>

<xsd:element name="rootPackage"

type="cty:CompanyDirectory"

maxOccurs="unbounded"/>

</xsd:sequence>

</xsd:extension>

</xsd:complexContent>

</xsd:complexType>

</xsd:element>

</xsd:schema>

PyEmofUC

XMI textual format for model serialization

J.M. Drake, P. López Martínez and C. Cuevas

Software Engineering and Real-Time (ISTR) - University of Cantabria

Table of Contents

XMI specification

Specific XMI specification in PyEmofUC

Schemas

Documents

Link of interest