Modelado de sistemas software

Notas del editor

• #32: Figura de [Rivas et al., 1997] Los conceptos utilizandos durante el modelado son varios, que potencialmente pueden provenir de diferentes disciplinas, pero cuando se modela, se quiere crear un modelo de un sistema, en lugar de muchas piezas que no está muy claro como relacionarlas. Para conseguir integrar modelos, el metamodelo subyacente debe estar integrado a través de las disciplinas también. Un ejemplo simple: considérese el concepto de un requisito. Independientemente de que se esté haciendo un diseño estructurado, OO o hardware, es el mismo concepto. Un metamodelo integrado permitirá representar los mismo conceptos a través de dominios diferentes utiilizando los mismos metaobjetos, una colección de fragmentos de metamodelos no integrados requerirán un esfuerzo adicional del vendedor de herramientas para integrar lo que el metamodelo no hace [Metamodel, 1997]
• #49: Figura de [Rivas et al., 1997] Los conceptos utilizandos durante el modelado son varios, que potencialmente pueden provenir de diferentes disciplinas, pero cuando se modela, se quiere crear un modelo de un sistema, en lugar de muchas piezas que no está muy claro como relacionarlas. Para conseguir integrar modelos, el metamodelo subyacente debe estar integrado a través de las disciplinas también. Un ejemplo simple: considérese el concepto de un requisito. Independientemente de que se esté haciendo un diseño estructurado, OO o hardware, es el mismo concepto. Un metamodelo integrado permitirá representar los mismo conceptos a través de dominios diferentes utiilizando los mismos metaobjetos, una colección de fragmentos de metamodelos no integrados requerirán un esfuerzo adicional del vendedor de herramientas para integrar lo que el metamodelo no hace [Metamodel, 1997]
• #50: (1b) El conjunto de objetos instanciados de las clases tienen todos las mismas propiedades y las relaciones definidas en la clase En este ejemplo, City es una clase y Houston es un objeto generado de City. En otras palabras, Houston es una instancia de City. Nótese que City, en su papel como plantillas de objetos, define los tipos de atributos que sus instancias pueden tener. Houston, como objeto generado de City, tiene valores específicos para estos atributos.
• #51: A similar situation occurs with relationships. A class defines the kinds of relationships which its instance may enter into with instances of other classes. Thus, extending the above example, a given city my have an ‘‘IsIn’’ relationship with a country. For example, Houston ‘‘IsIn’’ the USA. In its role as a template, the class City defines the kinds of relationships which its instances can enter into with instances of other classes. Thus in the above example, ‘‘IsIn’’ between City and Country is a template for ‘‘IsIn’’ relationships between instances of City and County (such as Houston and the USA). Relationships are often shown diagramatically as lines connecting classes, as illustrated above. However, it is also possible to represent them textually. To do so it is necessary to give the names of the related entities as well as the relationship name. Thus ‘‘Houston.IsIn.USA’’ is the full name of the relationship depicted on the right hand side of the above drawing, and ‘‘City.IsIn.Country’’ is the name of the relationship template shown on the left hand side. To reiterate, a class represents a template from which multiple object may be instantiated. These objects are related to the class by the IsInstanceOf relationship. A class also defines attribute templates and relationships templates which its instances may have instances of. These attributes and relationships may also be viewed as defining a membership test for belonging to the set of instances of the class. One common source of confusion in object modeling is that the term attribute is often used to refer both to attribute templates at the class level (eg. Pop) and attribute instances at the object level (e.g. Pop = 2M) . Thus, it would not be uncommon to use a phrase of the kind ‘‘City has attributes Pop and Area’’ and also ‘‘Houston has attributes Pop and Area’’. A similar situation applies to the term ‘‘relationship’’. This is frequently used loosely to mean both relationships templates and relationship instances. In other words, in current modeling methods relationships and attributes are rarely distinguished explicitly from relationship templates and attribute templates. This can be a major source of confusion when extending object modeling to meta-modeling.
• #52: (1a) Ver las entidades de ese nivel como si fueran objetos When viewed as an object, a class itself becomes an instance of another class. Thus, in the diagram fragment below, class City is an instance of the class, Class, which defines an attribute (template), NoOfInstances. Since it is an instance of the class, Class, City has a value for its instance of the attribute NoOfInstances, in this case 1.
• #53: As with regular objects, City can participate in relationships with other objects. The example below indicates that City is stored in a file, called CityFile, which has a size attribute with a value of 4 Kb. Naturally, the kind of attributes and relationships which City can have are defined by its class, Class. Notice that the attributes and relationships of City (when viewed as objects), are not passed on to (i.e. have no effect on) its instances (when it plays the role of a class). Thus, Houston, an instance of City, does not have (instances of) the attribute NoOfInstances or the relationship Stores. This is impossible, since NoOfInstances and Store are not templates, but actual attribute and relationship instances.
• #54: Por lo tanto, una descripción completa de tales entidades deberían tener dos dimensiones como se ilustra en la figura. A full description of a class such as City, therefore involves four elements · attribute values (eg. NoOfInstances = 1) · relationship values (eg. CityFile.Stores.City) · attribute templates (eg. Pop, Area) · relationship templates (eg. City.IsIn.Country)
• #56: This is the approach adopted in this submission. As illustrated below, the label ‘‘meta’’ is used to describe entities defined at the third level in the instantiation hierarchy, and the label ‘‘meta-meta’’ to describe entities defined at the fourth level. All data entities at the bottom level are instances of artifacts at the model level, which in turn are instances of entitities at the meta model level and so on up the IsInstanceOf (or meta) hierarchy. This use of terminology differs from the situation with inheritance hierarchies in which relative terminology is used. Thus, when the term superclass is used, it means the immediate parent of a given class. It is rarely, if ever, used in an absolute sense to mean classes at the second level (from the bottom) of the inheritance hierarchy. Unfortunately, the term meta is sometimes also used to represent a relative IsInstanceOf relationship between two classes, but this submission uses the term ‘‘meta’’ exclusively in an absolute sense.
• #57: Notice that that the entities at the extremes of the hierarchy can only be viewed from one perspective. Objects, at the bottom of the hierarchy do not have a template perspective (since they cannot be instantiated), while meta-meta-classes at the top of the hierarchy do not have an object perspective (since they have not been instantiated). Only those class in the middle can be viewed from both perspectives. However, this submission borrows a neat trick from CDIF [EIA/IS-107] to side step this problem at the top end. The tick is to make the entities at the top of the meta hierachy instances of other entities at the same level, possibly even themselves. Although this has the flavor of creating something from thin air (like particles in the ‘‘soup’’ of quantum mechanics) it does neatly terminate the meta hiearchy. Note also that because of the absolute naming scheme an entity is said to have values for attributes from the next meta-level. Thus, a class has value for meta-attributes (eg. NoIfInstances = 1), and a meta-class has value for meta-meta-attributes (eg. Shape = rectangle).