6.1 Data-flow models

  -. Show the proccessing steps as data flows through a system.
  -. Intrinsic part of many analysis methods.
  -. Simple and intuitive notation that customers can understand.
  -. Show end-to-end processing of data.
  -. Data-flow diaagrams (DFD)
	+. May be used to show processing at different levels of abstraction from
                     fairly detailed.
	+. May also be used for architectural description showing data interchange
                     between the sub-systems making up the system.
	+. Not a good way to describe system interfaces.

  6.2 Semantic data models

  -. Used to describe the logical structure of data processed by the system.
  -. Entity-relation model seets out the entities in the system, the relationships between
    these entities and the entity attributes. 
  -. Widely used in database design. Can readily be implemented using relational 
    databases.

  6.3 Object models

  -. Object models describe the system in terms of object classes.
  -. An object class is an abstraction over a set of objects with common attributes and
    the services(operations) provided by each object.
 -. Various object models may be produced.
	+. Inheritance models
	+. Aggregation models
	+. Service models
  -. Natural ways of reflecting the real-world entities manipulated by the system.
  -. More abstract entities are more difficult to model using this approach.
  -. Object class identification is recognised as a difficult process requiring a deep
    understanding of the applicaation domain.
  -. Object classes reflecting domain entities are reusable across systems.
  -. Inheritance models
       +. Organise the domain object classes into aa hierarchy.
       +. Classes at the top of the hierarchy reflect the common features of all classes.
       +. Object classes inheerit their attributes and services from one oor more 
          super-classes.
       +. Class hierarchy design is a difficult process if duplication in different branches is 
          to be avoided.  
  -. Multiple inheritance
  -. Object aggregation
  -. Service-usage models

  6.4 Data dictionaries

  -. A data dictionary is a list of names and associated descriptions of entities used 
    in the system.
  -. It repressents a shared repository of system information.
  -. It serves as
	+. A mechanism for name management. As a system model may be
	  developed by different people, there is potential for name clashes.
	+. A link from analysis to design and implementation.
  -. All names used in the system model, design and implementation should be entered
    in the data dictionary.
  -. Support software should be used to create, maintain and query the dictionary.
  -. The data dictionary may be integrated with CASE tools so that its construction and 
    maintenance may be partially automated.