CS 425/625 Software Engineering System Models

1
CS 425/625 Software Engineering System Models

• Based on Chapter 7 of the textbook Somm00 Ian
  Sommerville,
• Software Engineering, 6th Ed., Addison-Wesley,
  2000 and on
• Ch7 PowerPoint presentation available at the
  books web-site
• www.comp.lancs.ac.uk/computing/resources/IanS/SE6/
  Slides/index.html
• September 29, 2003

2
Outline

• System Models
• Contextual Models
• Behavioral Models
• Data Models
• Object Models

3
Introduction.

• System models are used to specify the system to
  be built in a more detailed way
• Typically, system models are graphical
  descriptions of the system, its services and its
  constraints
• Various perspectives for system modeling
• Context
• Behavior
• Structure

4
.Introduction

• The production of system models is usually guided
  by methods, e.g., SSA (Structured Systems
  Analysis), SADT (Structured Analysis and Design
  Technique), OMT (Object Modeling Technique), etc.
• In practice, various models, not bound to
  methods, could be combined to describe the system
  and its requirements
• Main types of system models
• Data Processing Models
• Composition Models
• Architectural Models
• Classification Models
• Stimulus-Response Models

5
Context Models

• Contextual models show the system and its
  environment
• Ideally, the boundaries between the system and
  its environment are identified clearly
• Dependencies between the computer-based system
  and systems from the environment are highlighted

6
.Context Models..

• Various models can be used for context modeling
• Very simple, high level architectural models show
  the system and its connections with environmental
  components
• High level process models indicate main process
  activities
• High-level data-flow diagrams depict data
  transformations and data transfers

7
..Context Models.

• Example of architectural model for system context
  specification an ATM system and its environment
  Fig. 7.1, Somm00

8
Context Models

• Example of process model for system context
  specification an equipment procurement system
  Fig. 7.2, Somm00

9
Behavioral Models.

• Behavioral models specify the overall behavior of
  the system
• Data-flow models (or data-flow diagrams, DFD) are
  suitable for data-driven systems
• State machine models (or stimulus-response
  models) are appropriate for event-driven systems

10
.Behavioral Models...

• Data-flow models show the system in terms of
• Data stores
• graphical representation rectangles
• Data transformations (data processing operations)
  
• graphical representation rounded rectangles
• Data movements (data transfers)
• graphical representation directed lines

11
..Behavioral Models..

• State machine models describe system behavior
  using two main modeling elements
• States
• represented using rounded rectangles
• Transitions
• shown as directed links between states

12
Behavioral Models....

• A DFD example an order processing system Fig.
  7.3, Somm00

13
.Behavioral Models

• State machine diagrams depict the behavior of the
  system in terms of responses to events
• Real-time systems are essentially event-driven
  systems
• The system is assumed to be in one of its
  possible states at any given moment of time. A
  transition to another state is triggered by the
  occurrence of an event
• David Harels statecharts are included in the UML
  (Unified Modeling Language) DFDs are not part of
  UML

14
..Behavioral Models..

• Example of state machine diagram A microwave
  oven controller Fig. 7.5, Somm00

15
...Behavioral Models.

• The statecharts notation used in UML allows
• Specification of composite states (states that
  have sub-states), thus supporting model
  decomposition
• Description of actions performed while in a
  state
• Entry actions
• State actions (activities)
• Exit actions
• Description of transitions
• Events that trigger the transition
• Conditions under which transitions occur
• Actions on transitions, if any

16
....Behavioral Models

• Example of composite state microwave oven Fig.
  7.7, Somm00

17
Semantic Data Models

• Data models describe the logical structure of
  data processed by the system
• The Entity-Relationship-Attribute model, ERA
  (also known as Entity-Relationship model, ER)
  shows
• Entities in the system
• Relationships between entities
• Attributes of entities and relationships
• Cardinality constraints (e.g., one or many)

18
.Semantic Data Models..

• Data dictionaries are repositories of information
  pertaining to the elements of the system models.
  They are useful to manage the namespace of the
  system models as well as to supply a
  cross-reference tool for analysis, design,
  implementation, and evolution.

19
..Semantic Data Models.

• An ERA example a software design description
  Fig. 7.8, Somm00

20
Semantic Data Models

• Data dictionary examples of data entries Fig.
  7.9, Somm00

21
Object Models..

• Objected-oriented modeling is centered around the
  concepts of object and class
• An object is an entity described in terms of
  attributes (data) and operations (functionality)
• A class is a template for objects that have
  similar features (attributes and operations).
• More exactly, the objects that belong to the same
  class have the same structure and behavior, e.g.,
  davidsAlarmClock is an object (instantiation) of
  the class AlarmClock

22
.Object Models.

• The distinguishing principles of object-oriented
  (OO) technology are
• Encapsulation
• Inheritance
• Polymorphism

23
..Object Models

• Objects provide more natural ways to model
  real-life entities
• Both structure (class diagrams) and behavior
  (e.g., interaction diagrams and state diagrams)
  can be described
• A class diagram shows
• Classes
• Relationships between classes
• Constraints
• Annotations

24
Object Models..

• The value of OO comes from
• Greater modeling power
• Increased reusability and extensibility
• Improved control of complexity

25
.Object Models.

• Example of class hierarchy a library system
  Fig. 7.10, Somm00

26
..Object Models

• Another example of class hierarchy Library
  users Fig. 7.11, Somm00

27
Object Models..

• Example of multiple inheritance Fig. 7.12,
  Somm00

28
.Object Models.

• Example of class structure with aggregation Fig.
  7.13, Somm00

29
..Object Models

• Example of sequence diagram Fig. 7.14, Somm00