©Ian Sommerville 1995 Software DesignSlide 1 Software Design u Deriving a solution which satisfies...
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©Ian Sommerville 1995 Software Design Slide 1
Software Design
Deriving a solution which satisfies software requirements
©Ian Sommerville 1995 Software Design Slide 2
Topics covered The design process and design methods Design strategies including object-oriented design
and functional decomposition - briefly Design quality attributes
©Ian Sommerville 1995 Software Design Slide 3
The design process Any design may be modelled as a directed
graph made up of entities with attributes which participate in relationships
The system should be described at several different levels of abstraction
Design takes place in overlapping stages. It is artificial to separate it into distinct phases but some separation is usually necessary
©Ian Sommerville 1995 Software Design Slide 4
Phases in the design process
Architecturaldesign
Abstractspecificatio
n
Interfacedesign
Componentdesign
Datastructuredesign
Algorithmdesign
Systemarchitecture
Softwarespecification
Interfacespecification
Componentspecification
Datastructure
specification
Algorithmspecification
Requirementsspecification
Design activities
Design products
©Ian Sommerville 1995 Software Design Slide 5
Design phases Architectural design Identify sub-systems Abstract specification Specify sub-systems Interface design Describe sub-system interfaces Component design Decompose sub-systems
into components Data structure design Design data structures to
hold problem data Algorithm design Design algorithms for problem
functions
©Ian Sommerville 1995 Software Design Slide 6
Hierarchical design structure
System level
Sub-systemlevel
©Ian Sommerville 1995 Software Design Slide 7
Top-down design In principle, top-down design involves starting
at the uppermost components in the hierarchy and working down the hierarchy level by level
In practice, large systems design is never truly top-down. Some branches are designed before others. Designers reuse experience (and sometimes components) during the design process
©Ian Sommerville 1995 Software Design Slide 8
Design methods Structured methods are sets of notations for
expressing a software design and guidelines for creating a design
Well-known methods include Structured Design (Yourdon), and JSD (Jackson Method)
Can be applied successfully because they support standard notations and ensure designs follow a standard form
Structured methods may be supported with CASE tools
©Ian Sommerville 1995 Software Design Slide 9
Method components Many methods support comparable views of a
system A data flow view (data flow diagrams) showing
data transformations An entity-relation view describing the logical
data structures A structural view showing system components
and their interactions
©Ian Sommerville 1995 Software Design Slide 10
Method deficiencies They are guidelines rather than methods in the
mathematical sense. Different designers create quite different system designs
They do not help much with the early, creative phase of design. Rather, they help the designer to structure and document his or her design ideas
©Ian Sommerville 1995 Software Design Slide 11
Design description Graphical notations. Used to display
component relationships Program description languages. Based on
programming languages but with more flexibility to represent abstract concepts
Informal text. Natural language description. All of these notations may be used in large
systems design
©Ian Sommerville 1995 Software Design Slide 12
Design strategies Functional design
• The system is designed from a functional viewpoint. The system state is centralised and shared between the functions operating on that state
Object-oriented design• The system is viewed as a collection of interacting
objects. The system state is de-centralised and each object manages its own state. Objects may be instances of an object class and communicate by exchanging methods
©Ian Sommerville 1995 Software Design Slide 13
Functional or DFD view of a compiler
AnalyseBuild
symboltable
Scansource
Generatecode
Symboltable
Outputerrors
Sourceprogram
Tokens Tokens Syntaxtree
Objectcode
ErrorindicatorSymbols Symbols
Errormessages
Poor example, see Safe Home Alarm System instead
©Ian Sommerville 1995 Software Design Slide 14
Object-oriented view of a compiler
Sourceprogram
Tokenstream
Symboltable
Syntaxtree
GrammarError
messages
Abstractcode
Objectcode
Scan Add
CheckGet
Build Print
Generate
Generate
Notice the main difference between this and previous diagram?
©Ian Sommerville 1995 Software Design Slide 15
Mixed-strategy design Although it is sometimes suggested that one
approach to design is superior, in practice, an object-oriented and a functional-oriented approach to design are complementary
Good software engineers should select the most appropriate approach for whatever sub-system is being designed
©Ian Sommerville 1995 Software Design Slide 16
Design quality Design quality is an elusive concept. Quality
depends on specific organisational priorities A 'good' design may be the most efficient, the
cheapest, the most maintainable, the most reliable, etc.
The attributes discussed here are concerned with the maintainability of the design
Quality characteristics are equally applicable to function-oriented and object-oriented designs
©Ian Sommerville 1995 Software Design Slide 17
Cohesion A measure of how well a component 'fits
together' A component should implement a single logical
entity or function Cohesion is a desirable design component
attribute as when a change has to be made, it is localised in a single cohesive component
Various levels of cohesion have been identified
©Ian Sommerville 1995 Software Design Slide 18
Cohesion levels
Coincidental cohesion (weak)• Parts of a component are simply bundled together
Logical association (weak)• Components which perform similar functions are
grouped• For example:
output text to screenoutput line to printeroutput record to file
• Seems ok • Problem is it carries out a range of similar but
different actions • No single well defined action
©Ian Sommerville 1995 Software Design Slide 19
Cohesion levels Temporal cohesion (weak)
• Components which are activated at the same time are grouped
• For example:clear screenopen fileInitialise total
• again not related • solution is to make initialisation module all other
specialised modules: call init_terminal call init_files call init_calculations
©Ian Sommerville 1995 Software Design Slide 20
Cohesion levels Communicational cohesion (medium)
• All the elements of a component operate on the same data
• e.g. display and log temperature Sequential cohesion (medium)
• The output for one part of a component is the input to another part
©Ian Sommerville 1995 Software Design Slide 21
Cohesion levels
Functional cohesion (strong)• optimal type of cohesion • performs a single well-defined action on a single data
object • e.g. calculate average • Each part of a component is necessary for the
execution of a single function Object cohesion (strong)
• Each operation provides functionality which allows object attributes to be modified or inspected
©Ian Sommerville 1995 Software Design Slide 22
Cohesion as a design attribute Not well-defined. Often difficult to classify
cohesion Inheriting attributes from super-classes
weakens cohesion To understand a component, the super-classes
as well as the component class must be examined
©Ian Sommerville 1995 Software Design Slide 23
A measure of the strength of the inter-connections between system components
Loose coupling means component changes are unlikely to affect other components
Shared variables or control information exchange lead to tight coupling
Loose coupling can be achieved by state decentralisation (as in objects) and component communication via parameters or message passing
Coupling
©Ian Sommerville 1995 Software Design Slide 24
Tight coupling
Module A Module B
Module C Module D
Shared dataarea
©Ian Sommerville 1995 Software Design Slide 25
Loose coupling
Module A
A’s data
Module B
B’s data
Module D
D’s data
Module C
C’s data
©Ian Sommerville 1995 Software Design Slide 26
Coupling levels Altering another modules code (LISP, Assembler) Modifying data within another module
• fault that appears in one module may be due to another • complicated understanding and debugging • can be done via global variables or pointers or call be
reference in C++ / Java Shared or global data
©Ian Sommerville 1995 Software Design Slide 27
Coupling levels Procedure call with a parameter that is a switch (or a
function pointer in C) • io (command, device buffer, length); • where command is 0,1,2 for read, write open; better to
use • read( device, buffer, length);
Procedure call with parameters that are pure data • ideal is call by value, where a small number of
parameters are used and a copy of the data is passed to the procedure invoked
• clear what information is being communicated
©Ian Sommerville 1995 Software Design Slide 28
Coupling levels Passing a serial data stream
• most ideal situation• one module passes stream of data to another• once passed data is outside control of process/module• like piping in UNIX• data can be thought of as a temporary intermediate file• only possible in languages that support concurrency
such as Ada and Erlang and Parallel C
©Ian Sommerville 1995 Software Design Slide 29
Object-oriented systems are loosely coupled because there is no shared state and objects communicate using message passing
However, an object class is coupled to its super-classes. Changes made to the attributes or operations in a super-class propagate to all sub-classes. Such changes must be carefully controlled
Coupling and Inheritance
©Ian Sommerville 1995 Software Design Slide 30
Cohesion, Coupling & Testing High Cohesion ensures each unit provides a set of
related capabilities and makes the tests of those capabilities easier to maintain
Low Coupling allows each unit to be effectively tested in isolation
©Ian Sommerville 1995 Software Design Slide 31
Related to several component characteristics• Cohesion. Can the component be understood on its
own?• Naming. Are meaningful names used?• Documentation. Is the design well-documented?• Complexity. Are complex algorithms used?
Informally, high complexity means many relationships between different parts of the design. hence it is hard to understand
Most design quality metrics are oriented towards complexity measurement. They are of limited use
Understandability
©Ian Sommerville 1995 Software Design Slide 32
A design is adaptable if:• Its components are loosely coupled• It is well-documented and the documentation is up to
date• There is an obvious correspondence between design
levels (design visibility)• Each component is a self-contained entity (tightly
cohesive) To adapt a design, it must be possible to trace the
links between design components so that change consequences can be analysed
Adaptability
©Ian Sommerville 1995 Software Design Slide 33
Design traceability
P O R
D
A
B
F
C
D Object interactionlevel
Object decompositionlevel
©Ian Sommerville 1995 Software Design Slide 34
Inheritance dramatically improves adaptability. Components may be adapted without change by deriving a sub-class and modifying that derived class
However, as the depth of the inheritance hierarchy increases, it becomes increasingly complex. It must be periodically reviewed and restructured• new trend in this area known as refactoring, associated
with the lightweight process of Extreme Programming (XP)
Adaptability and inheritance
Slide 35
Advantages of inheritance It is an abstraction mechanism which may be
used to classify entities - polymorphism It is a reuse mechanism at both the design and the
programming level - very useful in framework The inheritance graph is a source of
organisational knowledge about domains and systems
Slide 36
Problems with inheritance Object classes are not self-contained. they cannot
be understood without reference to their super-classes
Designers have a tendency to reuse the inheritance graph created during analysis. Can lead to significant inefficiency
The inheritance graphs of analysis, design and implementation have different functions and should be separately maintained
Slide 37
Inheritance and OOD There are differing views as to whether
inheritance is fundamental to OOD.• View 1. Identifying the inheritance hierarchy or network is a
fundamental part of object-oriented design. Obviously this can only be implemented using an OOPL.
• View 2. Inheritance is a useful implementation concept which allows reuse of attribute and operation definitions. Identifying an inheritance hierarchy at the design stage places unnecessary restrictions on the implementation
Inheritance introduces complexity and this is undesirable, especially in critical systems
Slide 38
UML associations Objects and object classes participate in relationships
with other objects and object classes In the UML, a generalised relationship is indicated by an
association Associations may be annotated with information that
describes the association Associations are general but may indicate that an attribute
of an object is an associated object or that a method relies on an associated object
Be careful to distinguish between a UML dependemcy and an association
Slide 39
An association model
EmployeeDepartment
Manager
is-member-of
is-managed-by
manages