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1
Software Design
Reference: Software Engineering, by Ian Sommerville, Ch. 12 & 13, 5th edition and Ch. 10, 6th edition
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Topics• What is Design?• Design Description • The Design Process
– Architectural Design
• Design Strategies– Functional– OO
• Design Quality– Component Cohesion– Component Coupling
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What is Design?
• Where informal ideas are transformed to detailed implementation descriptions
• It is a creative process
• There is no design “cookbook”
• It is learned by experience and study of existing systems
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Design DescriptionThree main design notations:
• Graphical notations– Display relationships between components– Relate the design to the real-world system
• Program Description Languages (PDLs)– Pseudocode
• Informal text– For anything that can’t be described formally
(e.g., design rationale, non-functional considerations)
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The Design Process
• Architectural Design– Subsystems and their relationships are
identified and documented
• Abstract Specification– Document an abstract specification of the
services provided by and constraints on each subsystem
• Interface Design– Document each subsystem’s interface
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The Design Process (con’t)
• Component Design– Break subsystems into components and
document their interfaces
• Data Structure Design– Specify the data structures used in the
system implementation
• Algorithm Design– Specify the implementation algorithms
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Architectural DesignHow a system is decomposed into subsystems
that provide some related set of services• Different “flavors” of architectural design
models:– Structural models– Control models– Others
• There is no one “correct” architectural design model.
• Designs may be hybrids of several models or even self-defined.
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Structural Model Examples
• Simple block diagram
• Domain-independent architectures– Repository Model– Client-Server Model– Event-driven Model– Many others ...
• Domain-specific architectures
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Simple Block Diagram
• Presents an overview of the system structure (subsystems) and their interconnections
• Good overview, but may not present enough information (e.g., connection to databases)
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Alarmcontroller
Voicesynthesizer
Movementsensors
Siren
Doorsensors
Telephonecaller
Externalcontrol centre
Intruder Alarm System
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Visionsystem
Objectidentification
system
Armcontroller
Grippercontroller
Packagingselectionsystem
Packingsystem
Conveyorcontroller
A Packing Robot Control System
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Repository Model
• Systems which use large amounts of data are organized around a shared database or repository
• Suited to applications where data is generated by one subsystem and used by others
• Example: a management information system
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Student Information Repository
CourseScheduleGenerator
TranscriptGenerator
GraduationCheckoutSystem
StudentRegistration
System
Grade ReportGenerator
A Student Information System
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Projectrepository
Designtranslator
Programeditor
Designeditor
Codegenerator
Designanalyser
Reportgenerator
A CASE Toolset
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Client-Server Model
• A distributed system model which shows how data and processing is distributed across a range of processors
• Servers offer services to other subsystems
• Clients call on the services offered by the servers
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Film and picture library
Catalogueserver
Catalogue
Videoserver
Film clipfiles
Pictureserver
Digitizedphotographs
Hypertextserver
Hypertextweb
Client 1 Client 2 Client 3 Client 4
Wide-bandwidth network
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Control Models
Are concerned with the control flow between subsystems.
Examples:– Centralized control: One subsystem has
overall responsibility for control and starts and stops other subsystems
– Event-driven: System is driven by externally generated events where the timing of the event is out of the control of the subsystem(s) that processes the event (e.g., spreadsheets, GUIs)
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Design Strategies
• Functional DesignSystem is designed from a functional
viewpoint, starting with a high-level view and refining this into a more detailed design. The system state is centralized and shared between the functions.
• Object-oriented DesignSystem is viewed as a collection of objects
rather than functions. The system state is decentralized. Each object manages its own information.
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Design Quality
What is “good” design? No general agreement, but ...
• Should correctly implement specification• Must be understandable
– Good naming conventions– Good internal and external documentation– Minimize complex algorithms
• Must be able to adapt to modification or addition of new functionality– High component cohesion– Low component coupling
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Component Cohesion
• A measure of the closeness of the relationships between the component’s components
• Component should implement a single logical function/task (functional) or implement a single logical entity (OO)
• We want strong cohesion
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Component Cohesion (con’t)7 levels of cohesion (Constantine &
Yourdan), weakest to strongest:• Coincidental Cohesion
– The parts of a component are not related but simply bundled into a single component
• Logical Association– Components that perform similar functions
such as input, error handling and so on are put together in a single component
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Component Cohesion (con’t)• Temporal Association
– All of the components that are activated at a single time, such as start up or shut down, are brought together
• Procedural Cohesion– The elements in a component make up a
single control sequence
• Communicational Cohesion– All of the elements of a component operate
on the same input data or produce the same output data
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Component Cohesion (con’t)
• Sequential Cohesion– The output from one element in the component
serves as input for some other element
• Functional Cohesion– Each part of the component is necessary for
the execution of a single function/task
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Component Cohesion (con’t)Cohesion applies to both functional and
OO design approaches:
• Cohesive Function– Performs a single task
• Cohesive Object– A single entity is represented and all the
operations on that entity are included with the object
So, which promotes strong cohesion better -- functional or OO design?
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Component Coupling
• A measure of the strength of the interconnections between components in a design
• Want components to be as independent as possible
• We want low coupling
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Component Coupling (con’t)
• Functional Design– No/little global data– No hard-coded constants– Nothing that causes one function to require
knowledge of another’s implementation
• OO Design– Inheritance by nature causes coupling between
base and derived classes– Multiple inheritance greatly increases coupling
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How Components Are Coupled• References from one component to another, such as invocation
• Amount of data passed from one component to another
• Amount of control one component has over another
• Degree of complexity of interface, e.g., one entry point vs. mutual entry points
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Goal is to Minimize Coupling• Enables us to change portion of system while disrupting rest of system little as possible
• Very low coupling might allow pull-out, plug-in replacement of only one component
• Loose coupling may require changing or replacing a few components
• High coupling may require widespread perturbations in system
• Low coupling reduces the number of components needing revision
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Types of Coupling
• Content: one component directly modifies data or control flow of another
• Common: Organizing data into a common store
• Control: Control flags passed as parameters between components
• Stamp: Data structures passed• Data: Only primitive data passed