SOFTWARE ENGINEERING (CO3001) Chapter 7 – Design and Implementation WEEK 10, 11, 12 Jan 2018 Chapter 7. Design and Implementation 2 Topics covered • Object-oriented design using the UML • Design patterns • Implementation issues • Open source development Jan 2018 Chapter 7. Design and Implementation 3 Development = Design + implementation • Software design: • identify software components and their relationships, based on a customer’s requirements. • Implementation: • realizing the design as a program.
• Software design and implementation activities are invariably inter-leaved. Design and Implementation 4 OBJECT-ORIENTED DESIGN USING THE UML Jan 2018 Chapter 7. Design and Implementation 5 An object-oriented design process using UML • Structured object-oriented design processes involve developing a number of different system models. • They require a lot of effort for development and maintenance of these models • for small systems, this may not be cost-effective.
• for large systems developed by different groups, design models are an important communication mechanism. Design and Implementation 6 Object-oriented design process • There are a variety of different object-oriented design processes that depend on the organization using the process. • Common activities in these processes include: • Define the context and modes of use of the system; • Design the system architecture; • Identify the principal system objects; • Develop design models; • Specify object interfaces. Design and Implementation 7 System context and interactions • Understanding the relationships between the developing software and its external environment • how to provide the required system functionality and how to structure the system to communicate with its environment.
• Understanding of the context also lets you establish the boundaries of the system. • System boundaries helps you decide what features are implemented in the system being designed and what features are in other associated systems. Design and Implementation 8 Context and interaction models • A system context model is a structural model that demonstrates the other systems in the environment of the system being developed. • An interaction model is a dynamic model that shows how the system interacts with its environment as it is used.
Design and Implementation 9 System context for the weather station Control 1 system 1 1 1.n Weather information station system 1 1.n Satellite 1 1 In this example, the system about the weather station is a system of weather stations as a weather information system, an onboard satellite system, and a control system. The cardinality information on the link shows that there is one control system but several weather stations, one satellite, and one general weather information system. Design and Implementation 10 Weather station use cases Report weather Report status Weather information system Restart Shutdown Reconfigure Control system Powersave Remote control Jan 2018 Chapter 7. Design and Implementation 11 Use case description - Report weather System Weather station Use case Report weather Actors Weather information system, Weather station Description The weather station sends a summary of the weather data that has been collected from the instruments in the collection period to the weather information system.
The data sent are the maximum, minimum, and average ground and air temperatures; the maximum, minimum, and average air pressures; the maximum, minimum, and average wind speeds; the total rainfall; and the wind direction as sampled at five-minute intervals. Stimulus The weather information system establishes a satellite communication link with the weather station and requests transmission of the data. Response The summarized data is sent to the weather information system. Comments Weather stations are usually asked to report once per hour but this frequency may differ from one station to another and may be modified in the future.
Design and Implementation 12 Architectural design • Once interactions between the system and its environment have been understood, we use this information for designing the system architecture. • Identify the major components that make up the system and their interactions, • and then may organize the components using an architectural pattern such as a layered or client-server model. Design and Implementation 13 High-level architecture of the weather station «subsystem» «subsystem» «subsystem» Fault manager Configuration manager Power manager Communication link «subsystem» «subsystem» «subsystem» Communications Data collection Instruments Jan 2018 Chapter 7. Design and Implementation 14 Architecture of data collection system Data collection Transmitter Receiver WeatherData Jan 2018 Chapter 7.
Design and Implementation 15 Object class identification • Any 'magic formula' for object identification? • NO Jan 2018 Chapter 7. Design and Implementation 16 Approaches to identification • Use a grammatical approach based on a natural language description of the system. • Base the identification on tangible things in the application domain. • Use a behavioural approach and identify objects based on what participates in what behaviour.
• Use a scenario-based analysis. The objects, attributes and methods in each scenario are identified. Design and Implementation 17 Example: Weather station description • A weather station is a package of software controlled instruments which collects data, performs some data processing and transmits this data for further processing. The instruments include air and ground thermometers, an anemometer, a wind vane, a barometer and a rain gauge.
Data is collected periodically. • When a command is issued to transmit the weather data, the weather station processes and summarises the collected data. The summarised data is transmitted to the mapping computer when a request is received. Design and Implementation 18 Weather station object classes • Object class identification in the weather station system may be based on the tangible hardware and data in the system: • Ground thermometer, Anemometer, Barometer • Application domain objects that are ‘hardware’ objects related to the instruments in the system.
• Weather station • The basic interface of the weather station to its environment. It therefore reflects the interactions identified in the use-case model. • Weather data • Encapsulates the summarized data from the instruments. Design and Implementation 19 Weather station object classes WeatherStation WeatherData identifier airTemperatures groundTemperatures reportWeather ( ) windSpeeds reportStatus ( ) windDirections powerSave (instruments) pressures remoteControl (commands) rainfall reconfigure (commands) restart (instruments) collect ( ) shutdown (instruments) summarize ( ) Ground Anemometer Barometer thermometer an_Ident bar_Ident gt_Ident windSpeed pressure temperature windDirection height get ( ) get ( ) get ( ) test ( ) test ( ) test ( ) Jan 2018 Chapter 7.
Design and Implementation 20 Design models • Design models show the objects and object classes and relationships between these entities. • Static models describe the static structure of the system in terms of object classes and relationships. • Dynamic models describe the dynamic interactions between objects. Design and Implementation 21 Examples of design models • Subsystem models that show logical groupings of objects into coherent subsystems.
• Sequence models that show the sequence of object interactions. • State machine models that show how individual objects change their state in response to events. • Other models include use-case models, aggregation models, generalisation models, etc. Design and Implementation 22 Subsystem models • Shows how the design is organised into logically related groups of objects.
• In the UML, these are shown using packages - an encapsulation construct. This is a logical model. The actual organisation of objects in the system may be different. Design and Implementation 23 Sequence models • Sequence models show the sequence of object interactions that take place • Objects are arranged horizontally across the top; • Time is represented vertically so models are read top to bottom; • Interactions are represented by labelled arrows, Different styles of arrow represent different types of interaction; • A thin rectangle in an object lifeline represents the time when the object is the controlling object in the system.
Design and Implementation 24 Sequence diagram describing data collection Weather information system :SatComms :WeatherStation :Commslink :WeatherData request (report) acknowledge reportWeather () acknowledge get (summary) summarize () send (report) acknowledge reply (report) acknowledge Jan 2018 Chapter 7. Design and Implementation 25 State diagrams • State diagrams are used to show how objects respond to different service requests and the state transitions triggered by these requests. • State diagrams are useful high-level models of a system or an object’s run-time behavior. • You don’t usually need a state diagram for all of the objects in the system.
Many of the objects in a system are relatively simple and a state model adds unnecessary detail to the design. Design and Implementation 26 Weather station state diagram Controlled Operation shutdown() remoteControl() reportStatus() restart() Testing Shutdown Running transmission done test complete configuration done reconfigure() Transmitting powerSave() clock collection done reportWeather() Configuring weather summary complete Summarizing Collecting Jan 2018 Chapter 7. Design and Implementation 27 Interface specification • Object interfaces have to be specified so that the objects and other components can be designed in parallel. • Designers should avoid designing the interface representation but should hide this in the object itself.
• Objects may have several interfaces which are viewpoints on the methods provided. • The UML uses class diagrams for interface specification but Java may also be used. Design and Implementation 28 Weather station interfaces «interface» «interface» Remote Control Reporting startInstrument(instrument): iStatus weatherReport (WS-Ident): Wreport stopInstrument (instrument): iStatus statusReport (WS-Ident): Sreport collectData (instrument): iStatus provideData (instrument ): string Jan 2018 Chapter 7. Design and Implementation 29 DESIGN PATTERNS Jan 2018 Chapter 7.
Design and Implementation 30 Design patterns • A design pattern is a way of reusing abstract knowledge about a problem and its solution. • A pattern is a description of the problem and the essence of its solution. • It should be sufficiently abstract to be reused in different settings. • Pattern descriptions usually make use of object-oriented characteristics such as inheritance and polymorphism.
Design and Implementation 31 Pattern elements • Name • A meaningful pattern identifier. • Not a concrete design but a template for a design solution that can be instantiated in different ways. • Consequences • The results and trade-offs of applying the pattern. Design and Implementation 32 The Observer pattern • Name • Observer.
• Description • Separates the display of object state from the object itself. • Problem description • Used when multiple displays of state are needed. • Solution description • See slide with UML description. • Consequences • Optimisations to enhance display performance are impractical.
Design and Implementation 33 The Observer pattern (1) Pattern Observer name Description Separates the display of the state of an object from the object itself and allows alternative displays to be provided. When the object state changes, all displays are automatically notified and updated to reflect the change. Problem In many situations, you have to provide multiple displays of state description information, such as a graphical display and a tabular display. Not all of these may be known when the information is specified.
All alternative presentations should support interaction and, when the state is changed, all displays must be updated. This pattern may be used in all situations where more than one display format for state information is required and where it is not necessary for the object that maintains the state information to know about the specific display formats used. Design and Implementation 34 The Observer pattern (2) Pattern name Observer Solution This involves two abstract objects, Subject and Observer, and two description concrete objects, ConcreteSubject and ConcreteObserver, which inherit the attributes of the related abstract objects. The abstract objects include general operations that are applicable in all situations.
The state to be displayed is maintained in ConcreteSubject, which inherits operations from Subject allowing it to add and remove Observers (each observer corresponds to a display) and to issue a notification when the state has changed. The ConcreteObserver maintains a copy of the state of ConcreteSubject and implements the Update() interface of Observer that allows these copies to be kept in step. The ConcreteObserver automatically displays the state and reflects changes whenever the state is updated. Consequences The subject only knows the abstract Observer and does not know details of the concrete class.
Therefore there is minimal coupling between these objects. Because of this lack of knowledge, optimizations that enhance display performance are impractical.