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Figures – Chapter 7 Chapter 7 – Design and Implementation Chapter 7 Design and Implementation 130/10/2014 Topics covered Object-oriented design using the UML Design patterns Implementation issues Open source development Chapter 7 Design and Implementation 230/10/2014 Design and implementation Software design and implementation is the stage in the software engineering process at which an executable software system is developed. Software design and implementation activities are invariably inter-leaved. Software design is a creative activity in which you identify software components and their relationships, based on a customer’s requirements. Implementation is the process of realizing the design as a program. Chapter 7 Design and Implementation 330/10/2014 Build or buy In a wide range of domains, it is now possible to buy off- the-shelf systems (COTS) that can be adapted and tailored to the users’ requirements. For example, if you want to implement a medical records system, you can buy a package that is already used in hospitals. It can be cheaper and faster to use this approach rather than developing a system in a conventional programming language. When you develop an application in this way, the design process becomes concerned with how to use the configuration features of that system to deliver the system requirements. Chapter 7 Design and Implementation 430/10/2014 Object-oriented design using the UML Chapter 7 Design and Implementation 530/10/2014 An object-oriented design process 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 and, for small systems, this may not be cost-effective. However, for large systems developed by different groups design models are an important communication mechanism. Chapter 7 Design and Implementation 630/10/2014 Process stages An OO system is made up of interacting objects that maintain their own local state and provide operations on that state. 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. Process illustrated here using a design for a wilderness weather station. Chapter 7 Design and Implementation 730/10/2014 System context and interactions Understanding the relationships between the software that is being designed and its external environment is essential for deciding 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. Setting the system boundaries helps you decide what features are implemented in the system being designed and what features are in other associated systems. Chapter 7 Design and Implementation 830/10/2014 Wilderness weather station The government of a country with large areas of wilderness decides to deploy several hundred weather stations in remote areas. Weather stations collect data from a set of instruments that measure temperature and pressure, sunshine, rainfall, wind speed and wind direction. Instruments measure weather parameters such as the wind speed and direction, the ground and air temperatures, the barometric pressure and the rainfall over a 24-hour period. Each of these instruments is controlled by a software system that takes parameter readings periodically and manages the data collected from the instruments. Chapter 1 Introduction 30/10/2014 9 Weather information system The weather station system This is responsible for collecting weather data, carrying out some initial data processing and transmitting it to the data management system. The data management and archiving system This system collects the data from all of the wilderness weather stations, carries out data processing and analysis and archives the data. The station maintenance system This system can communicate by satellite with all wilderness weather stations to monitor the health of these systems and provide reports of problems. Chapter 1 Introduction 30/10/2014 10 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. Can be documented using a simple block diagram, showing the entities and their associations. An interaction model is a dynamic model that shows how the system interacts with its environment as it is used. Can be represented with a use case model. • Use structured natural language to describe use cases. Chapter 7 Design and Implementation 1130/10/2014 System context for the weather station Chapter 7 Design and Implementation 1230/10/2014 Weather station use cases Chapter 7 Design and Implementation 1330/10/2014 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. Chapter 7 Design and Implementation 1430/10/2014 Architectural design Once interactions between the system and its environment have been understood, you use this information for designing the system architecture. You 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. The weather station is composed of independent subsystems that communicate by broadcasting messages on a common infrastructure. Chapter 7 Design and Implementation 1530/10/2014 High-level architecture of the weather station Chapter 7 Design and Implementation 1630/10/2014 Architecture of data collection system Chapter 7 Design and Implementation 1730/10/2014 Object class identification Identifying object classes is often a difficult part of object oriented design. There is no 'magic formula' for object identification. It relies on the skill, experience and domain knowledge of system designers. Object identification is an iterative process. You are unlikely to get it right first time. Chapter 7 Design and Implementation 1830/10/2014 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. Chapter 7 Design and Implementation 1930/10/2014 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 summarizes the collected data. The summarized data is transmitted to the mapping computer when a request is received. 30/10/2014 Chapter 7 Design and Implementation 20 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. Chapter 7 Design and Implementation 2130/10/2014 Weather station object classes Chapter 7 Design and Implementation 2230/10/2014 Design models Design models show the objects and object classes and relationships between these entities. There are two kinds of design model: Structural models describe the static structure of the system in terms of object classes and relationships. Dynamic models describe the dynamic interactions between objects. Three UML model types are particularly useful for adding detail to use case and architectural models: • Subsystem models • Sequence models • State machine models Chapter 7 Design and Implementation 2330/10/2014 Examples of design models Subsystem models that show logical groupings of objects into coherent subsystems. Represented using a form of class diagram with each system shown as a package with enclosed objects. This is a logical model. The actual organization of objects in the system may be different. Sequence models that show the sequence of object interactions. Represented as a UML sequence diagram State machine models that show how individual objects change their state in response to events. Represented as a UML state diagram Other models include use-case models, aggregation models, generalisation models, etc. Chapter 7 Design and Implementation 2430/10/2014 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. Chapter 7 Design and Implementation 2530/10/2014 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. Chapter 7 Design and Implementation 2630/10/2014 Sequence diagram describing data collection Chapter 7 Design and Implementation 2730/10/2014 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. Chapter 7 Design and Implementation 2830/10/2014 Weather station state diagram Chapter 7 Design and Implementation 2930/10/2014 Interface specification