Software Engineering Design and Implementation PDF

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 w...

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. 30/10/2014 Chapter 7 Design and Implementa3on 3 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. 30/10/2014 Chapter 7 Design and Implementa3on 4  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. 30/10/2014 Chapter 7 Design and Implementa3on 6 Process stages  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. 30/10/2014 Chapter 7 Design and Implementa3on 7 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. 30/10/2014 Chapter 7 Design and Implementa3on 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. 30/10/2014 Chapter 7 Design and Implementa3on 9 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. 30/10/2014 Chapter 7 Design and Implementa3on 13 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. 30/10/2014 Chapter 7 Design and Implementa3on 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. 30/10/2014 Chapter 7 Design and Implementa3on 17 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. 30/10/2014 Chapter 7 Design and Implementa3on 18 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. 30/10/2014 Chapter 7 Design and Implementa3on 20 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. 30/10/2014 Chapter 7 Design and Implementa3on 21 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. 30/10/2014 Chapter 7 Design and Implementa3on 22 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. 30/10/2014 Chapter 7 Design and Implementa3on 23 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. 30/10/2014 Chapter 7 Design and Implementa3on 25 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. 30/10/2014 Chapter 7 Design and Implementa3on 27 Implementation issues  Focus here is not on programming, although this is obviously important, but on other implementation issues that are often not covered in programming texts:  Reuse Most modern software is constructed by reusing existing components or systems. When you are developing software, you should make as much use as possible of existing code.  Configuration management During the development process, you have to keep track of the many different versions of each software component in a configuration management system.  Host-target development Production software does not usually execute on the same computer as the software development environment. Rather, you develop it on one computer (the host system) and execute it on a separate computer (the target system). 30/10/2014 Chapter 7 Design and Implementa3on 30 Reuse  From the 1960s to the 1990s, most new software was developed from scratch, by writing all code in a high- level programming language.  The only significant reuse or software was the reuse of functions and objects in programming language libraries.  Costs and schedule pressure mean that this approach became increasingly unviable, especially for commercial and Internet-based systems.  An approach to development based around the reuse of existing software emerged and is now generally used for business and scientific software. 30/10/2014 Chapter 7 Design and Implementa3on 31 Reuse levels  The abstraction level  At this level, you don’t reuse software directly but use knowledge of successful abstractions in the design of your software.  The object level  At this level, you directly reuse objects from a library rather than writing the code yourself.  The component level  Components are collections of objects and object classes that you reuse in application systems.  The system level  At this level, you reuse entire application systems. 30/10/2014 Chapter 7 Design and Implementa3on 32 Reuse costs  The costs of the time spent in looking for software to reuse and assessing whether or not it meets your needs.  Where applicable, the costs of buying the reusable software. For large off-the-shelf systems, these costs can be very high.  The costs of adapting and configuring the reusable software components or systems to reflect the requirements of the system that you are developing.  The costs of integrating reusable software elements with each other (if you are using software from different sources) and with the new code that you have developed. 30/10/2014 Chapter 7 Design and Implementa3on 34 Configuration management  Configuration management is the name given to the general process of managing a changing software system.  The aim of configuration management is to support the system integration process so that all developers can access the project code and documents in a controlled way, find out what changes have been made, and compile and link components to create a system.  See also Chapter 25. 30/10/2014 Chapter 7 Design and Implementa3on 35 Configuration management activities  Version management, where support is provided to keep track of the different versions of software components. Version management systems include facilities to coordinate development by several programmers.  System integration, where support is provided to help developers define what versions of components are used to create each version of a system. This description is then used to build a system automatically by compiling and linking the required components.  Problem tracking, where support is provided to allow users to report bugs and other problems, and to allow all developers to see who is working on these problems and when they are fixed. 30/10/2014 Chapter 7 Design and Implementa3on 36 Development platform tools  An integrated compiler and syntax-directed editing system that allows you to create, edit and compile code.  A language debugging system.  Graphical editing tools, such as tools to edit UML models.  Testing tools, such as Junit that can automatically run a set of tests on a new version of a program.  Project support tools that help you organize the code for different development projects. 30/10/2014 Chapter 7 Design and Implementa3on 38 Integrated development environments (IDEs)  Software development tools are often grouped to create an integrated development environment (IDE).  An IDE is a set of software tools that supports different aspects of software development, within some common framework and user interface.  IDEs are created to support development in a specific programming language such as Java. The language IDE may be developed specially, or may be an instantiation of a general-purpose IDE, with specific language-support tools. 30/10/2014 Chapter 7 Design and Implementa3on 39 Open source development  Open source development is an approach to software development in which the source code of a software system is published and volunteers are invited to participate in the development process  Its roots are in the Free Software Foundation (www.fsf.org), which advocates that source code should not be proprietary but rather should always be available for users to examine and modify as they wish.  Open source software extended this idea by using the Internet to recruit a much larger population of volunteer developers. Many of them are also users of the code. 30/10/2014 Chapter 7 Design and Implementa3on 41 Open source systems  The best-known open source product is, of course, the Linux operating system which is widely used as a server system and, increasingly, as a desktop environment.  Other important open source products are Java, the Apache web server and the mySQL database management system. 30/10/2014 Chapter 7 Design and Implementa3on 42 Open source issues  Should the product that is being developed make use of open source components?  Should an open source approach be used for the software’s development? 30/10/2014 Chapter 7 Design and Implementa3on 43 Open source business  More and more product companies are using an open source approach to development.  Their business model is not reliant on selling a software product but on selling support for that product.  They believe that involving the open source community will allow software to be developed more cheaply, more quickly and will create a community of users for the software. 30/10/2014 Chapter 7 Design and Implementa3on 44