Chapter 7 - Design and Implementation PDF

Chapter 7 – Design and Implementation Chapter 7 Design and Implementation 1 Topics covered  Object-oriented design using the UML  Design patterns  Implementation issues  Open source development Chapter 7 Design and Implementation 2 Design and implemen...

Chapter 7 – Design and Implementation Chapter 7 Design and Implementation 1 Topics covered  Object-oriented design using the UML  Design patterns  Implementation issues  Open source development Chapter 7 Design and Implementation 2 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 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. Chapter 7 Design and Implementation 4 Implementation issues Chapter 7 Design and Implementation 39 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). Chapter 7 Design and Implementation 40 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. Chapter 7 Design and Implementation 41 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. Chapter 7 Design and Implementation 42 Software reuse Chapter 7 Design and Implementation 43 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. Chapter 7 Design and Implementation 44 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. Chapter 7 Design and Implementation 45 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. Chapter 7 Design and Implementation 46 Host-target development  Most software is developed on one computer (the host), but runs on a separate machine (the target).  More generally, we can talk about a development platform and an execution platform.  A platform is more than just hardware.  It includes the installed operating system plus other supporting software such as a database management system or, for development platforms, an interactive development environment.  Development platform usually has different installed software than execution platform; these platforms may have different architectures. Chapter 7 Design and Implementation 48 Host-target development Chapter 7 Design and Implementation 49 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. Chapter 7 Design and Implementation 50 Open source development Chapter 7 Design and Implementation 53 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. Chapter 7 Design and Implementation 54 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. Chapter 7 Design and Implementation 55 Open source licensing  A fundamental principle of open-source development is that source code should be freely available, this does not mean that anyone can do as they wish with that code.  Legally, the developer of the code (either a company or an individual) still owns the code. They can place restrictions on how it is used by including legally binding conditions in an open source software license.  Some open source developers believe that if an open source component is used to develop a new system, then that system should also be open source.  Others are willing to allow their code to be used without this restriction. The developed systems may be proprietary and sold as closed source systems. Chapter 7 Design and Implementation 58 License models  The GNU General Public License (GPL). This is a so-called ‘reciprocal’ license that means that if you use open source software that is licensed under the GPL license, then you must make that software open source.  The GNU Lesser General Public License (LGPL) is a variant of the GPL license where you can write components that link to open source code without having to publish the source of these components.  The Berkley Standard Distribution (BSD) License. This is a non-reciprocal license, which means you are not obliged to re- publish any changes or modifications made to open source code. You can include the code in proprietary systems that are sold. Chapter 7 Design and Implementation 59 Key points  Software design and implementation are inter-leaved activities. The level of detail in the design depends on the type of system and whether you are using a plan-driven or agile approach.  The process of object-oriented design includes activities to design the system architecture, identify objects in the system, describe the design using different object models and document the component interfaces.  A range of different models may be produced during an object- oriented design process. These include static models (class models, generalization models, association models) and dynamic models (sequence models, state machine models).  Component interfaces must be defined precisely so that other objects can use them. A UML interface stereotype may be used to define interfaces. Chapter 7 Design and Implementation 61 Key points  When developing software, you should always consider the possibility of reusing existing software, either as components, services or complete systems.  Configuration management is the process of managing changes to an evolving software system. It is essential when a team of people are cooperating to develop software.  Most software development is host-target development. You use an IDE on a host machine to develop the software, which is transferred to a target machine for execution.  Open source development involves making the source code of a system publicly available. This means that many people can propose changes and improvements to the software. Chapter 7 Design and Implementation 62