Chapter 7 - Design and Implementation PDF

Chapter 7 – Design and Implementation Object-oriented design using the UML Design patterns Implementation issues Open source development 30/10/2014 Chapter 7 Design and Implementation 1 Design and implementation...

Chapter 7 – Design and Implementation Object-oriented design using the UML Design patterns Implementation issues Open source development 30/10/2014 Chapter 7 Design and Implementation 1 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 Implementation 2 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 Implementation 3  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 Implementation 4 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. 30/10/2014 Chapter 7 Design and Implementation 5 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 Implementation 6 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. 30/10/2014 Chapter 7 Design and Implementation 7 High-level architecture of the weather station 30/10/2014 Chapter 7 Design and Implementation 8 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 Implementation 9 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 Implementation 10 Examples of design models Subsystem models that show logical groupings of objects into coherent subsystems (packages). 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 Implementation 11 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. 30/10/2014 Chapter 7 Design and Implementation 12 Pattern elements Name A meaningful pattern identifier. Problem description. Solution description. 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. 30/10/2014 Chapter 7 Design and Implementation 13 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 Implementation 14 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. 30/10/2014 Chapter 7 Design and Implementation 15 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 Implementation 16 Open source business More and more product companies are using an open source approach to development. 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 Implementation 17 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. 30/10/2014 Chapter 7 Design and Implementation 18 Key points Software design and implementation are inter-leaved activities. 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. When developing software, you should always consider the possibility of reusing existing software, either as components, services or complete systems. Open source development involves making the source code of a system publicly available. 30/10/2014 Chapter 7 Design and Implementation 19