Software Design and Implementation Phases

Posted on Mar 5, 2025 in Software Engineering

Software Design and Implementation

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.

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.

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.

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.

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.

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.

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 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.

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).

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.

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.