System Design

Design concepts

A set of fundamental design concepts has evolved. They are:
  1. Abstraction - Abstraction is the process or result of generalization by reducing the information content of a concept or an observable phenomenon, typically in order to retain only information which is relevant for a particular purpose. (Generalize the problem and extract the key problem model. Do not puzzle in the decorated details)
  2. Refinement - It is the process of elaboration. A hierarchy is developed by decomposing a macroscopic statement of function in a step-wise fashion until programming language statements are reached. In each step, one or several instructions of a given program are decomposed into more detailed instructions. (Decomposing the problems to programming statements) Abstraction and Refinement are complementary concepts.
  3. Modularity - Software architecture is divided into components called modules. (Divide the problem into small functions)
  4. Software Architecture - It refers to the overall structure of the software and the ways in which that structure provides conceptual integrity for a system. A good software architecture will yield a good return on investment with respect to the desired outcome of the project, e.g. in terms of performance, quality, schedule and cost.
  5. Control Hierarchy - A program structure that represents the organization of a program component and implies a hierarchy of control.
  6. Structural Partitioning - The program structure can be divided both horizontally and vertically. Horizontal partitions define separate branches of modular hierarchy for each major program function. Vertical partitioning suggests that control and work should be distributed top down in the program structure.
  7. Data Structure - It is a representation of the logical relationship among individual elements of data. (Construct the logical relationship among data, representing as structs, class or extended as any basic or advanced data structure such as tree and graph)
  8. Software Procedure - It focuses on the processing of each modules individually
  9. Information Hiding - Modules should be specified and designed so that information contained within a module is inaccessible to other modules that have no need for such information


Design considerations

There are many aspects to consider in the design of a piece of software. The importance of each should reflect the goals the software is trying to achieve. Some of these aspects are:
  • Compatibility - The software is able to operate with other products that are designed for interoperability with another product. For example, a piece of software may be backward-compatible with an older version of itself.
  • Extensibility - New capabilities can be added to the software without major changes to the underlying architecture. (Interface and inheritance)
  • Fault-tolerance - The software is resistant to and able to recover from component failure.
  • Maintainability - A measure of how easily bug fixes or functional modifications can be accomplished. High maintainability can be the product of modularity and extensibility.
  • Modularity - the resulting software comprises well defined, independent components. That leads to better maintainability. The components could be then implemented and tested in isolation before being integrated to form a desired software system. This allows division of work in a software development project.
  • Reliability - The software is able to perform a required function under stated conditions for a specified period of time. ( Fully consideration of all possible edge cases)
  • Reusability - the software is able to add further features and modification with slight or no modification.
  • Robustness - The software is able to operate under stress or tolerate unpredictable or invalid input. For example, it can be designed with a resilience to low memory conditions.
  • Security - The software is able to withstand hostile acts and influences.
  • Usability - The software user interface must be usable for its target user/audience. Default values for the parameters must be chosen so that they are a good choice for the majority of the users.
  • Performance - The software performs its tasks within a user-acceptable time. The software does not consume too much memory. (The optimization. Considering trade-off between time and memory.)
  • Scalability - The software adapts well to increasing data or number of users.

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