Chiaki Tech

    1.1.B. Software Engineering Overview - Good Software

    1.1. Software Engineering Overview

    Part b: Good Software

    How Successful Have We Been?

    • Enabled performing tasks quickly and effectively
      • Word processing, spreadsheets, e-mail
    • Support advances in medicine, agriculture, transportation, multimedia education, and most other industries
    • Many good stories
    • However, software is not without problems

    Terminology for Describing Bugs

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    There is some specific terminology to this.

    • A fault is what exists in the software that can lead to a failure.
      • As software developers, we will be looking at faults within the source code, within the configuration, or within intergration issues with other applications.
      • This is how we will try to solve the faults.
    • Failure
      • This is what the user sees from using the application.
      • It is what is observable by running the program and running into problems.
    • Human error
      • This is what leads to a bug existing in the code and reflecting itself in the real world when the user tries to use the application.
      • Human error can lead to the fault, which then can lead to the failure in the application.

    Not all faults lead to a failure because of the way that users use a particular program, there will likely be functionality that never ends up being used in the real-world.
    To emphasise, human error can lead to fault, which can lead to a failure. These are classified as bugs or defects. The terminology (human error, fault or failure) will depend on the perspective of a developer, a tester or a user.

    Examples of Software Failure

    • In the USA, IRS hired Sperry Corportation to build an automated federal income tax form processing process
      • An extra $90M was needed to enhance the original $103M product, losing $40.2M on interests + $22.3M in overtime wages because refunds were not returned on time.
    • Malfunctioning code in Therac-25 killed several people.
    • Reliability constraints have caused cancellation of many safety critical systems.
      • Safety-critical: something whose failure poses a threat to life or health -> should have a reliability of 10^9 (fail no more than once in 10^9 hours of operation)

    What is Good Software?

    • Good software engineering must always include a strategy for producting quality software.

      • Part of the problem was not understanding the requirements correctly. (e.g: not understanding what was necessary for the IRS/Therac-25 which led to probems)
      • There are problems which are not necessarily mistakes made during the programming stage, but mistakes that preceded these which are the cause of these issues.

    • There are three main ways of considering quality.

      • The quality of the product
        • Is it reliable and robust, or is it bug-ridden?
      • The quality of the process
        • Is there a good process in place to develop the software that wuld lead it to be high quality?
        • Do we conduct tests early or do we leave them for later in the process?
      • The quality of the product in the context of the business environment
        • Software is there for a reason to solve a particular business matter, so we need to take this into account.

    • Quality of the product can mean performing well in many categories, such as:

      • Correctness
      • Reliability
      • Efficiency
      • Usability
      • Maintainability
      • Reusability

    The Quality of the Process

    • Quality of the development and maintenance process is as important as the product quality.

      • Without a good process, there may be some mistakes that slip through until the end of development that may not be noticed until it is too late.

    • The development process needs to be modeled, to address questions such as:

      • Where to find a particular kind of fault
      • How to find faults early
      • How to build in fault tolerance
      • What are alternative activities

    • Models for process improvement

      • SEI's Capability Maturity Model (CMM)
      • ISO 9000
      • Software Process Improvement and Capability dEtermination (SPICE)

    The Quality in the Context of the Business Environment

    • Business value is as important as technical value for the success of a product.
    • A common approach to quantify: return on investment (ROI)
      • ROI is interpreted in different terms: reducing costs, predicting savings, improving productivity, and costs (efforts and resources)