Software Standards Compliance 101: Using a formal requirements capture process
In the mid-1990s, a formal investigation was conducted into a series of fatal accidents with the Therac-25 radiotherapy machine. Led by Nancy Leveson of the University of Washington, the investigation resulted in a set of recommendations on how to create safety-critical software solutions in an objective manner. Since then, industries as disparate as aerospace, automotive and industrial control have encapsulated the practices and processes for creating safety- and/or security-critical systems in an objective manner into industry standards.
Although subtly different in wording and emphasis, the standards across industries follow a similar approach to ensuring the development of safe and/or secure systems. This common approach includes ten phases:
- Perform a system safety or security assessment
- Determine a target system failure rate
- Use the system target failure rate to determine the appropriate level of development rigor
- Use a formal requirements capture process
- Create software that adheres to an appropriate coding standard
- Trace all code back to their source requirements
- Develop all software and system test cases based on requirements
- Trace test cases to requirements
- Use coverage analysis to assess test completeness against both requirements and code
- For certification, collect and collate the process artifacts required to demonstrate that an appropriate level of rigor has been maintained.
Phase 4 is discussed in the main body of this article. As a key topic in many software development standards, this article will not focus on any particular software development standard, but will discuss several standards whose key concepts hinge on a robust requirements capture process.
Every software development standard, across every industry, incorporates requirements capture as a key component to the development process, and justifiably so. Several studies over the past 3 decades, including the “Chaos” reports published by the Standish Institute, seeking to identify the top causes of software project failure have revealed that one of the top causes of failure relates to the requirements elicitation, definition and management processes. Having a robust requirements capture and traceability process should therefore be considered the backbone to a robust software development process. But what are requirements?
There are many detailed technical definitions of the term “requirements”, all based around the concept that requirements are an expression of desired behavior. Probably the most succinct definition derives from a 1979 paper by Barry Boehm entitled Guidelines for Verifying and Validating Software Requirements and Design Specifications. In this paper, Boehm differentiates between validation and verification by defining each via the following questions:
Am I building the right product?”
Am I building the product right?”
Furthermore, he explains that the role of validation is to establish the fitness or worth of a software product for its operational mission. In other words, validation is used to determine whether a software product meets its requirements, so requirements can be defined as an expression of what the “right product” is.
Why are requirements so important? There are several levels of answer to this question. At its most basic, requirements form the foundation for a contract between a supplier and their customer. For the supplier, requirements provide the final reference for determining when the system is actually complete and that scope creep does not occur by implementing more than is expected. During development, requirements also form the baseline reference for verification; i.e. ensuring that the software components have been built right. For safety critical systems where software can cause personal injury (e.g. medical devices developed to meet the standard of the IEC 62304, Medical Device Software – Software Lifecycle Processes), requirements can also be a key component in defining the limits of a software developer’s liability as in the case of negligence lawsuits regulators will audit development and testing practices that are rooted in the system requirements.
Figure 1: Automate requirements to reduce costs and time. In this specific view, we can see that we want to verify that we have the necessary structural coverage. If the coverage is as expected, then we can observe that the status indicator turns from red (unverified) to green (verified). (Source: LDRA)
As a definition of “right product”, requirements need to encapsulate the needs of all of the stakeholders in a product. However, you can’t design a process that assumes requirements are stable as there will always be some degree of learning what the requirements really are while building the product. This concept is well understood in the development of avionic systems developed in accordance with the DO-178C standard, Software Considerations in Airborne Systems and Equipment Certification. Avionic systems requirements are divided into initial requirements, those that are provided as the original system definition, and derived requirements that are identified while building the product.