Correctness by construction in software development

Ensuring the correctness of code is a top priority. Traditionally, the dominant approach has been test-driven development (TDD), which consists in writing tests to validate the code after it has been written. The core idea is to provide examples of desired behaviour (in the form of tests), and check the code output against them. While this has its benefits and has proven effective in many cases, there is an alternative approach which follows a completely different paradigm: Correctness by Construction (CbC). CbC is rotten in formal methods and mathematical proofs, and it aims to prevent errors in software code rather than solely relying on detecting them through tests. In this blog post, we will explore what CbC entails, its advantages over traditional TDD, and why software professionals of all levels should consider embracing it as a valuable methodology, or at least learn about it.

Test-Driven Development

Test-Driven Development (TDD) is a software development approach that emphasises writing tests before writing the actual code. It follows a cyclical process where developers write a failing test case, write the minimum amount of code to pass the test, refactor the code to improve its design, and then repeat the process for the next test. The cycle is often referred to as "red-green-refactor". 

This has proven effective in many scenarios, but it has two major drawbacks. First of all, there is an overemphasis on testing, which might lead to a big unbalance between test code and production code. Secondly, and most importantly, the correctness of the written code relies entirely on the accuracy and correctness of the tests that have been written and bugs might stay undetected in case some of the tests are inaccurate or do not consider all the possible scenarios.

Correctness by Construction

Correctness by Construction (CbC) is a paradigm that focuses on preventing errors in software code rather than relying solely on detecting them through tests. It emphasises the notion that if code is constructed correctly from the beginning, it is less likely to contain defects and bugs. The approach involves using formal methods, such as mathematical proofs and model checking, to establish the correctness of the code before it is even implemented. Its roots can be traced back to the early days of computer science when pioneers like E. Dijkstra and T. Hoare emphasised the importance of formal verification and rigorous reasoning about program correctness. Their work laid the foundation for the development of formal methods, which have been successfully applied in critical domains such as aerospace, nuclear systems, and medical devices. Over the years, CbC has evolved and found its place in mainstream software development practices: researchers and industry practitioners have developed various formal verification techniques, such as model checking, theorem proving, and abstract interpretation, to ensure the correctness and reliability of complex software systems.

Advantages of Correctness by Construction

Improved Code Quality: By employing rigorous formal methods, CbC raises the bar for code quality. It minimises the chances of introducing bugs and ensures that code adheres to desired specifications. This, in turn, reduces the time spent debugging and maintaining the software in the long run.

Early Error Detection: One of the major advantages of CbC is the early detection of errors. Instead of discovering defects during runtime or through extensive testing, potential issues are identified and resolved at the design stage itself. This results in significant savings in time and effort, as developers can catch and fix problems before they propagate throughout the codebase.

Increased Confidence: By relying on formal proofs and mathematical models, Correctness by Construction instils a higher level of confidence in the software's correctness. It provides a solid foundation for developers to build upon and facilitates better collaboration among team members. The confidence gained from knowing that code has been thoroughly verified promotes a positive work environment and encourages innovation.

Long-term Maintainability: Correctness by Construction leads to more maintainable codebases. With its emphasis on design clarity and correctness, developers can better understand the code's structure and purpose. This makes maintenance tasks, such as refactoring or adding new features, easier and less error-prone. Over time, this can significantly reduce technical debt and improve the overall sustainability of the software.

Disadvantages of Correctness by Construction

CbC has two major disadvantages, which are due to the complexity and difficulty of accurately following this paradigm.

Learning Curve: Adopting Correctness by Construction requires a steep learning curve, as it involves acquiring formal methods knowledge and developing new skills, which are not always common in software developers. However, with the right resources and training, developers can gradually master these techniques and overcome the initial hurdles.

Time and Effort: Applying formal methods to validate code correctness can be time-consuming, particularly for complex systems. The process involves upfront investment in designing formal specifications, creating proofs, and conducting model checking. However, the time and effort saved on debugging and maintenance in the long term can bring a greater return than the initial investment.

Conclusions

Correctness by Construction is a paradigm that emphasises building software correctly from the beginning, and it is based on using formal methods to ensure its correctness. It has clear advantages, such as improved code quality and early error detection, but it comes with some drawbacks, mostly related to the steep learning curve and the time and effort required to master it and validating code with formal methods. Software professionals have a responsibility to deliver high-quality and reliable software to users. Correctness by Construction provides an effective approach to achieve this goal. While Test-driven Development remains valuable, embracing Correctness by Construction might help in proactively preventing defects and designing more robust software systems. By investing in formal methods and adhering to the principles of Correctness by Construction, developers might work towards elevating the quality and maintainability of code, and build more robust and reliable software.