Lehman’s Laws of Software Evolution

In the world of software engineering, the Lehman's Laws of Software Evolution are widely recognized as a fundamental framework for understanding how software systems evolve over time. These laws were introduced by British computer scientist, Meir Lehman, in the 1980s(!) in his paper “Programs, Life Cycles, and Laws of Software Evolution”, and they remain relevant to this day.

Lehman's Laws describe how software systems evolve and how they can be managed over their lifespan. In this blog post, we will explore these laws in detail and their significance in software development.

From the paper:

The first law of software evolution states that software systems must evolve or they will become progressively less useful. This law recognizes that software is not static and unchanging, but instead must adapt and grow to meet changing user needs and technological advancements. Failure to evolve software systems can result in decreased usefulness or even obsolescence. This means that software systems are never “DONE”, they only become obsolete.

The second law of software evolution states that the complexity of software systems will increase over time unless they are actively reduced. As software systems grow and evolve, they become more complex, which can make them more difficult to maintain, debug, and enhance. This complexity can also lead to an increase in defects and errors. To combat this, it is essential to take active steps to reduce complexity and streamline software systems. Yes, we are talking about technical debt.

Side note: Check out this interesting talk about technical debt:

The third law of software evolution states that software evolution is a continuous process. This law recognizes that software systems are not static and unchanging, but rather they are continuously evolving. As such, software development is an ongoing process that requires continual improvement and adaptation.

The fourth law of software evolution states that software systems are subject to feedback loops. As software systems evolve, they receive feedback from users, stakeholders, and other sources. This feedback can help guide the evolution of the software, ensuring that it continues to meet user needs and remains relevant in the face of changing technologies.

The fifth law of software evolution states that software systems are subject to both incremental and radical change. As software systems evolve, they can undergo incremental changes that build on existing functionality and improve the system gradually. Alternatively, software systems can undergo radical changes that fundamentally alter the system's architecture or functionality.

The sixth law of software evolution states that software evolution is constrained by organizational stability and the ability of developers to understand the system. This law recognizes that software development is an inherently human activity, and as such, the people involved in the process play a crucial role in shaping the evolution of the software.

The seventh law of software evolution states that the evolution of a software system is limited by its architecture. This law recognizes that the architecture of a software system plays a critical role in determining its evolution. A well-designed architecture can facilitate the evolution of the system, while a poorly designed architecture can limit its potential.

In conclusion, the Lehman's Laws of Software Evolution provide a valuable framework for understanding how software systems evolve over time. By recognizing the continuous nature of software development and the role of people, architecture, and feedback in shaping software evolution, developers can create more robust and adaptable software systems.