In the realm of software engineering, certain principles emerge from experience, often learned the hard way. A recent article highlights four key software design principles that can significantly impact the development process and the reliability of software systems. The first principle emphasizes the importance of maintaining a single source of truth. When data is stored in multiple locations, the risk of inconsistencies increases. For instance, in a frontend application displaying a bank balance, it is advisable to retrieve the balance directly from the server rather than storing it in multiple places. This approach minimizes synchronization issues and ensures that derived values, like a spendable balance, are calculated on-the-fly rather than stored separately. The overarching message is that derived data should be computed rather than duplicated to avoid potential bugs. The second principle challenges the conventional wisdom of "Don't Repeat Yourself" (DRY) by introducing the concept of "Please Repeat Yourself" (PRY). The author argues that striving for excessive reusability can lead to overly complex abstractions that lose their original purpose. Instead of forcing code into a single reusable class, it may be more effective to allow for some code duplication, which can simplify testing and maintenance. This principle acknowledges that while code reuse is valuable, it should not come at the cost of clarity and functionality. The third principle addresses the use of mocks in testing. While mocks can facilitate quick unit tests, they can also lead to issues when the mocked components do not accurately reflect the real dependencies. The author suggests that relying too heavily on mocks can compromise the reliability of tests, as they may not behave as expected in production. Instead, it is recommended to use real dependencies whenever possible, even if it means writing more comprehensive tests. This approach enhances the reliability of the software and reduces the likelihood of encountering issues in production. The final principle focuses on minimizing mutable state. The author argues that while caching and state management are essential in software development, it is crucial to evaluate what data truly needs to be stored versus what can be derived dynamically. By reducing mutable state, developers can avoid synchronization problems and streamline the development process. The principle advocates for a more straightforward approach, allowing for redundant calculations when necessary, as modern computing power can handle such tasks efficiently. These principles serve as valuable guidelines for software engineers, encouraging them to think critically about their design choices and the implications of those choices on the overall reliability and maintainability of their systems. Each principle highlights the importance of simplicity, clarity, and a thoughtful approach to software design, ultimately leading to more robust and effective software solutions.