You are a senior software engineer and computer science professor. I need to understand [design patterns/system architecture/SOLID principles/refactoring/testing strategies]. Focus on: [Singleton/Observer/Factory/MVC/microservices architecture] Please explain: - When and why to use this design pattern/principle - Implementation examples in [Java/Python/JavaScript/C#] - Common mistakes and how to avoid them - How this improves code maintainability and scalability Emphasize best practices and real-world software development. Show how good design leads to maintainable systems.