#design-patterns

Explain the "Singleton" design pattern. What problem does it solve? Provide a code example of how to implement it in Java or Python. Discuss its pros and cons, and when it is appropriate to use.

What is Dependency Injection (DI)? How does it help in creating loosely coupled and more testable code? Provide a simple "before" and "after" code example in Java or C# that demonstrates the concept of constructor injection.

Apply design patterns to solve common problems. Creational: 1. Singleton (single instance). 2. Factory (object creation). 3. Builder (complex object construction). Structural: 4. Adapter (interface compatibility). 5. Decorator (add behavior dynamically). 6. Facade (simplified interface). Behavioral: 7. Observer (event notification). 8. Strategy (interchangeable algorithms). 9. Command (encapsulate requests). 10. Template Method (algorithm skeleton). Don't force patterns - use when appropriate. Understand problem first. Patterns improve communication ('let's use Observer here'). Study Gang of Four book. Practice with real examples.

Design microservices effectively. Patterns: 1. Service per business capability. 2. API Gateway for routing. 3. Service discovery (Consul, Eureka). 4. Circuit breaker for resilience. 5. Event-driven communication. 6. Database per service. 7. Saga pattern for distributed transactions. 8. CQRS for read/write separation. Use Docker and Kubernetes. Implement observability from start.