Introduction to Design Patterns

Design patterns are reusable solutions to common problems in software design. They represent best practices evolved over time by experienced software developers. Understanding and applying these patterns is crucial for building robust enterprise systems.

Creational Patterns

Singleton Pattern

Ensures a class has only one instance and provides a global point of access to it. Use cases include configuration managers, logging services, and database connection pools.

public class ConfigurationManager {
    private static ConfigurationManager instance;
    private ConfigurationManager() {}
    
    public static ConfigurationManager getInstance() {
        if (instance == null) {
            instance = new ConfigurationManager();
        }
        return instance;
    }
}

Factory Pattern

Provides an interface for creating objects without specifying their concrete classes. Essential for dependency injection and creating families of related objects.

Builder Pattern

Separates the construction of complex objects from their representation. Perfect for objects with many optional parameters.

Structural Patterns

Adapter Pattern

Allows incompatible interfaces to work together. Commonly used when integrating legacy systems with modern applications.

Facade Pattern

Provides a simplified interface to a complex subsystem. Reduces coupling between clients and complex systems.

Decorator Pattern

Adds behavior to objects dynamically without affecting other objects of the same class. Used extensively in I/O streams and middleware.

Behavioral Patterns

Observer Pattern

Defines a one-to-many dependency between objects. When one object changes state, all dependents are notified automatically. Foundation for event-driven architectures.

Strategy Pattern

Defines a family of algorithms and makes them interchangeable. Clients can switch algorithms at runtime without modifying the context.

Command Pattern

Encapsulates a request as an object, allowing parameterization and queuing of requests. Essential for implementing undo/redo functionality.

Enterprise Patterns

Repository Pattern

Mediates between the domain and data mapping layers. Provides a collection-like interface for accessing domain objects.

Unit of Work Pattern

Maintains a list of objects affected by a business transaction and coordinates writing out changes.

CQRS Pattern

Separates read and write operations for a data store. Improves performance, scalability, and security.

Best Practices

Don't force patterns where they don't fit
Understand the problem before applying a pattern
Consider the trade-offs of each pattern
Document pattern usage in your codebase
Combine patterns when appropriate

Conclusion

Design patterns are powerful tools in a software architect's toolkit. Master these patterns, understand their trade-offs, and apply them judiciously to create maintainable, scalable enterprise systems.

Essential Design Patterns for Enterprise Systems