Major Software Design Patterns

Software Design Patterns: Building Flexible, Maintainable & Scalable Systems

Software design patterns are proven, reusable approaches to solving common software design problems. Instead of reinventing a solution every time, developers can use a well-understood pattern.

The major patterns can be understood in three broad groups:

1. Creational Patterns

How should objects be created?

These patterns deal with object creation, making the system more flexible and reducing dependency on specific classes.

• Factory Method - Creates objects through a common interface while allowing the actual implementation to be chosen at runtime.
• Abstract Factory - Creates families of related objects without specifying their concrete classes.
• Builder - Constructs complex objects step-by-step, particularly useful when an object has many optional properties.
• Prototype - Creates new objects by copying or cloning an existing object.
• Singleton - Ensures that a class has only one instance and provides a common access point to it.

Think: “How can I create objects without tightly coupling my code to specific implementations?”

2. Structural Patterns

How should objects and components be connected?

These patterns focus on composing classes and objects to build larger, flexible structures.

• Adapter - Allows two incompatible interfaces to work together.
• Bridge - Separates an abstraction from its implementation so both can evolve independently.
• Composite - Allows individual objects and groups of objects to be treated in the same way.
• Decorator - Adds new behavior to an object dynamically without changing its original class.
• Facade - Provides a simple interface over a complex subsystem.
• Flyweight - Reduces memory usage by sharing common objects.
• Proxy - Provides a substitute or intermediary that controls access to another object.

Think: “How can I combine components without creating tightly coupled code?”

3. Behavioral Patterns

How should objects communicate and behave?

These patterns define how objects interact, communicate, and distribute responsibilities.

• Strategy - Allows different algorithms or behaviors to be switched without changing the client code.
• Observer - Automatically notifies dependent objects when something changes.
• Command - Encapsulates an action as an object, making it possible to queue, log, or undo actions.
• State - Allows an object to change its behavior when its internal state changes.
• Chain of Responsibility - Passes a request through a sequence of handlers until one handles it.
• Mediator - Uses a central object to coordinate communication between multiple objects.
• Iterator - Provides a consistent way to traverse a collection without exposing its internal structure.
• Template Method - Defines the overall structure of an algorithm while allowing specific steps to be customized.

Think: “How should different objects communicate and share responsibilities?”

Most Commonly Used in Modern Applications

Factory → Strategy → Observer → Adapter → Decorator → Facade → Repository → Dependency Injection → Command → State

Other Important Enterprise Patterns

Beyond the classic Gang of Four (GoF) patterns, modern software architecture commonly uses additional patterns.

• Repository - Separates business logic from data-access logic.
  Business Logic → Repository → Database
• Dependency Injection - Provides dependencies to a class rather than having the class create them itself. Reduces coupling and improves testability.
• MVC - Separates an application into: Model → View → Controller
  Commonly used in web applications.
• CQRS (Command Query Responsibility Segregation) - Separates operations that change data (Commands /Writes) from operations that read data (Queries /Reads).
  
• Service Layer - Centralizes application/business operations behind a defined service interface.
• Event-Driven Pattern - Components communicate through events rather than directly calling each other.
  Event → Producer → Event Bus → Consumers

Resilience & Distributed-System Patterns

For modern distributed and cloud applications, some particularly important patterns are:

• Retry - Automatically retries transient failures.
• Timeout - Stops waiting for an unresponsive service after a defined period.
• Circuit Breaker - Temporarily stops calls to a failing service to prevent cascading failures.
• Bulkhead - Isolates resources so failure in one area doesn't bring down the entire system.
• Fallback - Provides an alternative response when the primary operation fails.
• Rate Limiting - Controls the number of requests allowed within a period.
• Saga - Manages distributed transactions across multiple services.

In short:

Creational = Create

Structural = Connect

Behavioral = Communicate

Enterprise = Organize

Resilience = Survive failures