Software Design Patterns – A Short, Practical Guide

When developers talk about “writing clean code” or “building good architecture,” it often sounds like a mix of buzzwords and abstract theory. But a lot of it boils down to something very practical: knowing design patterns and how to use them well.

Design patterns aren’t magic formulas. They’re just tried-and-true ways of solving problems that developers have been running into for decades. Learning them will not only make your day-to-day coding easier and more maintainable, but it can also seriously boost your confidence in technical discussions and interviews.

In this article, we’ll go through some of the most common ( probably 90% of them ) software design patterns in a short and digestible format. By the end, you’ll have a cheat sheet you can keep in your back pocket that useful both when you’re deep in a project and when you’re getting ready for interviews and need to refresh your knowledge.

#Pattern Categories at a Glance

Before we dive into the specific patterns, it helps to know that most design patterns fall into a few main categories. Think of these as “categories” of solutions and each category focuses on a different type of problem you might face when building software.

#Creational Patterns

Focus on object creation mechanisms, optimizing how objects are created and managed.

• Singleton: Ensures a class has only one instance and provides a global point of access.
• Factory Method: Defines an interface for creating an object, but lets subclasses decide which class to instantiate.
• Abstract Factory: Creates families of related or dependent objects without specifying their concrete classes.
• Builder: Separates construction of a complex object from its representation, allowing different representations.
• Prototype: Creates new objects by cloning an existing object.

#Structural Patterns

Deal with object composition and relationships to form larger structures.

• Adapter: Allows incompatible interfaces to work together by converting one interface into another.
• Decorator: Adds behavior to objects dynamically without affecting other objects of the same class.
• Facade: Provides a simplified interface to a complex subsystem.
• Proxy: Controls access to an object, adding a layer for additional functionality like lazy loading or access control.
• Composite: Composes objects into tree structures to represent part-whole hierarchies.
• Flyweight: Reduces memory usage by sharing as much data as possible with similar objects.

#Behavioral Patterns

Focus on communication between objects and how they interact.

• Observer: Defines a one-to-many dependency so that when one object changes state, its dependents are notified automatically.
• Strategy: Defines a family of algorithms, encapsulates each one, and makes them interchangeable.
• Command: Encapsulates a request as an object, allowing parameterization and queuing.
• Iterator: Provides a way to access elements of a collection sequentially without exposing its underlying representation.
• State: Allows an object to change its behavior when its internal state changes.
• Chain of Responsibility: Passes a request along a chain of handlers until one handles it.
• Mediator: Defines an object that encapsulates how a set of objects interact.
• Visitor: Separates an algorithm from the object structure it operates on.

#Concurrency Patterns

Help manage parallelism and asynchronous operations.

• Queue (Producer–Consumer): Separates task producers and consumers using a queue, allowing asynchronous processing and workload balancing.
• Thread Pool: Uses a fixed set of worker threads to execute multiple tasks efficiently, avoiding the overhead of creating new threads.
• Reactor: Handles service requests concurrently by dispatching events to registered handlers; foundational for Node.js event loop.
• Future / Promise: Represents a value that will be available in the future, allowing non-blocking asynchronous operations.
• Saga Pattern: Manages long-running distributed transactions by executing steps independently and using compensating actions on failure.

#Architectural Patterns

Broader patterns that guide overall system architecture.

• Model-View-Controller (MVC): Separates application logic, UI, and input.
• Microservices: Breaks an app into small, loosely coupled services.
• Event-Driven Architecture: Components communicate via events.
• Layered (n-tier) Architecture: Divides system into layers with specific responsibilities.
• CQRS (Command Query Responsibility Segregation): Separates read and write models.
• Event Sourcing: Stores system state as a sequence of events.