Design Patterns Selection Guide
When to Use Patterns
Creational Patterns
Use when object creation logic becomes complex or needs flexibility.
| Pattern | Use When |
|---|---|
| Factory Method | Creating objects without specifying exact class; subclasses decide |
| Abstract Factory | Creating families of related objects without concrete classes |
| Builder | Constructing complex objects step-by-step; many optional parameters |
| Prototype | Cloning existing objects; creation is expensive |
| Singleton | Exactly one instance needed globally (use sparingly) |
Structural Patterns
Use when composing objects into larger structures.
| Pattern | Use When |
|---|---|
| Adapter | Making incompatible interfaces work together |
| Bridge | Separating abstraction from implementation; both vary independently |
| Composite | Tree structures; treating individual objects and compositions uniformly |
| Decorator | Adding behavior dynamically without subclassing |
| Facade | Simplifying complex subsystem with unified interface |
| Flyweight | Many similar objects; sharing common state to save memory |
| Proxy | Controlling access; lazy loading, caching, access control |
Behavioral Patterns
Use when defining communication between objects.
| Pattern | Use When |
|---|---|
| Chain of Responsibility | Multiple handlers; request passed until handled |
| Command | Encapsulating requests as objects; undo/redo, queuing |
| Iterator | Sequential access without exposing underlying structure |
| Mediator | Reducing chaotic dependencies; centralized communication |
| Memento | Saving/restoring object state; undo functionality |
| Observer | One-to-many notifications; event systems |
| State | Behavior changes based on internal state |
| Strategy | Interchangeable algorithms; runtime selection |
| Template Method | Algorithm skeleton; subclasses override specific steps |
| Visitor | Adding operations to objects without modifying them |
Pattern Selection by Problem
| Problem | Recommended Pattern(s) |
|---|---|
| Object creation is complex | Builder, Factory Method |
| Need runtime algorithm selection | Strategy |
| Need to notify multiple objects | Observer |
| Incompatible interfaces | Adapter |
| Complex subsystem access | Facade |
| Adding features without subclassing | Decorator |
| Request handling chain | Chain of Responsibility |
| Undo/redo functionality | Command, Memento |
| State-dependent behavior | State |
| Tree/hierarchical structures | Composite |
Design Doc Integration
When specifying patterns in design docs:
markdown
## Design Patterns - **[Pattern Name]** - [Component/area] - [Brief rationale]
Example:
markdown
## Design Patterns - **Strategy** - PaymentProcessor - Runtime selection of payment providers - **Observer** - EventBus - Decoupled notification of order status changes - **Factory Method** - ServiceFactory - Creating service instances with dependencies
MCP Servers for Examples
Query language-specific MCP servers for implementation details:
- •
go-design-patterns- Go examples - •
ts-design-patterns- TypeScript examples - •
php-design-patterns- PHP examples