When to use which structural pattern in LLD - System Design Exercise

Design: Structural Design Patterns Usage Guide

Covers main structural patterns like Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy. Does not cover behavioral or creational patterns.

Functional Requirements

FR1: Explain common structural design patterns

FR2: Describe scenarios when each pattern is appropriate

FR3: Provide simple examples of use cases

FR4: Highlight benefits and trade-offs of each pattern

Non-Functional Requirements

NFR1: Use clear, simple language without jargon

NFR2: Focus on practical, real-world applicability

NFR3: Keep explanations beginner-friendly

NFR4: Avoid overly complex or theoretical details

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

Key Components

Client code using the pattern

Interfaces or abstract classes involved

Concrete implementations or wrappers

Relationships between objects (composition, inheritance)

Design Patterns

Adapter

Bridge

Composite

Decorator

Facade

Flyweight

Proxy

Reference Architecture

Structural Patterns Overview

+----------------+      +----------------+      +----------------+
|    Adapter     | ---> |    Bridge      | ---> |   Composite    |
+----------------+      +----------------+      +----------------+
        |                      |                       |
        v                      v                       v
+----------------+      +----------------+      +----------------+
|   Decorator    | ---> |    Facade      | ---> |   Flyweight    |
+----------------+      +----------------+      +----------------+
                             |
                             v
                         +-----------+
                         |   Proxy   |
                         +-----------+

Components

Adapter

Any OOP language

Convert interface of one class to another expected by client

Bridge

Any OOP language

Separate abstraction from implementation to vary independently

Composite

Any OOP language

Compose objects into tree structures to represent part-whole hierarchies

Decorator

Any OOP language

Add responsibilities to objects dynamically without subclassing

Facade

Any OOP language

Provide a simple interface to a complex subsystem

Flyweight

Any OOP language

Share common parts of objects to save memory

Proxy

Any OOP language

Control access to another object, add extra functionality

Request Flow

1. Client calls Adapter to use incompatible interface as expected.

2. Bridge separates abstraction and implementation; client uses abstraction.

3. Composite allows client to treat individual and composite objects uniformly.

4. Decorator wraps object to add features without changing original code.

5. Facade simplifies client interaction by hiding subsystem complexity.

6. Flyweight shares common data among many objects to reduce memory use.

7. Proxy controls access to real object, adding security or lazy loading.

Database Schema

Not applicable for structural patterns; focus is on object relationships and code structure.

Scaling Discussion

Bottlenecks

Adapter can add overhead if many conversions are chained.

Bridge may increase complexity if abstraction and implementation grow independently.

Composite trees can become large and slow to traverse.

Decorator layers can cause deep wrapping and debugging difficulty.

Facade may hide too much, limiting flexibility.

Flyweight requires careful management of shared state to avoid errors.

Proxy can introduce latency or bottlenecks if overused.

Solutions

Keep Adapter usage minimal and well-defined.

Maintain clear separation and documentation in Bridge pattern.

Optimize Composite tree traversal and caching results.

Limit Decorator layers and use clear naming conventions.

Design Facade to balance simplicity and access to features.

Manage Flyweight shared data carefully with immutability where possible.

Use Proxy selectively and monitor performance impact.

Interview Tips

Time: Spend 10 minutes explaining each pattern briefly, 15 minutes discussing use cases and trade-offs, 10 minutes answering questions.

Explain problem each pattern solves with simple examples.

Describe when to choose one pattern over another.

Discuss benefits like flexibility, maintainability, and reuse.

Mention potential downsides and how to mitigate them.

Show understanding of object relationships and design goals.