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

Design: Creational Design Patterns Usage Guide

Focus on the five main creational patterns: Singleton, Factory Method, Abstract Factory, Builder, Prototype. Exclude structural and behavioral patterns.

Functional Requirements

FR1: Explain when to use common creational design patterns

FR2: Provide clear criteria for choosing each pattern

FR3: Include examples of scenarios for each pattern

FR4: Support understanding for beginners and non-technical learners

Non-Functional Requirements

NFR1: Use simple language without jargon

NFR2: Keep explanations concise and relatable

NFR3: Focus on practical decision-making, not code details

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

❓ Question 6

Key Components

Singleton pattern for single instance control

Factory Method for flexible object creation

Abstract Factory for creating related object families

Builder for complex object construction

Prototype for cloning objects

Design Patterns

Singleton

Factory Method

Abstract Factory

Builder

Prototype

Reference Architecture

  +-------------------+       +-------------------+       +-------------------+
  |   Singleton       |       |   Factory Method   |       |  Abstract Factory  |
  |  (Single Instance)|       | (Create Objects)   |       | (Families of Obj)  |
  +---------+---------+       +---------+---------+       +---------+---------+
            |                           |                           |
            |                           |                           |
            v                           v                           v
  +-------------------+       +-------------------+       +-------------------+
  |    Builder        |       |    Prototype      |       |   Client Code      |
  | (Step-by-step     |       | (Clone Objects)   |       | (Uses Patterns)    |
  |  Construction)    |       +-------------------+       +-------------------+

Components

Singleton

Any OOP language

Ensure only one instance of a class exists and provide a global access point.

Factory Method

Any OOP language

Define an interface for creating an object but let subclasses decide which class to instantiate.

Abstract Factory

Any OOP language

Create families of related or dependent objects without specifying their concrete classes.

Builder

Any OOP language

Separate the construction of a complex object from its representation so the same construction process can create different representations.

Prototype

Any OOP language

Create new objects by copying an existing object, useful when object creation is costly.

Request Flow

1. Client needs an object and identifies the creation problem.

2. If only one instance is needed, use Singleton to get the instance.

3. If object creation varies by subclass, use Factory Method to delegate creation.

4. If a group of related objects is needed, use Abstract Factory to create them together.

5. If object construction is complex, use Builder to build it step-by-step.

6. If cloning existing objects is efficient, use Prototype to copy them.

7. Client uses the created object without worrying about creation details.

Database Schema

Not applicable for this conceptual design.

Scaling Discussion

Bottlenecks

Singleton can become a bottleneck if used in multithreaded environments without proper synchronization.

Factory Method and Abstract Factory can lead to many subclasses, increasing complexity.

Builder pattern may add overhead if object construction is simple.

Prototype requires careful cloning logic to avoid shared mutable state.

Solutions

Use thread-safe Singleton implementations or dependency injection to avoid bottlenecks.

Keep factory hierarchies manageable and use composition to reduce subclass explosion.

Use Builder only when object construction complexity justifies it.

Implement deep cloning carefully and document cloning behavior clearly.

Interview Tips

Time: Spend 10 minutes explaining each pattern briefly, 10 minutes discussing when to use them, and 5 minutes answering questions or giving examples.

Explain the problem each pattern solves in simple terms.

Describe real-life analogies (e.g., Singleton as a single remote control).

Discuss trade-offs and when not to use a pattern.

Show understanding of how patterns help manage object creation complexity.

Mention thread safety and complexity concerns where relevant.