Factory Method pattern in LLD - Scalability & System Analysis

Scalability Analysis - Factory Method pattern

Growth Table: Factory Method Pattern

Users	Objects Created	Complexity	Impact on System
100 users	Low (hundreds)	Simple	Factory handles creation easily, no performance issues
10,000 users	Moderate (thousands)	Manageable	Factory method may need optimization, caching objects helps
1,000,000 users	High (millions)	Complex	Object creation cost adds up, factory needs pooling or reuse
100,000,000 users	Very High (hundreds of millions)	Very Complex	Factory method alone insufficient; must combine with caching, pooling, and distributed creation

First Bottleneck

The first bottleneck is the object creation overhead in the factory method. As user count grows, creating many objects repeatedly consumes CPU and memory, slowing down the system.

Scaling Solutions

• Object Pooling: Reuse objects instead of creating new ones each time to reduce CPU and memory load.
• Caching: Cache created objects when possible to avoid repeated creation.
• Lazy Initialization: Create objects only when needed, not upfront.
• Horizontal Scaling: Distribute factory instances across multiple servers to share load.
• Asynchronous Creation: Create objects in background threads to avoid blocking main flow.

Back-of-Envelope Cost Analysis

Assuming each user triggers 1 object creation per second:

• At 1,000 users: 1,000 creations/sec, manageable by a single server.
• At 1,000,000 users: 1 million creations/sec, requires multiple servers and pooling.
• Memory per object ~1KB, so 1 million objects = ~1GB memory usage if all live simultaneously.
• CPU usage grows linearly with creation rate; pooling reduces CPU by reusing objects.

Interview Tip

Start by explaining what the Factory Method pattern does: it creates objects without specifying exact classes. Then discuss how object creation cost grows with users. Identify bottlenecks like CPU and memory. Suggest solutions like pooling and caching. Finally, mention horizontal scaling and asynchronous creation to handle very high loads.

Self Check

Your database handles 1000 QPS. Traffic grows 10x. What do you do first?

Answer: Since the database is the bottleneck, first add read replicas or caching to reduce load. For Factory Method pattern, similarly, first optimize object creation by adding pooling or caching before scaling horizontally.

Key Result

Factory Method pattern scales well at low user counts but faces CPU and memory bottlenecks at high loads due to object creation overhead. Solutions include object pooling, caching, and horizontal scaling.

Practice

1. What is the main purpose of the Factory Method pattern in system design?

easy

A. To enforce a single instance of a class throughout the application

B. To create objects directly without any abstraction

C. To define an interface for creating objects but let subclasses decide which class to instantiate

D. To provide a global point of access to a resource

Solution

1. Step 1: Understand the Factory Method pattern goal

   The Factory Method pattern aims to separate object creation from usage by defining an interface for creating objects.

2. Step 2: Identify the role of subclasses

   Subclasses decide which specific class to instantiate, allowing flexibility and easy extension.

3. Final Answer:

   To define an interface for creating objects but let subclasses decide which class to instantiate -> Option C

4. Quick Check:

   Factory Method = Interface for object creation + subclass decides [OK]

Hint: Factory Method lets subclasses choose object types [OK]

Common Mistakes:

• Confusing Factory Method with Singleton pattern
• Thinking Factory Method creates objects directly without abstraction
• Mixing Factory Method with global access patterns

2. Which of the following is the correct way to declare a Factory Method in an interface in a typical object-oriented design?

easy

A. public Product createProduct();

B. public static Product createProduct();

C. public void createProduct();

D. private Product createProduct();

Solution

1. Step 1: Understand Factory Method signature

   The Factory Method returns an object of a product type, so it must have a return type matching the product interface or class.

2. Step 2: Check access and static modifiers

   Factory Methods are usually instance methods (not static) and public to allow subclass overriding and usage.

3. Final Answer:

   public Product createProduct(); -> Option A

4. Quick Check:

   Factory Method returns product instance, public and non-static [OK]

Hint: Factory Method returns product, not void or static [OK]

Common Mistakes:

• Using void return type instead of product
• Making Factory Method static which breaks subclass override
• Declaring Factory Method private so subclasses can't override

3. Given the following code snippet, what will be the output?

abstract class Creator {
  abstract Product factoryMethod();
  void someOperation() {
    Product product = factoryMethod();
    print(product.operation());
  }
}

class ConcreteProductA implements Product {
  String operation() => "Product A operation";
}

class ConcreteCreatorA extends Creator {
  Product factoryMethod() => ConcreteProductA();
}

void main() {
  Creator creator = ConcreteCreatorA();
  creator.someOperation();
}

medium

A. Product operation

B. No output

C. Error: factoryMethod not implemented

D. Product A operation

Solution

1. Step 1: Trace the factoryMethod call in ConcreteCreatorA

   ConcreteCreatorA overrides factoryMethod to return ConcreteProductA instance.

2. Step 2: Call operation() on ConcreteProductA instance

   ConcreteProductA's operation() returns "Product A operation" which is printed.

3. Final Answer:

   Product A operation -> Option D

4. Quick Check:

   ConcreteCreatorA creates ConcreteProductA, prints its operation [OK]

Hint: Factory Method returns ConcreteProductA, prints its operation [OK]

Common Mistakes:

• Assuming abstract Creator can be instantiated
• Forgetting ConcreteCreatorA overrides factoryMethod
• Expecting generic Product operation output

4. Identify the error in the following Factory Method implementation:

abstract class Creator {
  Product factoryMethod();
}

class ConcreteCreator extends Creator {
  void factoryMethod() {
    print("Creating product");
  }
}

medium

A. ConcreteCreator's factoryMethod has wrong return type

B. Creator class should not be abstract

C. factoryMethod should be static

D. No error, code is correct

Solution

1. Step 1: Check method signature in abstract Creator

   factoryMethod returns Product type, so subclass must match return type.

2. Step 2: Check ConcreteCreator's factoryMethod signature

   It returns void, which conflicts with abstract method's Product return type.

3. Final Answer:

   ConcreteCreator's factoryMethod has wrong return type -> Option A

4. Quick Check:

   Return type mismatch causes error [OK]

Hint: Return type must match abstract method in subclass [OK]

Common Mistakes:

• Ignoring return type mismatch
• Thinking abstract class can't have abstract methods
• Assuming static needed for Factory Method

5. You are designing a system that supports multiple types of notifications: Email, SMS, and Push. Using the Factory Method pattern, how should you organize your classes to allow easy addition of new notification types without modifying existing code?

hard

A. Create separate unrelated classes for each notification type without a common interface

B. Create an abstract NotificationCreator with a factoryMethod returning Notification; implement subclasses EmailCreator, SMSCreator, PushCreator each creating respective Notification objects

C. Use static methods in Notification class to create different notification types

D. Create a single Notification class with if-else to handle all notification types

Solution

1. Step 1: Apply Factory Method pattern structure

   Define an abstract creator class with a factoryMethod that returns a Notification interface or abstract class.

2. Step 2: Implement concrete creators for each notification type

   Each subclass (EmailCreator, SMSCreator, PushCreator) overrides factoryMethod to create its specific Notification object.

3. Step 3: Ensure easy extension without modifying existing code

   Adding new notification types means adding new creator subclasses without changing existing ones, following open-closed principle.

4. Final Answer:

   Create an abstract NotificationCreator with a factoryMethod returning Notification; implement subclasses EmailCreator, SMSCreator, PushCreator each creating respective Notification objects -> Option B

5. Quick Check:

   Factory Method enables easy extension via subclassing [OK]

Hint: Use abstract creator + subclasses for each notification type [OK]

Common Mistakes:

• Using if-else inside one class, violating open-closed principle
• Using static methods, reducing flexibility
• Not using a common interface for notifications