Singleton pattern in LLD - System Design Guide

Problem Statement

When multiple parts of a program create their own instances of a class, it can lead to inconsistent states and wasted resources. This causes bugs that are hard to track and inefficient memory usage.

Solution

The Singleton pattern ensures that a class has only one instance throughout the program. It provides a global point of access to that instance, so all parts of the program use the same object, maintaining consistency and saving resources.

Architecture

Client A

↓

Singleton

Instance

↓

Client B

This diagram shows two clients accessing the same Singleton instance, ensuring only one object exists and is shared.

Trade-offs

✓ Pros

→

Ensures a single shared instance, preventing conflicting states.

→

Reduces memory usage by avoiding multiple object creations.

→

Provides a global access point simplifying object management.

✗ Cons

→

Can introduce hidden dependencies making testing harder.

→

May cause issues in multithreaded environments if not implemented carefully.

→

Violates single responsibility principle by controlling instance creation and business logic.

Use when exactly one instance of a class is needed across the entire application, such as configuration managers or logging services.

Avoid when multiple instances are required or in highly concurrent systems without thread-safe implementation.

Real World Examples

Netflix

Uses Singleton for centralized configuration management to ensure consistent settings across distributed services.

Amazon

Applies Singleton pattern in logging frameworks to maintain a single log manager instance.

LinkedIn

Employs Singleton for caching shared resources to optimize memory and access speed.

Code Example

The before code creates separate Logger instances, causing multiple objects. The after code uses __new__ to ensure only one SingletonLogger instance exists, shared by all.

LLD

### Before Singleton (no pattern)
class Logger:
    def __init__(self):
        self.logs = []

logger1 = Logger()
logger2 = Logger()
print(logger1 is logger2)  # False

### After Singleton pattern
class SingletonLogger:
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
            cls._instance.logs = []
        return cls._instance

logger1 = SingletonLogger()
logger2 = SingletonLogger()
print(logger1 is logger2)  # True

Alternatives

Dependency Injection

Instead of a class controlling its own instance, dependencies are provided externally, improving testability.

Use when: Choose when you want better modularity and easier unit testing.

Multiton

Allows a fixed number of instances identified by keys, unlike Singleton which allows only one.

Use when: Choose when you need controlled multiple instances rather than a single one.

Summary

Singleton pattern ensures only one instance of a class exists and provides a global access point to it.

It prevents inconsistent states and reduces resource usage by sharing the same object.

Careful implementation is needed to handle concurrency and testing challenges.

Practice

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

easy

A. To ensure only one instance of a class exists throughout the application

B. To allow multiple instances of a class with different states

C. To create a new instance every time it is requested

D. To inherit properties from multiple classes

Solution

1. Step 1: Understand Singleton pattern goal

   The Singleton pattern is designed to restrict a class to a single instance.

2. Step 2: Compare options with Singleton definition

   Only To ensure only one instance of a class exists throughout the application matches the purpose of Singleton by ensuring one instance exists.

3. Final Answer:

   To ensure only one instance of a class exists throughout the application -> Option A

4. Quick Check:

   Singleton = single instance [OK]

Hint: Singleton means one instance only, no duplicates [OK]

Common Mistakes:

• Thinking Singleton allows multiple instances
• Confusing Singleton with factory pattern
• Assuming Singleton creates new instances each call

2. Which of the following is the correct way to declare a Singleton instance method in a class?

easy

A. public static getInstance() { ... }

B. public void getInstance() { ... }

C. private static getInstance() { ... }

D. static getInstance() { ... }

Solution

1. Step 1: Identify Singleton instance method requirements

   The method to get the Singleton instance must be static and publicly accessible.

2. Step 2: Analyze options for correct access and static usage

   public static getInstance() { ... } is public and static, which is correct. public void getInstance() { ... } is not static, private static getInstance() { ... } is private, static getInstance() { ... } misses access modifier.

3. Final Answer:

   public static getInstance() { ... } -> Option A

4. Quick Check:

   Singleton instance method = public static [OK]

Hint: Singleton instance method must be public and static [OK]

Common Mistakes:

• Making getInstance() private or non-static
• Omitting access modifier
• Using void return type instead of instance

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

class Logger {
  static instance = null;
  constructor() {}
  static getInstance() {
    if (this.instance === null) {
      this.instance = new Logger();
    }
    return this.instance;
  }
}
const log1 = Logger.getInstance();
const log2 = Logger.getInstance();
console.log(log1 === log2);

medium

A. undefined

B. true

C. false

D. Error

Solution

1. Step 1: Understand Singleton instance creation

   The first call to getInstance() creates the instance; the second returns the same instance.

2. Step 2: Compare references of log1 and log2

   Both variables point to the same instance, so strict equality returns true.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Same instance check = true [OK]

Hint: Singleton returns same object reference every time [OK]

Common Mistakes:

• Assuming new instance created on each call
• Confusing equality with object content
• Expecting false due to separate variables

4. Identify the error in this Singleton implementation:

class Config {
  static instance;
  constructor() {}
  static getInstance() {
    if (!instance) {
      instance = new Config();
    }
    return instance;
  }
}

medium

A. No error, code is correct

B. Constructor should be public, not private

C. Missing 'this.' before 'instance' inside getInstance method

D. Static keyword is missing before getInstance method

Solution

1. Step 1: Check static variable access inside static method

   Static variables must be accessed with 'this.' prefix inside static methods.

2. Step 2: Identify missing 'this.' usage

   The code uses 'instance' instead of 'this.instance', causing a reference error.

3. Final Answer:

   Missing 'this.' before 'instance' inside getInstance method -> Option C

4. Quick Check:

   Static variable access requires 'this.' [OK]

Hint: Use 'this.' to access static variables inside static methods [OK]

Common Mistakes:

• Ignoring 'this.' prefix for static variables
• Changing constructor visibility incorrectly
• Assuming static keyword missing on method

5. You need to design a logging system using the Singleton pattern that supports multi-threaded access safely. Which approach best ensures thread safety while maintaining Singleton behavior?

hard

A. Use a global variable outside the class to store the instance

B. Create a new instance every time getInstance() is called

C. Make the constructor public and static

D. Use double-checked locking with a private static volatile instance variable

Solution

1. Step 1: Understand thread safety in Singleton

   In multi-threaded environments, multiple threads may create multiple instances if not synchronized.

2. Step 2: Evaluate options for thread-safe Singleton

   Double-checked locking with a volatile static instance ensures only one instance is created safely.

3. Final Answer:

   Use double-checked locking with a private static volatile instance variable -> Option D

4. Quick Check:

   Thread-safe Singleton = double-checked locking [OK]

Hint: Double-checked locking ensures thread-safe Singleton [OK]

Common Mistakes:

• Ignoring thread safety in Singleton
• Making constructor public defeats Singleton
• Using global variables breaks encapsulation