Singleton pattern in LLD - Architecture Diagram

System Overview - Singleton pattern

The Singleton pattern ensures a class has only one instance and provides a global point of access to it. It is commonly used when exactly one object is needed to coordinate actions across the system, such as a configuration manager or a logging service.

Architecture Diagram

  +-------------------+
  |    Client Code    |
  +---------+---------+
            |
            v
  +-------------------+
  | Singleton Class   |
  |  (Single Instance)|
  +---------+---------+
            |
            v
  +-------------------+
  |  Shared Resource  |
  +-------------------+

Components

Client Code

client

Requests access to the singleton instance

Singleton Class

service

Ensures only one instance exists and provides global access

Shared Resource

resource

The resource or data managed by the singleton instance

Request Flow - 5 Hops

Client Code → Singleton Class

Singleton Class → Singleton Class

Singleton Class → Singleton Class

Singleton Class → Client Code

Client Code → Shared Resource

Failure Scenario

Component Fails:Singleton Class

Impact:If the singleton instance creation fails, clients cannot access the shared resource, causing system features relying on it to fail.

Mitigation:Implement error handling during instance creation and fallback mechanisms or retries to ensure instance availability.

Architecture Quiz - 3 Questions

What is the main purpose of the Singleton Class in this architecture?

A To ensure only one instance exists and provide global access

B To handle multiple client requests simultaneously

C To store user data temporarily

D To balance load between servers

Design Principle

The Singleton pattern centralizes control by restricting instantiation to one object, ensuring consistent access to shared resources and preventing conflicts from multiple instances.

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