Singleton pattern in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why use the Singleton pattern?

Which of the following best explains the main purpose of the Singleton pattern in system design?

A To allow multiple instances of a class to be created with different configurations.

B To ensure a class has only one instance and provide a global point of access to it.

C To separate the interface from the implementation of a class.

D To enable objects to be created without specifying the exact class.

💡 Hint

Think about controlling the number of instances of a class.

The Singleton pattern restricts a class to a single instance and provides a global access point to that instance, ensuring controlled access and consistent state.

❓ Architecture

intermediate

Singleton implementation in a multithreaded environment

Which design approach correctly implements a thread-safe Singleton pattern to avoid multiple instances in a multithreaded system?

A Use eager initialization by creating the instance at class loading time, ensuring thread safety.

B Use lazy initialization without synchronization, creating the instance when first requested.

C Create a new instance every time the getInstance method is called.

D Use a static variable without any synchronization or initialization.

💡 Hint

Consider thread safety and when the instance is created.

Eager initialization creates the instance when the class loads, which is inherently thread-safe because the instance is created before any thread accesses it.

❓ scaling

advanced

Scaling Singleton in distributed systems

What is the main challenge when using the Singleton pattern in a distributed system with multiple servers?

A Using Singleton to create multiple instances for load balancing.

B Allowing each server to create its own independent Singleton instance.

C Avoiding the use of Singleton to reduce memory usage on each server.

D Ensuring that only one instance exists across all servers simultaneously.

💡 Hint

Think about instance uniqueness across multiple machines.

In distributed systems, the Singleton pattern must ensure a single instance across all servers, which is difficult because each server runs independently.

❓ tradeoff

advanced

Trade-offs of Singleton pattern usage

Which of the following is a common trade-off when using the Singleton pattern in system design?

A It guarantees high scalability in distributed environments.

B It simplifies testing by allowing easy instance replacement.

C It can introduce global state, making unit testing and debugging harder.

D It eliminates the need for synchronization in multithreaded code.

💡 Hint

Consider how global state affects testing and debugging.

Singleton introduces global state which can make unit testing difficult because tests may affect each other through shared state.

❓ component

expert

Singleton instance count in a complex system

Consider a system with three modules: A, B, and C. Each module uses the Singleton pattern to access a Logger class. The system runs on two servers. How many Logger instances exist at runtime?

A Two instances, one per server, shared by all modules on that server.

B One instance shared across all modules and servers.

C Three instances, one per module, shared across servers.

D Six instances, one per module per server.

💡 Hint

Think about Singleton scope and server boundaries.

Singleton ensures one instance per process. Since there are two servers (processes), each has its own Singleton instance shared by all modules on that server.