High-Level Design (HLD)

1️⃣ Fundamentals

What is the difference between horizontal and vertical scaling? When would you use each?
Explain Monolithic vs Microservices architecture. What are pros and cons of each?
How does a client-server architecture work? Can you design a basic client-server interaction flow?
How does DNS resolution work when you enter a URL in the browser?
How does HTTP work under the hood? Can you describe an end-to-end request flow?
What are the trade-offs of serverless vs serverful architectures?

2️⃣ Databases

SQL vs NoSQL — when to choose which?
What is Indexing? How does it improve performance? What are its downsides?
How does sharding work? What are the pros and cons of sharding a database?
What is replication? What are the different types of replication?
Explain eventual consistency and strong consistency in context of databases.
What are the CAP theorem implications on database design?
What is normalization? When would you denormalize your schema?
How do ACID properties ensure reliability in databases?
How would you scale a database horizontally? What challenges will you face?
How does a distributed database ensure fault tolerance?

3️⃣ Consistency & Availability

Explain CAP theorem with real-life examples.
What is eventual consistency? Can you give a use case where it is acceptable?
What is quorum in distributed systems? How does it relate to consistency and availability?
What are distributed consensus algorithms like Paxos or Raft? Why are they needed?
What is the difference between strong consistency, weak consistency, and eventual consistency?
What is the difference between read-your-writes and monotonic reads consistency models?
Explain isolation levels in relational databases with examples.
How does leader election work in distributed systems?

4️⃣ Caching

Why do we use caching? What are the benefits and risks?
What is CDN caching and how does it work?
What are the different cache eviction policies (LRU, LFU)? When would you use each?
What are cache consistency strategies? How do you ensure cache invalidation?
What is the difference between write-through, write-back, and write-around caching?
How does DNS caching improve performance?
How would you cache user sessions in a scalable web application?
What is cache stampede? How do you mitigate it?

5️⃣ Networking

What is the difference between TCP and UDP? When would you prefer one over the other?
How does HTTPS work internally? What is the role of SSL/TLS in HTTPS?
What is the difference between HTTP/1.1, HTTP/2, and HTTP/3?
How does WebSocket work? How is it different from HTTP?
How does a TLS handshake work? What is mutual TLS?
What is the role of a Content Delivery Network (CDN) in networking?
How does video streaming (like YouTube or Netflix) work over the internet?

6️⃣ Load Balancing

Why do we need load balancers? What are their key advantages?
What is the difference between Layer 4 and Layer 7 load balancing?
Explain various load balancing algorithms (Round Robin, Least Connections, IP Hashing).
What is a reverse proxy? How is it different from a forward proxy?
What is Consistent Hashing? How does it help in distributed systems?
How do you implement rate limiting at the API level?
What is a circuit breaker pattern? Why is it used?
How do you ensure high availability and fault tolerance using load balancers?

7️⃣ Message Queues & Event-Driven Architecture

What is the difference between message queues and streams? Give examples.
When would you use Kafka vs RabbitMQ?
What is the Pub/Sub model? How does it work?
What is event sourcing? How is it different from traditional CRUD?
Explain CQRS (Command Query Responsibility Segregation). What problem does it solve?
What are the advantages of asynchronous processing in large-scale systems?
How does exactly-once, at-least-once, and at-most-once delivery semantics work in messaging systems?
How does Kafka ensure message durability and fault tolerance?
What is backpressure in streaming systems? How do you handle it?

8️⃣ Microservices Architecture (Deep Dive)

What are the advantages and challenges of microservices over monolithic architecture?
What is service discovery? How does it work in a microservice ecosystem?
What is an API Gateway? What are its benefits and risks?
How do you handle inter-service communication — REST vs gRPC vs messaging?
What is a single point of failure? How do you avoid it in microservices?
How do you prevent cascading failures in a microservice architecture?
What is the role of Docker in microservices?
How does Kubernetes help in orchestration of microservices?
What are the challenges in migrating from monolith to microservices? How do you plan such a migration?
How do you manage distributed transactions in microservices (Sagas, 2PC)?
How do you monitor and debug a distributed microservices-based system?

9️⃣ Monitoring & Logging

What is the difference between monitoring and logging?
What are some key metrics you would monitor in a large distributed system?
How does distributed tracing work? Why is it important in microservices?
What are health checks and how do they help in system reliability?
What is anomaly detection in system monitoring? How can it be implemented?
How do you aggregate logs from multiple services/machines?
How do you design a centralized logging system?
What is an alerting system? What are best practices to avoid alert fatigue?

🔟 Security

What is the difference between authentication and authorization?
How does OAuth2 work? What are its different grant types?
What is the difference between JWT and session-based authentication?
How does Single Sign-On (SSO) work?
What is role-based access control (RBAC)? How do you implement it?
What is encryption at rest vs encryption in transit? Why are both important?
How do you secure APIs? What are common attack vectors (e.g., rate limiting, SQL injection)?
How do you prevent man-in-the-middle attacks in a distributed system?
How does OpenID Connect work in comparison with OAuth2?

1️⃣1️⃣ System Design Tradeoffs

How would you decide between SQL and NoSQL for a given application?
How do you balance consistency vs availability in a distributed system?
What is the trade-off between memory usage and latency in caching systems?
How do you design for cost vs performance in cloud infrastructure?
What are the trade-offs between push-based vs pull-based architectures?
When would you choose eventual consistency over strong consistency?
How do you handle failover in a distributed application (Active-Active vs Active-Passive)?
What is the difference between a data lake and a data warehouse? When would you use each?
How do you balance scalability vs maintainability in microservices architecture?

Perfect! Here’s Part 4 (Low-Level Design) covering OOP, Design Patterns, Concurrency, UML, APIs, and Common LLD Problems.

Low-Level Design (LLD)

1️⃣ Object-Oriented Programming (OOP)

What are the four basic principles of Object-Oriented Programming? Can you explain each one?
What is the difference between an interface and an abstract class? When would you use each?
Explain the concept of polymorphism with examples.
What is the difference between composition and inheritance? When would you prefer composition over inheritance?
What is dependency injection? How is it implemented in different programming languages?
Can you explain SOLID principles? Provide examples of how to apply them in real-world applications.
What is the difference between a deep copy and a shallow copy in object cloning?
What is the importance of encapsulation in software design?

2️⃣ Design Patterns

Creational Patterns

What is the Singleton pattern? When should it be used, and what are its potential issues?
How does the Factory pattern differ from the Abstract Factory pattern?
Can you explain the Builder pattern with an example? When is it useful?

Structural Patterns

What is the Adapter pattern? Can you provide an example where it can be used?
How does the Composite pattern work? Can you think of an example where it is applicable?
Explain the Proxy pattern. How can it help with performance or security?

Behavioral Patterns

What is the Observer pattern? How does it work and where is it useful?
How does the Strategy pattern work? Can you give an example of its usage?
Can you explain the Command pattern with an example?

3️⃣ Concurrency & Thread Safety

What is a thread? How is it different from a process?
What are race conditions? How can you avoid them in a multithreaded environment?
What is synchronization in Java? Can you explain how to implement it to ensure thread safety?
What are the potential pitfalls of multithreading and how do you mitigate them (e.g., deadlocks, starvation)?
How does the Executor Framework simplify thread management in Java?
What is the difference between a synchronized method and a synchronized block?
What is a thread pool and how does it improve performance?
What are thread-safe collections? Can you give examples of thread-safe collections in Java?
What is the purpose of the volatile keyword in Java? How does it help in multithreading?
How do you implement a thread-safe singleton in a multithreaded environment?

4️⃣ UML Diagrams

What is the purpose of a class diagram? What kind of information does it represent?
How do you represent relationships like inheritance and composition in a UML diagram?
What is a sequence diagram? How does it differ from a class diagram?
Can you explain an activity diagram with an example? How do you use it in system design?
What is a deployment diagram and what does it describe in the context of system architecture?
How would you design a component diagram for a microservice-based system?

5️⃣ APIs

What is the difference between REST, gRPC, and GraphQL? When would you use each?
How do you handle versioning in REST APIs?
Can you explain the concept of statelessness in REST APIs?
What are common HTTP status codes and their meanings (e.g., 200, 400, 404, 500)?
What is the role of authentication and authorization in API design? How do you implement OAuth2 in an API?
How would you design an API for a service like booking flights (e.g., POST /book, GET /book/{id})?
What is rate limiting? How would you implement it in a REST API?
What is the importance of pagination in APIs? How would you design pagination for a large dataset?
What are the differences between synchronous and asynchronous APIs?
What is Swagger/OpenAPI? How does it help in API documentation and testing?

6️⃣ Common LLD Problems

How would you design a parking lot system?
- What classes would you create? What attributes and methods would they have?
- How would you handle pricing, parking slots, and vehicle types?
Design a BookMyShow system.
- What components would be involved? How would you structure the data?
- How would you manage seat reservations, payments, and user interactions?
How would you design an elevator system?
- How would you handle multiple floors, multiple elevators, and user requests?
- What algorithms would you use for optimal elevator selection?
Design a Splitwise system.
- How would you model the debts, payments, and users? What is the structure of the data?
- How would you handle the scenario when someone clears a debt?
How would you design a file storage system like Dropbox or Google Drive?
- What are the main features you would include (upload, download, sharing)?
- How would you ensure scalability and fault tolerance for file storage?
How would you design a rate limiter?
- What are the strategies you would use (e.g., token bucket, leaky bucket)?
- How would you ensure that the rate limiting is efficient and does not degrade performance?
How would you design a caching system?
- What cache eviction policies would you use (LRU, LFU)?
- How would you manage cache consistency and invalidation?

Machine Coding Interview Questions

➔ Commonly Asked Machine Design Interview Questions (Divided into Levels)

𝗕𝗮𝘀𝗶𝗰 𝗟𝗲𝘃𝗲𝗹 𝗤𝘂𝗲𝘀𝘁𝗶𝗼𝗻𝘀:

What is system design and why is it important?
What are scalability, availability, and reliability?
What is load balancing, and why do we use it?
Explain caching and how it improves system performance.
What is a database index, and how does it help in queries?
What is the difference between vertical scaling and horizontal scaling?
What are the differences between SQL and NoSQL databases?
What is sharding and why is it useful?
Explain eventual consistency vs strong consistency.
What are the CAP theorem and its implications?

𝗠𝗼𝗱𝗲𝗿𝗮𝘁𝗲 𝗟𝗲𝘃𝗲𝗹 𝗤𝘂𝗲𝘀𝘁𝗶𝗼𝗻𝘀:

How would you design a URL shortening service like bitly?
How would you design a messaging/chat system?
How would you design a web crawler?
How do you design a rate limiter?
What is a queue and how do message queues work?
How would you design a news feed system like Facebook’s?
How do you design a notification system?
How do you handle database replication?
How would you design a system for uploading and serving large files?
How would you design an API rate limiting strategy?

𝗔𝗱𝘃𝗮𝗻𝗰𝗲𝗱 𝗟𝗲𝘃𝗲𝗹 𝗤𝘂𝗲𝘀𝘁𝗶𝗼𝗻𝘀:

How would you design a distributed cache system?
How would you design a global CDN (Content Delivery Network)?
How do you design an online collaborative document editor (like Google Docs)?
How do you design a fault-tolerant system?
How do you design a distributed transaction system?
How would you design a scalable search engine?
Explain the design of a real-time bidding system for ads.
How would you design a large-scale analytics platform?
How would you handle consistency in distributed systems?
How would you design a system to handle billions of user requests daily?