Top 20 System Design Questions

1. What is a Load Balancer?

Answer: A load balancer distributes incoming network traffic across multiple servers to ensure no single server becomes overwhelmed. This improves system reliability and availability. Load balancers can operate at different layers (Layer 4 - transport, Layer 7 - application) and can use various algorithms like round-robin, least connections, or IP hash.

2. Explain the CAP Theorem.

Answer: The CAP theorem states that in a distributed data store, it is impossible to simultaneously achieve Consistency (all nodes see the same data at the same time), Availability (every request receives a response), and Partition Tolerance (the system continues to operate despite network partitions). You can only achieve two out of the three.

3. What is Sharding?

Answer: Sharding is a database partitioning technique where data is horizontally partitioned into smaller, manageable pieces called shards. Each shard is stored on a separate database server to distribute the load and improve performance and scalability.

4. What are Microservices?

Answer: Microservices are an architectural style where a software application is composed of small, independent services that communicate over well-defined APIs. Each service is responsible for a specific business functionality, which allows for greater flexibility, scalability, and ease of maintenance.

5. Explain the difference between SQL and NoSQL databases.

Answer:

• SQL Databases: Relational databases that use structured query language (SQL) for defining and manipulating data. They are table-based and support ACID (Atomicity, Consistency, Isolation, Durability) properties.

• NoSQL Databases: Non-relational databases designed for unstructured data. They include document stores, key-value stores, wide-column stores, and graph databases. They offer scalability and flexibility but may sacrifice consistency.

6. What is a CDN (Content Delivery Network)?

Answer: A CDN is a network of geographically distributed servers that cache and deliver web content, such as images, videos, and scripts, to users based on their location. This reduces latency and improves load times by serving content from the nearest server.

7. What is a Message Queue?

Answer: A message queue is a communication mechanism used in distributed systems to send messages between services. It decouples the sender and receiver, allowing for asynchronous communication and better fault tolerance. Examples include RabbitMQ, Apache Kafka, and AWS SQS.

8. What is a Monolithic Architecture?

Answer: A monolithic architecture is a traditional software design where the entire application is built as a single, unified unit. All components are tightly coupled and run as a single process. While simple to develop and deploy, it can be difficult to scale and maintain as the application grows.

9. Explain the concept of Horizontal and Vertical Scaling.

Answer:

• Horizontal Scaling: Adding more machines or nodes to a system to distribute the load. This improves performance and fault tolerance.

• Vertical Scaling: Adding more resources (CPU, RAM) to a single machine to handle increased load. This has physical limitations and is less fault-tolerant than horizontal scaling.

10. What is a Database Index?

Answer: A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space. Indexes can be created on one or more columns and significantly enhance query performance.

11. What is a Reverse Proxy?

Answer: A reverse proxy is a server that sits between client devices and backend servers, forwarding client requests to the appropriate backend server. It can provide load balancing, security, caching, and SSL termination.

12. What is Consistent Hashing?

Answer: Consistent hashing is a technique used in distributed systems to distribute data across multiple nodes in a way that minimizes reorganization when nodes are added or removed. It ensures that only a small portion of the data needs to be redistributed when the cluster changes.

13. Explain the difference between Synchronous and Asynchronous Communication.

Answer:

• Synchronous Communication: The sender waits for the receiver to process the message and respond before continuing.

• Asynchronous Communication: The sender sends the message and continues processing without waiting for an immediate response from the receiver.

14. What is a Circuit Breaker Pattern?

Answer: The circuit breaker pattern is a design pattern used in microservices architectures to detect and handle failures. It prevents repeated requests to a failing service by "opening" the circuit and redirecting or rejecting requests until the service is restored.

15. What is the Role of a Cache in System Design?

Answer: A cache stores frequently accessed data in a fast-access layer (memory) to reduce the time it takes to retrieve data. It improves performance by reducing the load on databases and backend services.

16. What is Data Replication?

Answer: Data replication involves copying and maintaining database data in multiple locations to ensure high availability, fault tolerance, and improved performance. Replication can be synchronous or asynchronous.

17. Explain the concept of Eventual Consistency.

Answer: Eventual consistency is a consistency model used in distributed systems where updates to a data store may not be immediately visible to all nodes. However, given enough time, all nodes will eventually become consistent.

18. What is a Primary Key in a Database?

Answer: A primary key is a unique identifier for a record in a database table. It ensures that each record can be uniquely identified and retrieved. Primary keys must contain unique values and cannot contain nulls.

19. What is a Singleton Pattern?

Answer: The singleton pattern is a design pattern that restricts the instantiation of a class to a single instance. It is used to ensure that a class has only one instance and provides a global point of access to it.

20. What is the difference between HTTP and HTTPS?

Answer:

• HTTP (Hypertext Transfer Protocol): The standard protocol for transferring web pages over the internet. It is unencrypted.

• HTTPS (Hypertext Transfer Protocol Secure): An extension of HTTP that uses SSL/TLS to encrypt the data transferred between the client and server, ensuring secure communication.