What is asked in system design interview?

In a system design interview, you are typically asked to design a large-scale, complex system that solves real-world problems. The questions focus on how you break down the problem, structure the architecture, and make decisions about scalability, performance, fault tolerance, and trade-offs. The goal is to assess your ability to design systems that are robust, scalable, and efficient while handling practical constraints.

Here’s a breakdown of what is commonly asked in a system design interview:

1. High-Level Architecture Design

a. What components will you include in your design?

• The interviewer will ask you to design a system from scratch, requiring you to identify the major components, such as:
  • Frontend: Client-side components, mobile apps, or web interfaces.
  • Backend: APIs, microservices, or services that handle business logic.
  • Databases: SQL, NoSQL, or distributed databases for storing data.
  • Caching: Use of caches to reduce database load and improve performance.
  • Load Balancers: Tools to distribute traffic across multiple servers.
  • External Services: Services like third-party authentication, external APIs, etc.

Example: Design a URL shortener. Identify components like a frontend where users input long URLs, a backend service to generate and store short URLs, and a database to store the URL mappings.

2. Clarification of Requirements

a. What are the functional and non-functional requirements?

• Functional requirements: What are the key features the system must support? For example, in a messaging app, should it support group chats or just one-to-one messaging?
• Non-functional requirements: What are the constraints on scalability, availability, latency, and performance? What kind of traffic is expected, and what’s the acceptable response time?

Example: Design a news feed system. Clarify if the feed needs to update in real-time, handle millions of users, or support ranking algorithms to prioritize posts.

3. Scalability and Performance Considerations

a. How will your system handle increased traffic and data volume?

• The interviewer will expect you to address scalability—how the system will grow as traffic increases. Key points include:
  • Horizontal vs. vertical scaling: How will you add more servers or increase server capacity?
  • Database partitioning (sharding): How will you break up data across multiple servers?
  • Caching: How will you use caching to reduce load on databases and improve read performance?

Example: Design a video streaming platform. Discuss how you will scale video storage, deliver video content globally via CDNs (Content Delivery Networks), and handle millions of concurrent users watching videos.

4. Database Design and Data Management

a. How will you store and retrieve data?

• The interviewer will ask you how you will design your database schema or data storage model. Consider:
  • SQL vs. NoSQL: When will you use relational databases (SQL) for structured data and NoSQL databases for flexible, unstructured data?
  • Indexing: How will you index data to speed up lookups while balancing write performance?
  • Data partitioning and replication: How will you distribute data across multiple databases and ensure data is replicated for high availability?

Example: Design a social media platform. You’ll need to design a database schema that stores user profiles, posts, comments, and likes efficiently.

5. Trade-offs and System Bottlenecks

a. What trade-offs will you make between consistency, availability, and performance?

• You’ll be expected to explain trade-offs between consistency and availability (CAP theorem) in distributed systems.
  • Consistency: Should the data be consistent across all servers (e.g., in financial transactions)?
  • Availability: Should the system always respond, even if some data is stale (e.g., in social media feeds)?
  • Performance vs. Cost: Can you balance low latency with cost-effectiveness?

Example: Design a payment processing system. You need strong consistency for financial transactions, meaning you’ll need to sacrifice some availability to ensure that all servers are in sync.

6. Handling Failure and Fault Tolerance

a. How will your system handle failures and maintain availability?

• You’ll need to demonstrate how the system handles failures, such as server crashes, network partitions, or database outages. This involves:
  • Replication: Replicating data across multiple servers or regions for availability.
  • Failover: Implementing failover mechanisms so that if one component fails, another takes over.
  • Disaster Recovery: How will you ensure that the system recovers from catastrophic failures?

Example: Design a ride-hailing service. You’ll need to discuss how the system continues to function if a server or database fails, ensuring users can still request rides.

7. Load Balancing and Traffic Management

a. How will you distribute traffic across your servers?

• The interviewer will expect you to explain how your system will handle traffic spikes and balance the load across multiple servers.
  • Load balancers: How will you use load balancers to evenly distribute requests across servers?
  • Auto-scaling: How will the system automatically add or remove servers based on traffic?

Example: Design a search engine. Discuss how you’ll balance traffic between indexing servers, search query servers, and handle sudden spikes in traffic.

8. Caching Strategies

a. How will you use caching to improve performance?

• The interviewer will expect you to optimize the system’s performance by using caching to reduce the load on databases and speed up responses.
  • Write-through vs. write-back caching: When to update the cache immediately or delay updates.
  • Cache invalidation: How will you keep the cache in sync with the database?

Example: Design an e-commerce platform. Discuss how you’ll cache product information to reduce database reads and use a TTL (time-to-live) strategy to keep the cache fresh.

9. API Design and Communication

a. How will your components communicate with each other?

• The interviewer may ask how you will design the APIs between different components and how they’ll communicate (e.g., using REST or gRPC).
  • Synchronous vs. asynchronous communication: When should the system wait for a response, and when should it handle tasks in the background using message queues like Kafka or RabbitMQ?

Example: Design an online marketplace. Discuss how the product listing service, order processing service, and payment service will communicate via APIs.

10. Security and Data Privacy

a. How will you secure the system?

• You may be asked how you will ensure security and data privacy in your design. Key points to consider:
  • Encryption: How will you encrypt data at rest (in the database) and in transit (between services)?
  • Authentication and Authorization: How will users be securely authenticated (e.g., via OAuth) and authorized to access certain resources (e.g., role-based access control)?
  • Data privacy: How will you protect sensitive user data and ensure compliance with privacy regulations (e.g., GDPR)?

Example: Design a banking system. Explain how you’ll ensure secure transactions through SSL encryption and protect user accounts through multifactor authentication.

11. Handling Edge Cases

a. What edge cases will you consider, and how will you handle them?

• You’ll need to think about edge cases that may break the system or cause unexpected behavior, such as:
  • Traffic spikes: How will the system handle sudden increases in traffic (e.g., viral events)?
  • Data inconsistency: What happens if the cache and the database get out of sync?
  • Network partitions: How will the system behave if parts of the network are unreachable?

Example: Design a real-time collaboration tool (e.g., Google Docs). Discuss how the system will handle multiple users editing the same document simultaneously and how conflicts will be resolved.

12. Monitoring and Observability

a. How will you monitor the system and respond to issues?

• The interviewer may ask how you’ll monitor the system to detect issues early and respond to failures or performance bottlenecks. This could include:
  • Logging: What logs will you collect to track errors and user activity?
  • Metrics: What metrics will you monitor (e.g., request latency, error rates, traffic)?
  • Alerting: How will you set up alerts for critical system failures or performance degradation?

Example: Design a content delivery network (CDN). Discuss how you’ll monitor network latency, traffic distribution, and cache hit/miss rates, and trigger alerts if performance drops below a threshold.

Recommended Courses

• Grokking System Design Fundamentals
• Grokking the System Design Interview
• Grokking the Advanced System Design Interview

Conclusion

In a system design interview, you will be asked to design complex systems while addressing scalability, performance, fault tolerance, and security. The focus is on how you break down the problem, propose an architecture, and handle real-world constraints such as traffic spikes, system failures, and trade-offs between consistency and availability. The goal is to assess your ability to design a robust, scalable, and maintainable system that meets the requirements.

Key Areas to Focus On:

1. Clarifying requirements and setting up the system’s functional and non-functional goals.
2. High-level architecture and identifying core components.
3. Designing for scalability, fault tolerance, and performance.
4. Database design, caching, and handling data consistency.
5. Communicating your thought process clearly and handling feedback effectively.