⭐ Introduction
System Design interviews test how you think, not how fast you code.
Interviewers evaluate:
✔ Architecture thinking
✔ Scalability & performance
✔ OOPS + SOLID usage
✔ Real-world trade-offs
✔ Communication skills
This article covers most asked system design interview questions with clear explanations and design approach, not just theory.
🔹 1. What is System Design?
System Design is the process of defining architecture, components, data flow, and interactions to meet business and technical requirements.
It includes:
- High-Level Design (HLD)
- Low-Level Design (LLD)
🔹 2. Difference Between HLD and LLD
| HLD | LLD |
|---|---|
| Big picture | Detailed design |
| Architecture | Class & method level |
| Tech-agnostic | Language-specific |
| Focus on scalability | Focus on implementation |
🔹 3. How Do You Start a System Design Interview?
Always follow this order:
1️⃣ Clarify requirements
2️⃣ Identify users & use cases
3️⃣ Define system boundaries
4️⃣ Estimate scale (users, data)
5️⃣ Design core components
6️⃣ Design data storage
7️⃣ Handle scalability
8️⃣ Discuss trade-offs
🔹 4. What Are Functional vs Non-Functional Requirements?
Functional
- Login
- Place order
- Send notification
Non-Functional
- Performance
- Scalability
- Security
- Availability
- Reliability
🔹 5. Design a URL Shortener (Like Bitly)
Core Requirements
- Shorten long URLs
- Redirect short URLs
- Track clicks
Key Components
- API Layer
- URL Encoder
- Database
- Cache (Redis)
- Load Balancer
Concepts Tested
✔ Hashing
✔ Caching
✔ Scalability
✔ Database indexing
🔹 6. Design a Notification System
Requirements
- Email, SMS, Push
- Retry on failure
- Asynchronous processing
Design Approach
- Interface-based notification service
- Queue (Kafka/RabbitMQ)
- Worker services
OOPS Used
✔ Interface
✔ Polymorphism
✔ Strategy Pattern
🔹 7. How Do You Handle Scalability?
Scalability strategies:
- Horizontal scaling
- Load balancing
- Caching
- Database sharding
- Asynchronous processing
🔹 8. What Is Load Balancing and Why Is It Needed?
Load balancing distributes traffic across multiple servers to:
✔ Improve availability
✔ Increase throughput
✔ Prevent overload
🔹 9. Database: SQL vs NoSQL — When to Use?
| SQL | NoSQL |
|---|---|
| Structured data | Flexible schema |
| ACID | Eventual consistency |
| Complex queries | High scalability |
| Banking systems | Logging, caching |
🔹 10. What Is Caching? Where Would You Use It?
Caching stores frequently accessed data in memory.
Used for:
- User sessions
- Product catalogs
- Configuration data
Popular tools:
- Redis
- Memcached
🔹 11. What Is a Microservices Architecture?
Microservices split a system into small independent services.
Benefits
✔ Independent scaling
✔ Faster deployment
✔ Fault isolation
Challenges
❌ Network latency
❌ Distributed debugging
🔹 12. Monolith vs Microservices
| Monolith | Microservices |
|---|---|
| Single codebase | Multiple services |
| Simple | Complex |
| Hard to scale | Easy to scale |
| Fast initially | Better long-term |
🔹 13. How Do You Design a Scalable Login System?
Key points:
- Authentication service
- Token-based auth (JWT)
- Secure password storage
- Rate limiting
- Caching sessions
🔹 14. What Is API Gateway?
API Gateway:
✔ Single entry point
✔ Authentication
✔ Rate limiting
✔ Routing
Used in microservices.
🔹 15. How Do You Ensure High Availability?
- Redundant servers
- Failover strategy
- Health checks
- Load balancing
- Data replication
🔹 16. What Is CAP Theorem?
A distributed system can guarantee only two of:
- Consistency
- Availability
- Partition tolerance
🔹 17. How Do You Handle Failures in a System?
- Retry mechanism
- Circuit breaker
- Timeouts
- Fallback responses
- Monitoring & alerts
🔹 18. What Is Rate Limiting?
Limits number of requests per user/IP.
Prevents:
✔ Abuse
✔ DDoS attacks
🔹 19. Design a File Upload System
Key considerations:
- Chunk uploads
- File size limits
- CDN for delivery
- Virus scanning
- Metadata storage
🔹 20. What Is Idempotency?
An operation that gives the same result even if called multiple times.
Important for:
- Payments
- Order creation
- APIs
🔹 21. How Do You Design for Security?
- Authentication & Authorization
- HTTPS
- Encryption
- Input validation
- Role-based access
🔹 22. What Is Event-Driven Architecture?
Components communicate via events, not direct calls.
Benefits:
✔ Loose coupling
✔ Scalability
✔ Asynchronous processing
🔹 23. How Do You Store Logs in a Large System?
- Centralized logging
- Structured logs
- Log aggregation tools
🔹 24. What Is Observability?
Ability to understand system behavior using:
- Logs
- Metrics
- Traces
🔹 25. Common System Design Mistakes
❌ Jumping into coding
❌ Ignoring scale
❌ Over-engineering
❌ No trade-off discussion
❌ Not clarifying requirements
🎯 Interview Tip (VERY IMPORTANT)
Always explain:
✔ Why you chose a solution
✔ What trade-offs exist
✔ How it scales
✔ What you’d improve later
🎉 Conclusion
System Design interviews test your engineering maturity.
If you can:
✔ Structure your approach
✔ Explain trade-offs
✔ Apply OOPS + SOLID
✔ Think in scalability
You are interview-ready.