System Design Interview: Complete Guide for 2025
Learn how to approach system design interviews step-by-step. Covers scalability, databases, caching, and common design problems asked at FAANG.
System design interviews assess your ability to design large-scale distributed systems. This guide provides a framework for approaching any system design question.
System Design Interview Framework
Follow this 4-step framework:
- Step 1: Clarify Requirements (5 min) - Ask questions, define scope
- Step 2: High-Level Design (10 min) - Draw main components
- Step 3: Deep Dive (20 min) - Detail specific components
- Step 4: Wrap Up (5 min) - Discuss trade-offs, bottlenecks
Key Concepts to Master
Scalability: Horizontal vs Vertical
Vertical scaling (scale up) adds more power to existing machines. Horizontal scaling (scale out) adds more machines. Horizontal scaling is preferred for high availability and is more cost-effective at scale.
Database Choices: SQL vs NoSQL
Choose based on requirements:
- SQL: ACID compliance, complex queries, relationships (PostgreSQL, MySQL)
- NoSQL Document: Flexible schema, denormalized data (MongoDB)
- NoSQL Key-Value: Simple lookups, caching (Redis, DynamoDB)
- NoSQL Wide-Column: Time-series, analytics (Cassandra, HBase)
- NoSQL Graph: Relationships, social networks (Neo4j)
Caching Strategies
Cache-aside (lazy loading): App checks cache, loads from DB on miss. Write-through: Write to cache and DB simultaneously. Write-behind: Write to cache, async write to DB. Cache invalidation is the hard part - use TTL or event-driven invalidation.
Common System Design Questions
Design a URL Shortener (like bit.ly)
Key components:
- URL encoding: Base62 encoding (a-z, A-Z, 0-9)
- Database: NoSQL for high write throughput
- Caching: Redis for hot URLs
- Load balancer: Distribute traffic across servers
- Analytics: Track clicks, referrers, locations
Design a Chat Application (like WhatsApp)
Key components:
- WebSocket servers for real-time communication
- Message queue (Kafka) for reliability
- Database: Cassandra for message storage
- Presence service: Track online status
- Push notifications: FCM/APNs for offline users
- Media storage: S3 + CDN for images/videos
Design a News Feed (like Facebook/Twitter)
Key components:
- Fan-out approaches: Push (write) vs Pull (read)
- Timeline service: Aggregates posts for users
- Ranking algorithm: Relevance, recency, engagement
- Caching: Pre-computed feeds for active users
- Database: Graph DB for social connections
Practice system design with our AI interviewer. Get feedback on your approach, communication, and technical depth.
Practice with AI Mock Interviews
Put your knowledge to the test with our AI interviewer.