System Design Interview Prep

You are a senior system design interviewer at a top tech company. Walk through a complete system design for the described problem.

**Design Problem:** [e.g., Design Twitter, Design a URL shortener, Design a chat system, Design a ride-sharing service]
**Scope:** [What specific features to focus on]
**Scale:** [Expected users, requests per second, data volume]

**Walk through the design using this structure:**

1. **Requirements Clarification (5 minutes):**
   - Functional requirements (what the system does)
   - Non-functional requirements (latency, availability, consistency, durability)
   - Constraints and assumptions
   - Back-of-envelope calculations:
     - Storage requirements (per day, per year)
     - Bandwidth requirements
     - Read vs. write ratio
     - QPS (queries per second) for each operation

2. **High-Level Design (10 minutes):**
   - System architecture diagram description (components and data flow)
   - API design (REST endpoints with request/response schemas)
   - Data model (key entities, relationships, storage choice justification)
   - Choose appropriate technologies for each component with reasoning

3. **Deep Dive (15 minutes):**
   - Database design: Schema, sharding strategy, replication, indexing
   - Caching strategy: What to cache, cache invalidation, cache-aside vs. write-through
   - Message queues: When and why to use async processing
   - CDN strategy for static content
   - Search functionality (if applicable)
   - Notification system (if applicable)

4. **Scalability (5 minutes):**
   - Horizontal scaling strategy
   - Database scaling (read replicas, sharding, partitioning)
   - Load balancing (algorithm choice, health checks)
   - Rate limiting and throttling
   - Auto-scaling policies

5. **Reliability and Fault Tolerance (5 minutes):**
   - Single points of failure and how to eliminate them
   - Replication and redundancy
   - Graceful degradation strategy
   - Circuit breaker patterns
   - Data consistency model (strong vs. eventual)

6. **Monitoring and Operations:**
   - Key metrics to monitor
   - Alerting strategy
   - Deployment strategy (blue-green, canary)

**Output**: Complete system design walkthrough with diagrams, calculations, and trade-off discussions.