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name: system-design description: Master system design with architecture patterns, component design, scalability planning, and creating robust, maintainable systems.

System Design

Design robust, scalable systems using proven architecture patterns and best practices for enterprise applications.

When to Use This Skill

• New system design
• Architecture refactoring
• Scaling existing systems
• Technology modernization
• Integration planning
• Performance optimization
• Architecture reviews
• Technical interviews

Core Concepts

1. C4 Model Architecture Diagram

Copy## Level 1: System Context

[Users] → [E-Commerce System] → [Payment Gateway]
                ↓
          [Email Service]

## Level 2: Container Diagram

E-Commerce System:
- Web Application (React, Node.js)
- API Server (Node.js, Express)
- Database (PostgreSQL)
- Cache (Redis)
- Message Queue (RabbitMQ)
- Search Engine (Elasticsearch)

## Level 3: Component Diagram (API Server)

API Server:
- Authentication Service
- Product Catalog Service
- Order Management Service
- Payment Service
- Notification Service

## Level 4: Code (Authentication Service)

Classes:
- AuthController
- AuthService
- TokenManager
- UserRepository

2. Capacity Planning

Copy## System Capacity Planning

**Requirements:**
- 1M active users
- Peak: 10K concurrent users
- 100K transactions/day
- 3 second max response time
- 99.9% uptime

**Calculations:**

**Traffic:**
- Requests/sec (peak): 10K users × 10 req/min / 60 = 1,667 RPS
- Database queries: 1,667 × 3 avg queries = 5,000 QPS
- Storage/day: 100K × 5KB avg = 500MB/day
- Storage/year: 500MB × 365 = 180GB

**Server Capacity:**
- App servers: 1,667 RPS / 200 RPS per server = 9 servers
- With redundancy (3x): 27 servers
- Database: Master-replica setup, read replicas for queries

**Network:**
- Bandwidth: 1,667 req × 50KB avg = 83MB/s = 664 Mbps
- CDN for static assets

**Costs (AWS estimate):**
- EC2 (27 × c5.xlarge): $3,500/month
- RDS (db.r5.2xlarge): $1,200/month
- Load Balancer: $200/month
- CloudFront CDN: $500/month
- Total: ~$5,400/month

3. Trade-Off Analysis

Copy## Architecture Decision: Monolith vs Microservices

**Monolith:**
✅ Pros:
- Simpler deployment
- Easier local development
- Single codebase
- Simpler testing
- Lower operational overhead

❌ Cons:
- Scaling all-or-nothing
- Technology lock-in
- Longer deployment cycles
- Harder to maintain at scale

**Microservices:**
✅ Pros:
- Independent scaling
- Technology flexibility
- Team autonomy
- Fault isolation
- Faster deployments

❌ Cons:
- Complex infrastructure
- Distributed system challenges
- Network latency
- Data consistency
- Higher operational overhead

**Decision:** Start with modular monolith, extract microservices as needed
**Rationale:** Team size (5 devs), early stage, faster time-to-market

Best Practices

1. Understand requirements - Functional and non-functional
2. Start simple - YAGNI (You Aren't Gonna Need It)
3. Design for failure - Assume components will fail
4. Think in layers - Presentation, business, data
5. Loose coupling - Minimize dependencies
6. High cohesion - Related functionality together
7. Document decisions - ADRs (Architecture Decision Records)
8. Iterate and evolve - Continuous improvement

Resources

• Designing Data-Intensive Applications: Martin Kleppmann
• System Design Interview: Alex Xu
• C4 Model: https://c4model.com