system-design

# System Design Skill

## Quick Reference

| Pattern | Best For | Complexity | Scaling |
|---------|----------|------------|---------|
| **Monolith** | Startups, MVPs | Low | Limited |
| **Microservices** | Large teams | High | Excellent |
| **Serverless** | Event-driven | Medium | Auto |
| **Event-Driven** | High throughput | High | Excellent |

---

## Scalability Progression

```
Level 1: Single Server
    │
    ▼ Bottleneck: CPU/Memory
Level 2: Load Balancer + Multiple Servers
    │
    ▼ Bottleneck: Database reads
Level 3: Caching Layer (Redis)
    │
    ▼ Bottleneck: Database writes
Level 4: Read Replicas
    │
    ▼ Bottleneck: Single DB limits
Level 5: Sharding / Partitioning
    │
    ▼ Bottleneck: Cross-shard queries
Level 6: CQRS + Event Sourcing
```

---

## Architecture Decision Tree

```
What's your team size and product stage?
│
├─► Team < 10, product unclear
│   └─► Monolith (start simple)
│
├─► Team > 10, clear domain boundaries
│   └─► Microservices
│
├─► Variable workloads, pay-per-use
│   └─► Serverless
│
└─► High throughput, async workflows
    └─► Event-Driven
```

---

## API Design

### REST Best Practices
```
GET    /api/v1/users              # List
GET    /api/v1/users/{id}         # Get
POST   /api/v1/users              # Create
PUT    /api/v1/users/{id}         # Replace
PATCH  /api/v1/users/{id}         # Update
DELETE /api/v1/users/{id}         # Delete

GET    /api/v1/users/{id}/orders  # Nested
```

### HTTP Status Codes
| Code | Meaning | Use When |
|------|---------|----------|
| 200 | OK | GET/PUT/PATCH success |
| 201 | Created | POST success |
| 204 | No Content | DELETE success |
| 400 | Bad Request | Invalid input |
| 401 | Unauthorized | No/invalid auth |
| 403 | Forbidden | No permission |
| 404 | Not Found | Resource missing |
| 429 | Too Many Requests | Rate limited |
| 500 | Server Error | Server failure |

---

## Database Selection

| Use Case | Best Choice | Notes |
|----------|-------------|-------|
| Transactions | Po