API Caching Strategy Architect

You are a performance engineer specializing in API caching and content delivery optimization.

## CONTEXT
I need to implement caching for my API.

**Requirements:**
- API Type: {{API_TYPE}}
- Data Freshness Needs: {{FRESHNESS}}
- Traffic Volume: {{TRAFFIC}}
- Infrastructure: {{INFRASTRUCTURE}}

## TASK
Design a comprehensive caching strategy:

### 1. HTTP CACHING HEADERS
```
Cache-Control: public, max-age=3600, stale-while-revalidate=86400
ETag: "abc123"
Last-Modified: Wed, 21 Oct 2024 07:28:00 GMT
Vary: Accept-Encoding, Authorization
```

### 2. CACHING LAYERS
- Browser cache
- CDN cache
- Application cache (Redis/Memcached)
- Database query cache

### 3. CACHE INVALIDATION
- Time-based expiration
- Event-based invalidation
- Tag-based invalidation
- Manual purge mechanisms

### 4. ENDPOINT CLASSIFICATION
| Endpoint | Cache Strategy | TTL | Invalidation |
|----------|----------------|-----|--------------|
| /users/:id | Private | 5min | On update |
| /products | Public | 1hr | On change |
| /search | No cache | 0 | N/A |

### 5. CONDITIONAL REQUESTS
- ETag implementation
- If-None-Match handling
- 304 Not Modified responses

### 6. IMPLEMENTATION
Code examples for caching middleware.