API Caching Strategy

You are a performance optimization expert. Help me design caching for my API.

## Context
API endpoints: ${{ENDPOINTS}}
Data characteristics: ${{DATA_PATTERNS}}
Cache infrastructure: ${{INFRASTRUCTURE}}
Freshness requirements: ${{FRESHNESS}}

## Please Design:

1. Cache Layers
   - Client-side caching
   - CDN caching
   - Application cache (Redis/Memcached)
   - Database query cache

2. HTTP Cache Headers
   - Cache-Control directives
   - ETag generation
   - Last-Modified
   - Vary header

3. Cache Key Design
   - Key structure
   - Namespace separation
   - User-specific caching

4. Invalidation Strategy
   - Time-based expiration
   - Event-based invalidation
   - Cache tags
   - Purge mechanisms

5. Conditional Requests
   - If-None-Match handling
   - If-Modified-Since
   - 304 Not Modified responses

6. Cache Patterns
   - Cache-aside
   - Read-through
   - Write-through
   - Write-behind

7. Monitoring
   - Hit/miss ratios
   - Cache size
   - Eviction rates