# Cache overview
# Things to consider
# Layers
# Read & Write
# Eviction
- Least Recently used (LRU)
- Most recently used (MRU)
- Least Frequently used (LFU)
- Time-to-live (TTL)
- Two-tiered caching
# Problems
- Thunder hurd problem: a large amount of keys expire at the same time
- Cache Penetration: data empty from cache & DB
- Cache breakdown: hot key expires => a large amount of queries hit DB
- Cache crash