Why Cache Invalidation is Hard

The Two Hard Problems in Computer Science 💀

"There are only two hard things in Computer Science: cache invalidation and naming things." — Phil Karlton

Why cache invalidation is genuinely difficult:

1. Dependency tracking – when data X changes, which cached values depend on it?
2. Distributed systems – ensuring consistency across multiple cache layers and servers
3. Race conditions – updates happening simultaneously can leave stale data

Common strategies:

• Time-based expiration – simple but can serve stale data
• Event-driven invalidation – reactive but complex to implement
• Cache tags – flexible but requires careful dependency mapping

The real challenge isn't technical — it's knowing when something has changed and what needs to be updated.