Cache Invalidation

The process of removing or marking cached data as stale when the underlying content changes. Cache invalidation is notoriously difficult ('There are only two hard things in computer science: cache invalidation and naming things'). Strategies include time-based expiration (max-age), version-based URLs (file.v2.js), purge APIs (CDN-specific), and tag-based invalidation. Improper invalidation leads to stale content being served to users.