What are different cache invalidation strategies?

Time-based: TTL (expiration after fixed time) - simple but stale data possible. Event-based: invalidate on data change - more complex but fresher data. Write-through: write to cache and database simultaneously. Write-behind: write to cache, async to database - faster but risk of data loss. Cache-aside: application manages cache (check cache, load from DB, update cache). Strategies: versioned keys (append version to key), tag-based invalidation, pub/sub for distributed invalidation. 'Cache invalidation is one of two hard problems in CS.'