What are common caching strategies and how do you handle cache invalidation?

Common caching patterns: - **Cache-aside:** app reads from cache, falls back to DB, then populates cache. - **Read-through:** cache layer loads from DB on miss. - **Write-through / write-behind:** writes go through cache (sync/async) before DB. **Invalidation options:** - TTL/expiration - explicit deletes on writes - versioned keys (e.g., `user:v3:123`) **Interview tip:** explain trade-offs between freshness, complexity, and performance.