How do you handle cache invalidation when multiple services can update the same data?

Cache invalidation is the hardest problem. Strategies: 1) Event-driven invalidation: when any service updates product, publish ProductUpdated event to message bus (Kafka). All services subscribed delete their cached product version. 2) TTL-based staleness: accept up to 5-minute stale data. Set TTL=300. Auto-expired even if event missed. 3) Cache versioning: store version with cached object. On upd