A Key Expired in Redis, You Won't Believe What Happened Next

This post details an investigation into Redis key expiration consistency issues on read replicas in a Redis 2.x ElastiCache setup. It reveals that slaves do not expire keys unless explicitly told by the master, and that the passive expiration mechanism is too slow at scale (taking over 110 hours to guarantee no expired keys for a dataset of ~5.6 million keys). The findings led to recommendations to upgrade Redis versions more frequently and pay closer attention to cache invalidation strategies during software design.