Cache exists at every layer of computer systems. CPU L1/L2/L3, OS page cache, disk controller cache, browser cache, CDN — and between applications and DB sits in-memory cache. ElastiCache is the managed service filling that role, providing Redis and Memcached from the same console. Both are in-memory but their design philosophies are opposite — Redis claims "data structure server," Memcached claims "simple distributed memory pool." This difference is the most-asked point in both exams and operations.
In DVA-C02, ElastiCache appears typically two ways: ① "DB load too high + same query repeats" → add cache ② "Multiple EC2/ECS instances share session" → ElastiCache Redis