====== Redis (ElastiCache) ======

**What it is:** An in-memory key-value store with advanced data structures.

**What it’s for:**
  * Caching + sessions + pub/sub + leaderboards.
  * More features than Memcached.

**Key ideas:**
  * Supports persistence (optional) and replication in managed setups.
  * Common for session store.

**Hard words:**
  * *data structure* /ˈdeɪtə ˈstrʌktʃər/: cấu trúc dữ liệu
  * *persistence* /pərˈsɪstəns/: lưu bền (không mất hoàn toàn)
  * *replication* /ˌreplɪˈkeɪʃn/: nhân bản/sao chép