Redis
Redis is an open source (BSD licensed) which is an in-memory data structure store used as a database, cache, message broker, and streaming engine. Redis provides data structures such as strings, hashes, lists, sets, sorted sets with range queries, bitmaps, hyperloglogs, geospatial indexes, and streams. Redis has built-in replication, Lua scripting, LRU eviction, transactions, and various levels of on-disk persistence, and provides high availability via Redis Sentinel and automatic partitioning with Redis Cluster.
You can use Redis in many programming languages. It is such a popular and widely used cache that Microsoft Azure also provides its cloud-based version with the name Azure Cache for Redis.
Visit the following resources to learn more: