Redis

Redis (renamed from "Redis Community Edition" to "Redis Open Source" as of the 8.0 release) is an in-memory data structure server most often used as a cache, but it also functions as a general-purpose data structure store, session store, message broker/queue system, and - through Redis Search - a search and vector query engine for AI applications (semantic caching, RAG retrieval, and short/long-term LLM memory).

Because it keeps data primarily in memory and uses efficient data structures (strings, lists, sets, hashes, sorted sets, streams, and more), it achieves very low, often sub-millisecond latency, and its modules API allows extending it with additional commands and capabilities beyond what ships by default.

Licensing has changed across versions: releases through 7.2.x are BSD-3-Clause, while 8.0.x and later are offered under a choice of RSALv2, SSPLv1, or AGPLv3. Beyond self-hosting, Redis Ltd. also offers Redis Software (self-managed, enterprise-focused) and Redis Cloud (fully managed on AWS, Azure, or Google Cloud).