Key-Value Stores
The simplest data model: opaque values addressed by key. The useful distinction is not the model but whether the store is a server you talk to over the network or a library you embed in your process.
Redis
A server, and the only one most projects need here. In-memory, with data structures (lists, sets, sorted sets, streams) rather than opaque blobs, which is why it ends up used as a cache, a queue and a lock manager as much as a database. See Message Queues.
Check the version before you assume the licence. LICENSE.txt states that from Redis 8 the
project is tri-licensed - your choice of the Redis Source Available License v2, the Server Side
Public License v1, or AGPLv3 - while “Redis Open Source 7.2 and prior releases remain subject to
the BSDv3 clause license”.
Embedded
- LevelDB - an ordered key-value library from Google, BSD-3, not a server. Widely used as a storage engine inside other databases.
Dormant
- MemcacheDB - a persistent memcached. The repository has not been touched since 2009; listed only because it still turns up in search results. For a cache, use memcached or Redis.