Key-Value Databases
In terms of complexity, on the extreme end of most complexity would lie a relational database. On the other end of the spectrum would be a key-value database.
It resembles a dictionary or a hash table that gets used in most programming languages. You set key-value pairs, then you can retrieve the value by using the key.
Redis
Redis is an in-memory, key-value store that works with tremendous speed. It is often used as a cache or a database specifically for more ephemeral data. Data such as user sessions, shopping carts, or other data that is not critical to the application but receives a lot of traffic.