Redis is an in-memory, scriptable database used for operating on multiple types of data structures in realtime. Redis can be used as a cache or for managing high-frequency realtime operations with guaranteed consistency.
We use Redis Streams for our durable consumers (similar to Kafka), for caching content, rate limiting API requests, and in-memory databases like the matchmaker/parties/etc.
We run Redis in two main places: an ephemera cache used for data we don't care about persisting and a persistent pool used for data we want to make sure never gets lost.