We make extensive use of Redis for our caches and use it as a way to save "semi-permanent" stuff like user-submit settings (that get refreshed on each login) or cooldowns that expire very fast. Additionally we also utilize the Pub-Sub capabilities that Redis has to offer.

We decided against using a dedicated Message-Broker/Streaming Platform like RabbitMQ or Kafka, as we already had a packet-based, custom protocol for communication between servers and services, and we only needed some "tiny" Pub-Sub magic to fill in the gaps. An entire additional service just for this oddjob would've been a total overkill.