Redis

Redis is great for data that needs to be accessed by key quickly.
Especially if that data is temporary with the TTL feature, like a cache a ticket locking system.

It would be a great distributed lock for distributed system that needs a shared memory.

Redis has a priority queue thing by time? Sorted set.

redis can do atomic transactions with lua scripting

Redis can handle more or less 100k rps

Redis clusters handle a lot for you, sharding & replication