It's both. It's best used when it's being used as a message broker, but any sufficiently advanced message broker will need many of the features of a database –

It's both. It's best used when it's being used as a message broker, but any sufficiently advanced message broker will need many of the features of a database – durability of messages, querying in various ways, etc. I think it's reasonable to think of it as a very specialised database.