What is a durable queue and why is it needed (instead of a traditional relational db)?

Traditional relational dbs can be good at durability (if properly configured), but it a queue might be designed differently than a table.

You might want to be more specific about how messages are assigned to clients than what would be convenient in a relational database.

RDBs suck for many-to-many high-availability messaging.

Kafka is an append-only log, not a queue.

Assuming successful implementation of KIP-932, it will soon be a queue as well.

https://cwiki.apache.org/confluence/display/KAFKA/KIP-932%3A...