Built-in replication for broker

Dear developers,

In the paper "Kafka: A Distributed Messaging System for Log Processing, Jay Kreps, Neha Narkhede, Jun Rao from LinkedIn, at NetDB workshop 2011",

you mentioned this:
"If a broker goes down, any message stored on it not yet consumed becomes unavailable. If the storage system on a broker is permanently damaged, any unconsumed message is lost forever. In the future, we plan to add built-in replication in Kafka to redundantly store each me ssage on multiple brokers."

I'm curious, does the built-in replication has been implemented or it's still in progress? Currently, is there anyway to recover the died broker and unconsumed messages?

Thanks,
Haoming