Cassandra + Zookeeper, what is the current state?

[Yang]

I read from the Facebook cassandra paper that zookeeper "is used
....." for certain things ( membership and Rack-aware placement)

but I pulled 0.7.0-beta2 source and couldn't grep out anything with
"Zk" or "Zoo",  nor any files with "Zk/Zoo" in the names


is Zookeeper really used? docs/blog posts from online search kind of
give conflicting answers


Thanks
Yang