Users running clients using versions >= 2.0.7 and < 2.0.10, and versions >= 2.1.2 and < 2.1.6, may see the error "NoHostAvailableException" when the nodes are not unreachable.
The following error is typical of this scenario
com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (no host was tried)
There jira below was implemented to add behaviour to mark hosts as "down" on read timeouts. This has some unintended side-effects which caused the driver to sometimes mark nodes down there were still available
Upgrade to the java driver 2.0.10 or 2.1.6