You need to wait for the servers to be up again before restarting the
next one. nodetool ring on one of the servers you aren't restarting
will tell you when it is back up. You can also watch for "Starting up
server gossip" in the log file to know when it is starting to join the
cluster again.
So restarting cassandra servers has a tendency to cause a lot of exceptions
like "MaximumRetryException: Retried 6 times. Last failure was
UnavailableException()" and "TApplicationException: Internal error
processing batch_mutate" (using pycassa). If I restart the servers too
quickly, I get "all s
