The cassandra nodes corresponding to the data center might be down.
First check the cassandra status by below :
==>{INST_DIR} /apigee4/etc/init.d/apigee-cassandra status
If it is stopped then try to start it again
==> {INST_DIR}/ apigee4/etc/init.d/apigee-cassandra start
If after restart also it shows stopped then this might be because of a lingering cassandra daemon process on the servers.
We can check this by: ps -ef | grep cassandra
We will be able to see cassandra daemon process running if it's the case then try to explicitly kill the daemon process {require root access} and try to start the cassandra node again.
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