Sequence of logs:

Before logs:

2020-07-29 11:33:00:736*[ERROR]*transformconvertreg (1/2)*RequestHandler*onFailure*Unexpected error while querying nosqldb-0.nosqldb.default.svc.cluster.local/host2
com.datastax.driver.core.exceptions.ConnectionException: [nosqldb-0.nosqldb.default.svc.cluster.local/host2:9042] Pool is CLOSING
at com.datastax.driver.core.HostConnectionPool.borrowConnection(HostConnectionPool.java:241)
at com.datastax.driver.core.RequestHandler$SpeculativeExecution.query(RequestHandler.java:397)
at com.datastax.driver.core.RequestHandler$SpeculativeExecution.findNextHostAndQuery(RequestHandler.java:360)

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Posted log in datastax community..

2020-07-29 11:33:00:747*[ERROR]*transformconvertreg (2/2)*RequestHandler*onFailure*Unexpected error while querying nosqldb-2.nosqldb.default.svc.cluster.local/host1
com.datastax.driver.core.exceptions.ConnectionException: [nosqldb-2.nosqldb.default.svc.cluster.local/host1:9042] Pool is CLOSING
at com.datastax.driver.core.HostConnectionPool.borrowConnection(HostConnectionPool.java:241)
at com.datastax.driver.core.RequestHandler$SpeculativeExecution.query(RequestHandler.java:397)
at com.datastax.driver.core.RequestHandler$SpeculativeExecution.findNextHostAndQuery(RequestHandler.java:360)

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after logs:

2020-07-29 11:33:00:771*[ERROR]*transformconvertreg (2/2)*RegistrationRecordConvertor*processElement
java.lang.RuntimeException: Error preparing queries for accessor DetailsAccessor
	at com.datastax.driver.mapping.AccessorMapper.prepare(AccessorMapper.java:63)
	at com.datastax.driver.mapping.MappingManager.getAccessor(MappingManager.java:359)
	at com.datastax.driver.mapping.MappingManager.createAccessor(MappingManager.java:321)
	at com.test.cassandra.dao.DetailsDao.fetchDetailsImpl(DetailsDao.java:87)
	at com.test.cloud.util.DAOUtils.fetchDetails(DAOUtils.java:130)
	at com.test.cloud.processing.operators.RegistrationRecordConvertor.processElement(RegistrationRecordConvertor.java:94)
	at com.test.cloud.processing.operators.RegistrationRecordConvertor.processElement(RegistrationRecordConvertor.java:45)
	at org.apache.flink.streaming.api.operators.KeyedProcessOperator.processElement(KeyedProcessOperator.java:85)
	at org.apache.flink.streaming.runtime.io.StreamInputProcessor.processInput(StreamInputProcessor.java:202)
	at org.apache.flink.streaming.runtime.tasks.OneInputStreamTask.run(OneInputStreamTask.java:105)
	at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:300)
	at org.apache.flink.runtime.taskmanager.Task.run(Task.java:711)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.util.concurrent.ExecutionException: com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (tried: nosqldb-2.nosqldb.default.svc.cluster.local/host1:9042 (com.datastax.driver.core.exceptions.ConnectionException: [nosqldb-2.nosqldb.default.svc.cluster.local/host1:9042] Pool is CLOSING))
	at com.google.common.util.concurrent.AbstractFuture.getDoneValue(AbstractFuture.java:502)
	at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:461)
	at com.google.common.util.concurrent.AbstractFuture$TrustedFuture.get(AbstractFuture.java:83)
	at com.datastax.driver.mapping.AccessorMapper.prepare(AccessorMapper.java:58)
	... 12 common frames omitted
Caused by: com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (tried: nosqldb-2.nosqldb.default.svc.cluster.local/host1:9042 (com.datastax.driver.core.exceptions.ConnectionException: [nosqldb-2.nosqldb.default.svc.cluster.local/host1:9042] Pool is CLOSING))


2020-07-29 11:33:00:783*[ERROR]*transformconvertreg (2/2)*RegistrationRecordConvertor*processElement
java.lang.RuntimeException: Error preparing queries for accessor DetailsAccessor
	at com.datastax.driver.mapping.AccessorMapper.prepare(AccessorMapper.java:63)
	at com.datastax.driver.mapping.MappingManager.getAccessor(MappingManager.java:359)
	at com.datastax.driver.mapping.MappingManager.createAccessor(MappingManager.java:321)
	at com.test.cassandra.dao.DetailsDao.fetchDetailsImpl(DetailsDao.java:87)
	at com.test.cloud.util.DAOUtils.fetchDetails(DAOUtils.java:130)