in flink-connector-cassandra/src/main/java/org/apache/flink/streaming/connectors/cassandra/CassandraSinkBase.java [146:157]
public void invoke(IN value) throws Exception {
checkAsyncErrors();
tryAcquire(1);
final ListenableFuture<V> result;
try {
result = send(value);
} catch (Throwable e) {
semaphore.release();
throw e;
}
Futures.addCallback(result, callback);
}