If the org.apache.hadoop.hdfs.server.namenode.ha.AdaptiveFailoverProxyProvider class is unavailable when a cluster of MRS 3.x connects to NameNodes using HDFS, the cause is that the HDFS active/standby switchover class of the cluster is configured improperly. To solve the problem, perform the following operations:
The hadoop-plugins-xxx.jar package is stored in the HDFS client directory, for example, $HADOOP_HOME/share/hadoop/common/lib/hadoop-plugins-8.0.2-302023.jar.