The client does not know which is active / standy namenode it will only usess dfs.nameservices . DFS Client will determine which NameNode is the current Active, by using the either of the java classess which ever is implemented ConfiguredFailoverProxyProvider and the RequestHedgingProxyProvider, then subsequently it will invoke the ids and followed by the host identification by using the properties in hdfs-site.xml ( please refer the below link for the properties . https://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-hdfs/HDFSHighAvailabilityWithQJM.html
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