HDFS-9112. Improve error message for Haadmin when multiple name service IDs are configured. Contributed by Anu Engineer.

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -965,6 +965,9 @@ Release 2.8.0 - UNRELEASED
     HDFS-5795. RemoteBlockReader2#checkSuccess() shoud print error status.
     (Xiao Chen via Yongjun Zhang)
 
+    HDFS-9112. Improve error message for Haadmin when multiple name service IDs
+    are configured. (Anu Engineer via jing9)
+
   OPTIMIZATIONS
 
     HDFS-8026. Trace FSOutputSummer#writeChecksumChunks rather than

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/tools/NNHAServiceTarget.java

@@ -18,6 +18,7 @@
 package org.apache.hadoop.hdfs.tools;
 
 import java.net.InetSocketAddress;
+import java.util.Arrays;
 import java.util.Map;
 
 import org.apache.hadoop.classification.InterfaceAudience;
@@ -34,6 +35,8 @@ import org.apache.hadoop.net.NetUtils;
 
 import com.google.common.base.Preconditions;
 
+import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_NAMESERVICES;
+
 /**
  * One of the NN NameNodes acting as the target of an administrative command
  * (e.g. failover).
@@ -56,12 +59,19 @@ public class NNHAServiceTarget extends HAServiceTarget {
   public NNHAServiceTarget(Configuration conf,
       String nsId, String nnId) {
     Preconditions.checkNotNull(nnId);
-    
+
     if (nsId == null) {
       nsId = DFSUtil.getOnlyNameServiceIdOrNull(conf);
       if (nsId == null) {
-        throw new IllegalArgumentException(
-            "Unable to determine the nameservice id.");
+        String errorString = "Unable to determine the name service ID.";
+        String[] dfsNames = conf.getStrings(DFS_NAMESERVICES);
+        if ((dfsNames != null) && (dfsNames.length > 1)) {
+          errorString = "Unable to determine the name service ID. " +
+              "This is an HA configuration with multiple name services " +
+              "configured. " + DFS_NAMESERVICES + " is set to " +
+              Arrays.toString(dfsNames) + ". Please re-run with the -ns option.";
+        }
+        throw new IllegalArgumentException(errorString);
       }
     }
     assert nsId != null;