YARN-11442. Refactor FederationInterceptorREST Code.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-common/src/main/java/org/apache/hadoop/yarn/server/federation/utils/FederationStateStoreFacade.java

@@ -26,6 +26,7 @@
 import java.util.Map;
 import java.util.concurrent.TimeUnit;
 import java.util.Random;
+import java.util.Collection;
 
 import javax.cache.Cache;
 import javax.cache.CacheManager;
@@ -1211,4 +1212,19 @@ public boolean deregisterSubCluster(SubClusterId subClusterId,
     }
     return false;
   }
+
+  /**
+   * Get active subclusters.
+   *
+   * @return We will return a list of active subclusters as a Collection.
+   */
+  public Collection<SubClusterInfo> getActiveSubClusters() {
+    try {
+      Map<SubClusterId, SubClusterInfo> subClusterMap = getSubClusters(true);
+      return subClusterMap.values();
+    } catch (Exception e) {
+      LOG.error("getSubClustersCollection failed.", e);
+      return null;
+    }
+  }
 }