add metrics for symlink cluster count and total hosts count

d2/src/main/java/com/linkedin/d2/balancer/simple/SimpleLoadBalancerState.java

@@ -694,6 +694,11 @@ public int getTrackerClientCount(String clusterName)
     return count;
   }
 
+  public Set<String> getClusters()
+  {
+    return _uriProperties.keySet();
+  }
+
   public Set<String> getServicesForCluster(String clusterName)
   {
     Set<String> services = _servicesPerCluster.get(clusterName);

d2/src/main/java/com/linkedin/d2/balancer/strategies/relative/RelativeLoadBalancerStrategy.java

@@ -120,6 +120,11 @@ public int getFirstValidPartitionId()
     return _stateUpdater.getFirstValidPartitionId();
   }
 
+  public int getTotalHostsInAllPartitions()
+  {
+    return _stateUpdater.getTotalHostsInAllPartitions();
+  }
+
   /**
    * Exposed for testings
    */

d2/src/main/java/com/linkedin/d2/balancer/strategies/relative/StateUpdater.java

@@ -153,6 +153,17 @@ PartitionState getPartitionState(int partitionId)
     return _partitionLoadBalancerStateMap.get(partitionId);
   }
 
+  /**
+   * Return the total tracker clients in all partitions regardless of their statuses.
+   */
+  int getTotalHostsInAllPartitions()
+  {
+    return _partitionLoadBalancerStateMap.values().stream()
+        .map(partitionState -> partitionState.getTrackerClients().size())
+        .mapToInt(Integer::intValue)
+        .sum();
+  }
+
   /**
    * Return the first valid partition id. This is mainly used for monitoring at least one valid partition.
    */

d2/src/main/java/com/linkedin/d2/discovery/stores/zk/SymlinkUtil.java

@@ -64,4 +64,9 @@ public static boolean containsSymlink(String path)
   {
     return (firstSymlinkIndex(path) < 0) ? false : true;
   }
+
+  public static boolean isSymlinkNodeOrPath(String nodeNameOrPath)
+  {
+    return nodeNameOrPath != null && nodeNameOrPath.indexOf(SYMLINK_PREFIX) >= 0;
+  }
 }

d2/src/main/java/com/linkedin/d2/jmx/RelativeLoadBalancerStrategyJmx.java

@@ -145,6 +145,28 @@ public double getNthPercentileLatencyRelativeFactor(double pct)
     return avgLatency == 0 ? 0 : nthLatency / avgLatency;
   }
 
+  @Override
+  public int getTotalHostsInAllPartitionsCount()
+  {
+    if (isPartitionDataUnavailable())
+    {
+      return DEFAULT_INT_METRICS;
+    }
+
+    return _strategy.getTotalHostsInAllPartitions();
+  }
+
+  @Override
+  public int getTotalHostsCount()
+  {
+    if (isPartitionDataUnavailable())
+    {
+      return DEFAULT_INT_METRICS;
+    }
+
+    return _strategy.getPartitionState(_strategy.getFirstValidPartitionId()).getTrackerClientStateMap().size();
+  }
+
   @Override
   public int getUnhealthyHostsCount()
   {

d2/src/main/java/com/linkedin/d2/jmx/RelativeLoadBalancerStrategyJmxMBean.java

@@ -50,6 +50,18 @@ public interface RelativeLoadBalancerStrategyJmxMBean
    */
   double getNthPercentileLatencyRelativeFactor(double pct);
 
+  /**
+   *
+   * @return the number of total hosts in all partitions regardless of their status
+   */
+  int getTotalHostsInAllPartitionsCount();
+
+  /**
+   *
+   * @return the number of total hosts regardless of their status
+   */
+  int getTotalHostsCount();
+
   /**
    *
    * @return the number of unhealthy hosts

d2/src/main/java/com/linkedin/d2/jmx/SimpleLoadBalancerStateJmx.java

@@ -17,6 +17,7 @@
 package com.linkedin.d2.jmx;
 
 import com.linkedin.d2.balancer.clients.TrackerClient;
+import com.linkedin.d2.discovery.stores.zk.SymlinkUtil;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.util.ArrayList;
@@ -41,6 +42,12 @@ public int getClusterCount()
     return _state.getClusterCount();
   }
 
+  @Override
+  public long getSymlinkClusterCount()
+  {
+    return _state.getClusters().stream().filter(SymlinkUtil::isSymlinkNodeOrPath).count();
+  }
+
   @Override
   public int getClusterListenCount()
   {

d2/src/main/java/com/linkedin/d2/jmx/SimpleLoadBalancerStateJmxMBean.java

@@ -25,6 +25,8 @@ public interface SimpleLoadBalancerStateJmxMBean
 
   int getClusterCount();
 
+  long getSymlinkClusterCount();
+
   int getServiceCount();
 
   long getVersion();

d2/src/main/java/com/linkedin/d2/xds/XdsToD2PropertiesAdaptor.java

@@ -19,7 +19,6 @@
 import com.google.common.collect.HashBiMap;
 import com.google.common.collect.MapDifference;
 import com.google.common.collect.Maps;
-import com.google.protobuf.InvalidProtocolBufferException;
 import com.linkedin.d2.balancer.dualread.DualReadStateManager;
 import com.linkedin.d2.balancer.properties.ClusterProperties;
 import com.linkedin.d2.balancer.properties.ClusterPropertiesJsonSerializer;
@@ -31,6 +30,7 @@
 import com.linkedin.d2.discovery.PropertySerializationException;
 import com.linkedin.d2.discovery.event.PropertyEventBus;
 import com.linkedin.d2.discovery.event.ServiceDiscoveryEventEmitter;
+import com.linkedin.d2.discovery.stores.zk.SymlinkUtil;
 import indis.XdsD2;
 import io.grpc.Status;
 import java.nio.charset.StandardCharsets;
@@ -54,7 +54,6 @@ public class XdsToD2PropertiesAdaptor
   private static final String D2_CLUSTER_NODE_PREFIX = "/d2/clusters/";
   private static final String D2_SERVICE_NODE_PREFIX = "/d2/services/";
   private static final String D2_URI_NODE_PREFIX = "/d2/uris/";
-  private static final char SYMLINK_NODE_IDENTIFIER = '$';
   private static final char PATH_SEPARATOR = '/';
   private static final String NON_EXISTENT_CLUSTER = "NonExistentCluster";
 
@@ -144,7 +143,7 @@ public void listenToCluster(String clusterName)
   {
     // if cluster name is a symlink, watch for D2SymlinkNode instead
     String resourceName = D2_CLUSTER_NODE_PREFIX + clusterName;
-    if (isSymlinkNode(clusterName))
+    if (SymlinkUtil.isSymlinkNodeOrPath(clusterName))
     {
       listenToSymlink(clusterName, resourceName);
     }
@@ -163,7 +162,7 @@ public void listenToUris(String clusterName)
   {
     // if cluster name is a symlink, watch for D2SymlinkNode instead
     String resourceName = D2_URI_NODE_PREFIX + clusterName;
-    if (isSymlinkNode(clusterName))
+    if (SymlinkUtil.isSymlinkNodeOrPath(clusterName))
     {
       listenToSymlink(clusterName, resourceName);
     }
@@ -188,11 +187,6 @@ public void listenToService(String serviceName)
     });
   }
 
-  private static boolean isSymlinkNode(String nodeNameOrPath)
-  {
-    return nodeNameOrPath != null && nodeNameOrPath.indexOf(SYMLINK_NODE_IDENTIFIER) >= 0;
-  }
-
   private void listenToSymlink(String name, String fullResourceName)
   {
     // use full resource name ("/d2/clusters/$FooClusterMater", "/d2/uris/$FooClusterMaster") as the key