/**
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
    * with the License.  You may obtain a copy of the License at
    *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.hadoop.hbase.master.balancer;
  
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.Comparator;
  import java.util.Deque;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.Map.Entry;
  import java.util.NavigableMap;
  import java.util.Random;
  import java.util.Set;
  import java.util.TreeMap;
  
  import org.apache.commons.lang.NotImplementedException;
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.ClusterStatus;
  import org.apache.hadoop.hbase.HBaseIOException;
  import org.apache.hadoop.hbase.HDFSBlocksDistribution;
  import org.apache.hadoop.hbase.HRegionInfo;
  import org.apache.hadoop.hbase.RegionLoad;
  import org.apache.hadoop.hbase.ServerName;
  import org.apache.hadoop.hbase.client.RegionReplicaUtil;
  import org.apache.hadoop.hbase.master.LoadBalancer;
  import org.apache.hadoop.hbase.master.MasterServices;
  import org.apache.hadoop.hbase.master.RackManager;
  import org.apache.hadoop.hbase.master.RegionPlan;
  import org.apache.hadoop.hbase.master.balancer.BaseLoadBalancer.Cluster.Action.Type;
  import org.apache.hadoop.util.StringUtils;
  
  import com.google.common.annotations.VisibleForTesting;
  import com.google.common.base.Joiner;
  import com.google.common.collect.ArrayListMultimap;
  import com.google.common.collect.Lists;
  import com.google.common.collect.Sets;
  
  /**
   * The base class for load balancers. It provides the the functions used to by
   * {@link org.apache.hadoop.hbase.master.AssignmentManager} to assign regions
   * in the edge cases. It doesn't provide an implementation of the actual
   * balancing algorithm.
   *
   */
  public abstract class BaseLoadBalancer implements LoadBalancer {
    private static final int MIN_SERVER_BALANCE = 2;
    private volatile boolean stopped = false;
  
    private static final List<HRegionInfo> EMPTY_REGION_LIST = new ArrayList<HRegionInfo>(0);
  
    protected final RegionLocationFinder regionFinder = new RegionLocationFinder();
  
    private static class DefaultRackManager extends RackManager {
      @Override
      public String getRack(ServerName server) {
        return UNKNOWN_RACK;
      }
    }
  
    /**
     * The constructor that uses the basic MetricsBalancer
     */
    protected BaseLoadBalancer() {
      metricsBalancer = new MetricsBalancer();
    }
  
    /**
     * This Constructor accepts an instance of MetricsBalancer,
     * which will be used instead of creating a new one
     */
    protected BaseLoadBalancer(MetricsBalancer metricsBalancer) {
      this.metricsBalancer = (metricsBalancer != null) ? metricsBalancer : new MetricsBalancer();
    }
  
    /**
     * An efficient array based implementation similar to ClusterState for keeping
    * the status of the cluster in terms of region assignment and distribution.
    * LoadBalancers, such as StochasticLoadBalancer uses this Cluster object because of
    * hundreds of thousands of hashmap manipulations are very costly, which is why this
    * class uses mostly indexes and arrays.
    *
    * Cluster tracks a list of unassigned regions, region assignments, and the server
    * topology in terms of server names, hostnames and racks.
    */
   protected static class Cluster {
     ServerName[] servers;
     String[] hosts; // ServerName uniquely identifies a region server. multiple RS can run on the same host
     String[] racks;
     boolean multiServersPerHost = false; // whether or not any host has more than one server
 
     ArrayList<String> tables;
     HRegionInfo[] regions;
     Deque<RegionLoad>[] regionLoads;
     private RegionLocationFinder regionFinder;
 
     int[][] regionLocations; //regionIndex -> list of serverIndex sorted by locality
 
     int[]   serverIndexToHostIndex;      //serverIndex -> host index
     int[]   serverIndexToRackIndex;      //serverIndex -> rack index
 
     int[][] regionsPerServer;            //serverIndex -> region list
     int[][] regionsPerHost;              //hostIndex -> list of regions
     int[][] regionsPerRack;              //rackIndex -> region list
     int[][] primariesOfRegionsPerServer; //serverIndex -> sorted list of regions by primary region index
     int[][] primariesOfRegionsPerHost;   //hostIndex -> sorted list of regions by primary region index
     int[][] primariesOfRegionsPerRack;   //rackIndex -> sorted list of regions by primary region index
 
     int[][] serversPerHost;              //hostIndex -> list of server indexes
     int[][] serversPerRack;              //rackIndex -> list of server indexes
     int[]   regionIndexToServerIndex;    //regionIndex -> serverIndex
     int[]   initialRegionIndexToServerIndex;    //regionIndex -> serverIndex (initial cluster state)
     int[]   regionIndexToTableIndex;     //regionIndex -> tableIndex
     int[][] numRegionsPerServerPerTable; //serverIndex -> tableIndex -> # regions
     int[]   numMaxRegionsPerTable;       //tableIndex -> max number of regions in a single RS
     int[]   regionIndexToPrimaryIndex;   //regionIndex -> regionIndex of the primary
     boolean hasRegionReplicas = false;   //whether there is regions with replicas
 
     Integer[] serverIndicesSortedByRegionCount;
     Integer[] serverIndicesSortedByLocality;
 
     Map<String, Integer> serversToIndex;
     Map<String, Integer> hostsToIndex;
     Map<String, Integer> racksToIndex;
     Map<String, Integer> tablesToIndex;
     Map<HRegionInfo, Integer> regionsToIndex;
     float[] localityPerServer;
 
     int numServers;
     int numHosts;
     int numRacks;
     int numTables;
     int numRegions;
 
     int numMovedRegions = 0; //num moved regions from the initial configuration
     Map<ServerName, List<HRegionInfo>> clusterState;
 
     protected final RackManager rackManager;
 
     protected Cluster(
         Map<ServerName, List<HRegionInfo>> clusterState,
         Map<String, Deque<RegionLoad>> loads,
         RegionLocationFinder regionFinder,
         RackManager rackManager) {
       this(null, clusterState, loads, regionFinder,
         rackManager);
     }
 
     @SuppressWarnings("unchecked")
     protected Cluster(
         Collection<HRegionInfo> unassignedRegions,
         Map<ServerName, List<HRegionInfo>> clusterState,
         Map<String, Deque<RegionLoad>> loads,
         RegionLocationFinder regionFinder,
         RackManager rackManager) {
 
       if (unassignedRegions == null) {
         unassignedRegions = EMPTY_REGION_LIST;
       }
 
       serversToIndex = new HashMap<String, Integer>();
       hostsToIndex = new HashMap<String, Integer>();
       racksToIndex = new HashMap<String, Integer>();
       tablesToIndex = new HashMap<String, Integer>();
 
       //TODO: We should get the list of tables from master
       tables = new ArrayList<String>();
       this.rackManager = rackManager != null ? rackManager : new DefaultRackManager();
 
       numRegions = 0;
 
       List<List<Integer>> serversPerHostList = new ArrayList<List<Integer>>();
       List<List<Integer>> serversPerRackList = new ArrayList<List<Integer>>();
       this.clusterState = clusterState;
       this.regionFinder = regionFinder;
 
       // Use servername and port as there can be dead servers in this list. We want everything with
       // a matching hostname and port to have the same index.
       for (ServerName sn : clusterState.keySet()) {
         if (serversToIndex.get(sn.getHostAndPort()) == null) {
           serversToIndex.put(sn.getHostAndPort(), numServers++);
         }
         if (!hostsToIndex.containsKey(sn.getHostname())) {
           hostsToIndex.put(sn.getHostname(), numHosts++);
           serversPerHostList.add(new ArrayList<Integer>(1));
         }
 
         int serverIndex = serversToIndex.get(sn.getHostAndPort());
         int hostIndex = hostsToIndex.get(sn.getHostname());
         serversPerHostList.get(hostIndex).add(serverIndex);
 
         String rack = this.rackManager.getRack(sn);
         if (!racksToIndex.containsKey(rack)) {
           racksToIndex.put(rack, numRacks++);
           serversPerRackList.add(new ArrayList<Integer>());
         }
         int rackIndex = racksToIndex.get(rack);
         serversPerRackList.get(rackIndex).add(serverIndex);
       }
 
       // Count how many regions there are.
       for (Entry<ServerName, List<HRegionInfo>> entry : clusterState.entrySet()) {
         numRegions += entry.getValue().size();
       }
       numRegions += unassignedRegions.size();
 
       regionsToIndex = new HashMap<HRegionInfo, Integer>(numRegions);
       servers = new ServerName[numServers];
       serversPerHost = new int[numHosts][];
       serversPerRack = new int[numRacks][];
       regions = new HRegionInfo[numRegions];
       regionIndexToServerIndex = new int[numRegions];
       initialRegionIndexToServerIndex = new int[numRegions];
       regionIndexToTableIndex = new int[numRegions];
       regionIndexToPrimaryIndex = new int[numRegions];
       regionLoads = new Deque[numRegions];
       regionLocations = new int[numRegions][];
       serverIndicesSortedByRegionCount = new Integer[numServers];
       serverIndicesSortedByLocality = new Integer[numServers];
       localityPerServer = new float[numServers];
 
       serverIndexToHostIndex = new int[numServers];
       serverIndexToRackIndex = new int[numServers];
       regionsPerServer = new int[numServers][];
       regionsPerHost = new int[numHosts][];
       regionsPerRack = new int[numRacks][];
       primariesOfRegionsPerServer = new int[numServers][];
       primariesOfRegionsPerHost = new int[numHosts][];
       primariesOfRegionsPerRack = new int[numRacks][];
 
       int tableIndex = 0, regionIndex = 0, regionPerServerIndex = 0;
 
       for (Entry<ServerName, List<HRegionInfo>> entry : clusterState.entrySet()) {
         int serverIndex = serversToIndex.get(entry.getKey().getHostAndPort());
 
         // keep the servername if this is the first server name for this hostname
         // or this servername has the newest startcode.
         if (servers[serverIndex] == null ||
             servers[serverIndex].getStartcode() < entry.getKey().getStartcode()) {
           servers[serverIndex] = entry.getKey();
         }
 
         if (regionsPerServer[serverIndex] != null) {
           // there is another server with the same hostAndPort in ClusterState.
           // allocate the array for the total size
           regionsPerServer[serverIndex] = new int[entry.getValue().size() + regionsPerServer[serverIndex].length];
         } else {
           regionsPerServer[serverIndex] = new int[entry.getValue().size()];
         }
         primariesOfRegionsPerServer[serverIndex] = new int[regionsPerServer[serverIndex].length];
         serverIndicesSortedByRegionCount[serverIndex] = serverIndex;
         serverIndicesSortedByLocality[serverIndex] = serverIndex;
       }
 
       hosts = new String[numHosts];
       for (Entry<String, Integer> entry : hostsToIndex.entrySet()) {
         hosts[entry.getValue()] = entry.getKey();
       }
       racks = new String[numRacks];
       for (Entry<String, Integer> entry : racksToIndex.entrySet()) {
         racks[entry.getValue()] = entry.getKey();
       }
 
       for (Entry<ServerName, List<HRegionInfo>> entry : clusterState.entrySet()) {
         int serverIndex = serversToIndex.get(entry.getKey().getHostAndPort());
         regionPerServerIndex = 0;
 
         int hostIndex = hostsToIndex.get(entry.getKey().getHostname());
         serverIndexToHostIndex[serverIndex] = hostIndex;
 
         int rackIndex = racksToIndex.get(this.rackManager.getRack(entry.getKey()));
         serverIndexToRackIndex[serverIndex] = rackIndex;
 
         for (HRegionInfo region : entry.getValue()) {
           registerRegion(region, regionIndex, serverIndex, loads, regionFinder);
 
           regionsPerServer[serverIndex][regionPerServerIndex++] = regionIndex;
           regionIndex++;
         }
       }
       for (HRegionInfo region : unassignedRegions) {
         registerRegion(region, regionIndex, -1, loads, regionFinder);
         regionIndex++;
       }
 
       for (int i = 0; i < serversPerHostList.size(); i++) {
         serversPerHost[i] = new int[serversPerHostList.get(i).size()];
         for (int j = 0; j < serversPerHost[i].length; j++) {
           serversPerHost[i][j] = serversPerHostList.get(i).get(j);
         }
         if (serversPerHost[i].length > 1) {
           multiServersPerHost = true;
         }
       }
 
       for (int i = 0; i < serversPerRackList.size(); i++) {
         serversPerRack[i] = new int[serversPerRackList.get(i).size()];
         for (int j = 0; j < serversPerRack[i].length; j++) {
           serversPerRack[i][j] = serversPerRackList.get(i).get(j);
         }
       }
 
       numTables = tables.size();
       numRegionsPerServerPerTable = new int[numServers][numTables];
 
       for (int i = 0; i < numServers; i++) {
         for (int j = 0; j < numTables; j++) {
           numRegionsPerServerPerTable[i][j] = 0;
         }
       }
 
       for (int i=0; i < regionIndexToServerIndex.length; i++) {
         if (regionIndexToServerIndex[i] >= 0) {
           numRegionsPerServerPerTable[regionIndexToServerIndex[i]][regionIndexToTableIndex[i]]++;
         }
       }
 
       numMaxRegionsPerTable = new int[numTables];
       for (int serverIndex = 0 ; serverIndex < numRegionsPerServerPerTable.length; serverIndex++) {
         for (tableIndex = 0 ; tableIndex < numRegionsPerServerPerTable[serverIndex].length; tableIndex++) {
           if (numRegionsPerServerPerTable[serverIndex][tableIndex] > numMaxRegionsPerTable[tableIndex]) {
             numMaxRegionsPerTable[tableIndex] = numRegionsPerServerPerTable[serverIndex][tableIndex];
           }
         }
       }
 
       for (int i = 0; i < regions.length; i ++) {
         HRegionInfo info = regions[i];
         if (RegionReplicaUtil.isDefaultReplica(info)) {
           regionIndexToPrimaryIndex[i] = i;
         } else {
           hasRegionReplicas = true;
           HRegionInfo primaryInfo = RegionReplicaUtil.getRegionInfoForDefaultReplica(info);
           regionIndexToPrimaryIndex[i] =
               regionsToIndex.containsKey(primaryInfo) ?
               regionsToIndex.get(primaryInfo):
               -1;
         }
       }
 
       for (int i = 0; i < regionsPerServer.length; i++) {
         primariesOfRegionsPerServer[i] = new int[regionsPerServer[i].length];
         for (int j = 0; j < regionsPerServer[i].length; j++) {
           int primaryIndex = regionIndexToPrimaryIndex[regionsPerServer[i][j]];
           primariesOfRegionsPerServer[i][j] = primaryIndex;
         }
         // sort the regions by primaries.
         Arrays.sort(primariesOfRegionsPerServer[i]);
       }
 
       // compute regionsPerHost
       if (multiServersPerHost) {
         for (int i = 0 ; i < serversPerHost.length; i++) {
           int numRegionsPerHost = 0;
           for (int j = 0; j < serversPerHost[i].length; j++) {
             numRegionsPerHost += regionsPerServer[serversPerHost[i][j]].length;
           }
           regionsPerHost[i] = new int[numRegionsPerHost];
           primariesOfRegionsPerHost[i] = new int[numRegionsPerHost];
         }
         for (int i = 0 ; i < serversPerHost.length; i++) {
           int numRegionPerHostIndex = 0;
           for (int j = 0; j < serversPerHost[i].length; j++) {
             for (int k = 0; k < regionsPerServer[serversPerHost[i][j]].length; k++) {
               int region = regionsPerServer[serversPerHost[i][j]][k];
               regionsPerHost[i][numRegionPerHostIndex] = region;
               int primaryIndex = regionIndexToPrimaryIndex[region];
               primariesOfRegionsPerHost[i][numRegionPerHostIndex] = primaryIndex;
               numRegionPerHostIndex++;
             }
           }
           // sort the regions by primaries.
           Arrays.sort(primariesOfRegionsPerHost[i]);
         }
       }
 
       // compute regionsPerRack
       if (numRacks > 1) {
         for (int i = 0 ; i < serversPerRack.length; i++) {
           int numRegionsPerRack = 0;
           for (int j = 0; j < serversPerRack[i].length; j++) {
             numRegionsPerRack += regionsPerServer[serversPerRack[i][j]].length;
           }
           regionsPerRack[i] = new int[numRegionsPerRack];
           primariesOfRegionsPerRack[i] = new int[numRegionsPerRack];
         }
 
         for (int i = 0 ; i < serversPerRack.length; i++) {
           int numRegionPerRackIndex = 0;
           for (int j = 0; j < serversPerRack[i].length; j++) {
             for (int k = 0; k < regionsPerServer[serversPerRack[i][j]].length; k++) {
               int region = regionsPerServer[serversPerRack[i][j]][k];
               regionsPerRack[i][numRegionPerRackIndex] = region;
               int primaryIndex = regionIndexToPrimaryIndex[region];
               primariesOfRegionsPerRack[i][numRegionPerRackIndex] = primaryIndex;
               numRegionPerRackIndex++;
             }
           }
           // sort the regions by primaries.
           Arrays.sort(primariesOfRegionsPerRack[i]);
         }
       }
     }
 
     /** Helper for Cluster constructor to handle a region */
     private void registerRegion(HRegionInfo region, int regionIndex, int serverIndex,
         Map<String, Deque<RegionLoad>> loads, RegionLocationFinder regionFinder) {
       String tableName = region.getTable().getNameAsString();
       if (!tablesToIndex.containsKey(tableName)) {
         tables.add(tableName);
         tablesToIndex.put(tableName, tablesToIndex.size());
       }
       int tableIndex = tablesToIndex.get(tableName);
 
       regionsToIndex.put(region, regionIndex);
       regions[regionIndex] = region;
       regionIndexToServerIndex[regionIndex] = serverIndex;
       initialRegionIndexToServerIndex[regionIndex] = serverIndex;
       regionIndexToTableIndex[regionIndex] = tableIndex;
 
       // region load
       if (loads != null) {
         Deque<RegionLoad> rl = loads.get(region.getRegionNameAsString());
         // That could have failed if the RegionLoad is using the other regionName
         if (rl == null) {
           // Try getting the region load using encoded name.
           rl = loads.get(region.getEncodedName());
         }
         regionLoads[regionIndex] = rl;
       }
 
       if (regionFinder != null) {
         //region location
         List<ServerName> loc = regionFinder.getTopBlockLocations(region);
         regionLocations[regionIndex] = new int[loc.size()];
         for (int i=0; i < loc.size(); i++) {
           regionLocations[regionIndex][i] =
               loc.get(i) == null ? -1 :
                 (serversToIndex.get(loc.get(i).getHostAndPort()) == null ? -1
                     : serversToIndex.get(loc.get(i).getHostAndPort()));
         }
       }
     }
 
     /** An action to move or swap a region */
     public static class Action {
       public static enum Type {
         ASSIGN_REGION,
         MOVE_REGION,
         SWAP_REGIONS,
         NULL,
       }
 
       public Type type;
       public Action (Type type) {this.type = type;}
       /** Returns an Action which would undo this action */
       public Action undoAction() { return this; }
       @Override
       public String toString() { return type + ":";}
     }
 
     public static class AssignRegionAction extends Action {
       public int region;
       public int server;
       public AssignRegionAction(int region, int server) {
         super(Type.ASSIGN_REGION);
         this.region = region;
         this.server = server;
       }
       @Override
       public Action undoAction() {
         // TODO implement this. This action is not being used by the StochasticLB for now
         // in case it uses it, we should implement this function.
         throw new NotImplementedException();
       }
       @Override
       public String toString() {
         return type + ": " + region + ":" + server;
       }
     }
 
     public static class MoveRegionAction extends Action {
       public int region;
       public int fromServer;
       public int toServer;
 
       public MoveRegionAction(int region, int fromServer, int toServer) {
         super(Type.MOVE_REGION);
         this.fromServer = fromServer;
         this.region = region;
         this.toServer = toServer;
       }
       @Override
       public Action undoAction() {
         return new MoveRegionAction (region, toServer, fromServer);
       }
       @Override
       public String toString() {
         return type + ": " + region + ":" + fromServer + " -> " + toServer;
       }
     }
 
     public static class SwapRegionsAction extends Action {
       public int fromServer;
       public int fromRegion;
       public int toServer;
       public int toRegion;
       public SwapRegionsAction(int fromServer, int fromRegion, int toServer, int toRegion) {
         super(Type.SWAP_REGIONS);
         this.fromServer = fromServer;
         this.fromRegion = fromRegion;
         this.toServer = toServer;
         this.toRegion = toRegion;
       }
       @Override
       public Action undoAction() {
         return new SwapRegionsAction (fromServer, toRegion, toServer, fromRegion);
       }
       @Override
       public String toString() {
         return type + ": " + fromRegion + ":" + fromServer + " <-> " + toRegion + ":" + toServer;
       }
     }
 
     @edu.umd.cs.findbugs.annotations.SuppressWarnings(value="NM_FIELD_NAMING_CONVENTION",
         justification="Mistake. Too disruptive to change now")
     public static final Action NullAction = new Action(Type.NULL);
 
     public void doAction(Action action) {
       switch (action.type) {
       case NULL: break;
       case ASSIGN_REGION:
         // FindBugs: Having the assert quietens FB BC_UNCONFIRMED_CAST warnings
         assert action instanceof AssignRegionAction: action.getClass();
         AssignRegionAction ar = (AssignRegionAction) action;
         regionsPerServer[ar.server] = addRegion(regionsPerServer[ar.server], ar.region);
         regionMoved(ar.region, -1, ar.server);
         break;
       case MOVE_REGION:
         assert action instanceof MoveRegionAction: action.getClass();
         MoveRegionAction mra = (MoveRegionAction) action;
         regionsPerServer[mra.fromServer] = removeRegion(regionsPerServer[mra.fromServer], mra.region);
         regionsPerServer[mra.toServer] = addRegion(regionsPerServer[mra.toServer], mra.region);
         regionMoved(mra.region, mra.fromServer, mra.toServer);
         break;
       case SWAP_REGIONS:
         assert action instanceof SwapRegionsAction: action.getClass();
         SwapRegionsAction a = (SwapRegionsAction) action;
         regionsPerServer[a.fromServer] = replaceRegion(regionsPerServer[a.fromServer], a.fromRegion, a.toRegion);
         regionsPerServer[a.toServer] = replaceRegion(regionsPerServer[a.toServer], a.toRegion, a.fromRegion);
         regionMoved(a.fromRegion, a.fromServer, a.toServer);
         regionMoved(a.toRegion, a.toServer, a.fromServer);
         break;
       default:
         throw new RuntimeException("Uknown action:" + action.type);
       }
     }
 
     /**
      * Return true if the placement of region on server would lower the availability
      * of the region in question
      * @param server
      * @param region
      * @return true or false
      */
     boolean wouldLowerAvailability(HRegionInfo regionInfo, ServerName serverName) {
       if (!serversToIndex.containsKey(serverName.getHostAndPort())) {
         return false; // safeguard against race between cluster.servers and servers from LB method args
       }
       int server = serversToIndex.get(serverName.getHostAndPort());
       int region = regionsToIndex.get(regionInfo);
 
       int primary = regionIndexToPrimaryIndex[region];
 
       // there is a subset relation for server < host < rack
       // check server first
 
       if (contains(primariesOfRegionsPerServer[server], primary)) {
         // check for whether there are other servers that we can place this region
         for (int i = 0; i < primariesOfRegionsPerServer.length; i++) {
           if (i != server && !contains(primariesOfRegionsPerServer[i], primary)) {
             return true; // meaning there is a better server
           }
         }
         return false; // there is not a better server to place this
       }
 
       // check host
       if (multiServersPerHost) { // these arrays would only be allocated if we have more than one server per host
         int host = serverIndexToHostIndex[server];
         if (contains(primariesOfRegionsPerHost[host], primary)) {
           // check for whether there are other hosts that we can place this region
           for (int i = 0; i < primariesOfRegionsPerHost.length; i++) {
             if (i != host && !contains(primariesOfRegionsPerHost[i], primary)) {
               return true; // meaning there is a better host
             }
           }
           return false; // there is not a better host to place this
         }
       }
 
       // check rack
       if (numRacks > 1) {
         int rack = serverIndexToRackIndex[server];
         if (contains(primariesOfRegionsPerRack[rack], primary)) {
           // check for whether there are other racks that we can place this region
           for (int i = 0; i < primariesOfRegionsPerRack.length; i++) {
             if (i != rack && !contains(primariesOfRegionsPerRack[i], primary)) {
               return true; // meaning there is a better rack
             }
           }
           return false; // there is not a better rack to place this
         }
       }
       return false;
     }
 
     void doAssignRegion(HRegionInfo regionInfo, ServerName serverName) {
       if (!serversToIndex.containsKey(serverName.getHostAndPort())) {
         return;
       }
       int server = serversToIndex.get(serverName.getHostAndPort());
       int region = regionsToIndex.get(regionInfo);
       doAction(new AssignRegionAction(region, server));
     }
 
     void regionMoved(int region, int oldServer, int newServer) {
       regionIndexToServerIndex[region] = newServer;
       if (initialRegionIndexToServerIndex[region] == newServer) {
         numMovedRegions--; //region moved back to original location
       } else if (oldServer >= 0 && initialRegionIndexToServerIndex[region] == oldServer) {
         numMovedRegions++; //region moved from original location
       }
       int tableIndex = regionIndexToTableIndex[region];
       if (oldServer >= 0) {
         numRegionsPerServerPerTable[oldServer][tableIndex]--;
       }
       numRegionsPerServerPerTable[newServer][tableIndex]++;
 
       //check whether this caused maxRegionsPerTable in the new Server to be updated
       if (numRegionsPerServerPerTable[newServer][tableIndex] > numMaxRegionsPerTable[tableIndex]) {
         numRegionsPerServerPerTable[newServer][tableIndex] = numMaxRegionsPerTable[tableIndex];
       } else if (oldServer >= 0 && (numRegionsPerServerPerTable[oldServer][tableIndex] + 1)
           == numMaxRegionsPerTable[tableIndex]) {
         //recompute maxRegionsPerTable since the previous value was coming from the old server
         for (int serverIndex = 0 ; serverIndex < numRegionsPerServerPerTable.length; serverIndex++) {
           if (numRegionsPerServerPerTable[serverIndex][tableIndex] > numMaxRegionsPerTable[tableIndex]) {
             numMaxRegionsPerTable[tableIndex] = numRegionsPerServerPerTable[serverIndex][tableIndex];
           }
         }
       }
 
       // update for servers
       int primary = regionIndexToPrimaryIndex[region];
       if (oldServer >= 0) {
         primariesOfRegionsPerServer[oldServer] = removeRegion(
           primariesOfRegionsPerServer[oldServer], primary);
       }
       primariesOfRegionsPerServer[newServer] = addRegionSorted(
         primariesOfRegionsPerServer[newServer], primary);
 
       // update for hosts
       if (multiServersPerHost) {
         int oldHost = oldServer >= 0 ? serverIndexToHostIndex[oldServer] : -1;
         int newHost = serverIndexToHostIndex[newServer];
         if (newHost != oldHost) {
           regionsPerHost[newHost] = addRegion(regionsPerHost[newHost], region);
           primariesOfRegionsPerHost[newHost] = addRegionSorted(primariesOfRegionsPerHost[newHost], primary);
           if (oldHost >= 0) {
             regionsPerHost[oldHost] = removeRegion(regionsPerHost[oldHost], region);
             primariesOfRegionsPerHost[oldHost] = removeRegion(
               primariesOfRegionsPerHost[oldHost], primary); // will still be sorted
           }
         }
       }
 
       // update for racks
       if (numRacks > 1) {
         int oldRack = oldServer >= 0 ? serverIndexToRackIndex[oldServer] : -1;
         int newRack = serverIndexToRackIndex[newServer];
         if (newRack != oldRack) {
           regionsPerRack[newRack] = addRegion(regionsPerRack[newRack], region);
           primariesOfRegionsPerRack[newRack] = addRegionSorted(primariesOfRegionsPerRack[newRack], primary);
           if (oldRack >= 0) {
             regionsPerRack[oldRack] = removeRegion(regionsPerRack[oldRack], region);
             primariesOfRegionsPerRack[oldRack] = removeRegion(
               primariesOfRegionsPerRack[oldRack], primary); // will still be sorted
           }
         }
       }
     }
 
     int[] removeRegion(int[] regions, int regionIndex) {
       //TODO: this maybe costly. Consider using linked lists
       int[] newRegions = new int[regions.length - 1];
       int i = 0;
       for (i = 0; i < regions.length; i++) {
         if (regions[i] == regionIndex) {
           break;
         }
         newRegions[i] = regions[i];
       }
       System.arraycopy(regions, i+1, newRegions, i, newRegions.length - i);
       return newRegions;
     }
 
     int[] addRegion(int[] regions, int regionIndex) {
       int[] newRegions = new int[regions.length + 1];
       System.arraycopy(regions, 0, newRegions, 0, regions.length);
       newRegions[newRegions.length - 1] = regionIndex;
       return newRegions;
     }
 
     int[] addRegionSorted(int[] regions, int regionIndex) {
       int[] newRegions = new int[regions.length + 1];
       int i = 0;
       for (i = 0; i < regions.length; i++) { // find the index to insert
         if (regions[i] > regionIndex) {
           break;
         }
       }
       System.arraycopy(regions, 0, newRegions, 0, i); // copy first half
       System.arraycopy(regions, i, newRegions, i+1, regions.length - i); // copy second half
       newRegions[i] = regionIndex;
 
       return newRegions;
     }
 
     int[] replaceRegion(int[] regions, int regionIndex, int newRegionIndex) {
       int i = 0;
       for (i = 0; i < regions.length; i++) {
         if (regions[i] == regionIndex) {
           regions[i] = newRegionIndex;
           break;
         }
       }
       return regions;
     }
 
     void sortServersByRegionCount() {
       Arrays.sort(serverIndicesSortedByRegionCount, numRegionsComparator);
     }
 
     int getNumRegions(int server) {
       return regionsPerServer[server].length;
     }
 
     boolean contains(int[] arr, int val) {
       return Arrays.binarySearch(arr, val) >= 0;
     }
 
     private Comparator<Integer> numRegionsComparator = new Comparator<Integer>() {
       @Override
       public int compare(Integer integer, Integer integer2) {
         return Integer.valueOf(getNumRegions(integer)).compareTo(getNumRegions(integer2));
       }
     };
 
     void sortServersByLocality() {
       Arrays.sort(serverIndicesSortedByLocality, localityComparator);
     }
 
     float getLocality(int server) {
       return localityPerServer[server];
     }
 
     private Comparator<Integer> localityComparator = new Comparator<Integer>() {
       @Override
       public int compare(Integer integer, Integer integer2) {
         float locality1 = getLocality(integer);
         float locality2 = getLocality(integer2);
         if (locality1 < locality2) {
           return -1;
         } else if (locality1 > locality2) {
           return 1;
         } else {
           return 0;
         }
       }
     };
 
     int getLowestLocalityRegionServer() {
       if (regionFinder == null) {
         return -1;
       } else {
         sortServersByLocality();
         // We want to find server with non zero regions having lowest locality.
         int i = 0;
         int lowestLocalityServerIndex = serverIndicesSortedByLocality[i];
         while (localityPerServer[lowestLocalityServerIndex] == 0
             && (regionsPerServer[lowestLocalityServerIndex].length == 0)) {
           i++;
           lowestLocalityServerIndex = serverIndicesSortedByLocality[i];
         }
         if (LOG.isTraceEnabled()) {
           LOG.trace("Lowest locality region server with non zero regions is "
             + servers[lowestLocalityServerIndex].getHostname() + " with locality "
             + localityPerServer[lowestLocalityServerIndex]);
         }
         return lowestLocalityServerIndex;
       }
     }
 
     int getLowestLocalityRegionOnServer(int serverIndex) {
       if (regionFinder != null) {
         float lowestLocality = 1.0f;
         int lowestLocalityRegionIndex = -1;
         if (regionsPerServer[serverIndex].length == 0) {
           // No regions on that region server
           return -1;
         }
         for (int j = 0; j < regionsPerServer[serverIndex].length; j++) {
           int regionIndex = regionsPerServer[serverIndex][j];
           HDFSBlocksDistribution distribution = regionFinder
               .getBlockDistribution(regions[regionIndex]);
           float locality = distribution.getBlockLocalityIndex(servers[serverIndex].getHostname());
           // skip empty region
           if (distribution.getUniqueBlocksTotalWeight() == 0) {
             continue;
           }
           if (locality < lowestLocality) {
             lowestLocality = locality;
             lowestLocalityRegionIndex = j;
           }
         }
         if (lowestLocalityRegionIndex == -1) {
           return -1;
         }
         if (LOG.isTraceEnabled()) {
           LOG.trace("Lowest locality region is "
               + regions[regionsPerServer[serverIndex][lowestLocalityRegionIndex]]
                   .getRegionNameAsString() + " with locality " + lowestLocality
               + " and its region server contains " + regionsPerServer[serverIndex].length
               + " regions");
         }
         return regionsPerServer[serverIndex][lowestLocalityRegionIndex];
       } else {
         return -1;
       }
     }
 
     float getLocalityOfRegion(int region, int server) {
       if (regionFinder != null) {
         HDFSBlocksDistribution distribution = regionFinder.getBlockDistribution(regions[region]);
         return distribution.getBlockLocalityIndex(servers[server].getHostname());
       } else {
         return 0f;
       }
     }
 
     /**
      * Returns a least loaded server which has better locality for this region
      * than the current server.
      */
     int getLeastLoadedTopServerForRegion(int region, int currentServer) {
       if (regionFinder != null) {
         List<ServerName> topLocalServers = regionFinder.getTopBlockLocations(regions[region],
           servers[currentServer].getHostname());
         int leastLoadedServerIndex = -1;
         int load = Integer.MAX_VALUE;
         for (ServerName sn : topLocalServers) {
           if (!serversToIndex.containsKey(sn.getHostAndPort())) {
             continue;
           }
           int index = serversToIndex.get(sn.getHostAndPort());
           if (regionsPerServer[index] == null) {
             continue;
           }
           int tempLoad = regionsPerServer[index].length;
           if (tempLoad <= load) {
             leastLoadedServerIndex = index;
             load = tempLoad;
           }
         }
         if (leastLoadedServerIndex != -1) {
           LOG.debug("Pick the least loaded server " + servers[leastLoadedServerIndex].getHostname()
             + " with better locality for region " + regions[region]);
         }
         return leastLoadedServerIndex;
       } else {
         return -1;
       }
     }
 
     void calculateRegionServerLocalities() {
       if (regionFinder == null) {
         LOG.warn("Region location finder found null, skipping locality calculations.");
         return;
       }
       for (int i = 0; i < regionsPerServer.length; i++) {
         HDFSBlocksDistribution distribution = new HDFSBlocksDistribution();
         if (regionsPerServer[i].length > 0) {
           for (int j = 0; j < regionsPerServer[i].length; j++) {
             int regionIndex = regionsPerServer[i][j];
             distribution.add(regionFinder.getBlockDistribution(regions[regionIndex]));
           }
         } else {
           LOG.debug("Server " + servers[i].getHostname() + " had 0 regions.");
         }
         localityPerServer[i] = distribution.getBlockLocalityIndex(servers[i].getHostname());
       }
     }
 
     @VisibleForTesting
     protected void setNumRegions(int numRegions) {
       this.numRegions = numRegions;
     }
 
     @VisibleForTesting
     protected void setNumMovedRegions(int numMovedRegions) {
       this.numMovedRegions = numMovedRegions;
     }
 
     @edu.umd.cs.findbugs.annotations.SuppressWarnings(value="SBSC_USE_STRINGBUFFER_CONCATENATION",
         justification="Not important but should be fixed")
     @Override
     public String toString() {
       String desc = "Cluster{" +
           "servers=[";
           for(ServerName sn:servers) {
              desc += sn.getHostAndPort() + ", ";
           }
           desc +=
           ", serverIndicesSortedByRegionCount="+
           Arrays.toString(serverIndicesSortedByRegionCount) +
           ", regionsPerServer=[";
 
           for (int[]r:regionsPerServer) {
             desc += Arrays.toString(r);
           }
           desc += "]" +
           ", numMaxRegionsPerTable=" +
           Arrays.toString(numMaxRegionsPerTable) +
           ", numRegions=" +
           numRegions +
           ", numServers=" +
           numServers +
           ", numTables=" +
           numTables +
           ", numMovedRegions=" +
           numMovedRegions +
           '}';
       return desc;
     }
   }
 
   // slop for regions
   protected float slop;
   protected Configuration config;
   protected RackManager rackManager;
   private static final Random RANDOM = new Random(System.currentTimeMillis());
   private static final Log LOG = LogFactory.getLog(BaseLoadBalancer.class);
 
   public static final String TABLES_ON_MASTER =
     "hbase.balancer.tablesOnMaster";
 
   protected final Set<String> tablesOnMaster = new HashSet<String>();
   protected MetricsBalancer metricsBalancer = null;
   protected ClusterStatus clusterStatus = null;
   protected ServerName masterServerName;
   protected MasterServices services;
 
   protected static String[] getTablesOnMaster(Configuration conf) {
     String valueString = conf.get(TABLES_ON_MASTER);
     if (valueString != null) {
       valueString = valueString.trim();
     }
     if (valueString == null || valueString.equalsIgnoreCase("none")) {
       return null;
     }
     return StringUtils.getStrings(valueString);
   }
 
   /**
    * Check if configured to put any tables on the active master
    */
   public static boolean tablesOnMaster(Configuration conf) {
     String[] tables = getTablesOnMaster(conf);
     return tables != null && tables.length > 0;
   }
 
   public static boolean userTablesOnMaster(Configuration conf) {
     String[] tables = getTablesOnMaster(conf);
     if (tables == null || tables.length == 0) {
       return false;
     }
     for (String tn:tables) {
       if (!tn.startsWith("hbase:")) {
         return true;
       }
     }
     return false;
   }
 
   @Override
   public void setConf(Configuration conf) {
     setSlop(conf);
     if (slop < 0) slop = 0;
     else if (slop > 1) slop = 1;
 
     this.config = conf;
     String[] tables = getTablesOnMaster(conf);
     if (tables != null && tables.length > 0) {
       Collections.addAll(tablesOnMaster, tables);
     }
     this.rackManager = new RackManager(getConf());
     regionFinder.setConf(conf);
   }
 
   protected void setSlop(Configuration conf) {
     this.slop = conf.getFloat("hbase.regions.slop", (float) 0.2);
   }
 
   /**
    * Check if a region belongs to some small system table.
    * If so, the primary replica may be expected to be put on the master regionserver.
    */
   public boolean shouldBeOnMaster(HRegionInfo region) {
     return tablesOnMaster.contains(region.getTable().getNameAsString())
         && region.getReplicaId() == HRegionInfo.DEFAULT_REPLICA_ID;
   }
 
   /**
    * Balance the regions that should be on master regionserver.
    */
   protected List<RegionPlan> balanceMasterRegions(
       Map<ServerName, List<HRegionInfo>> clusterMap) {
     if (masterServerName == null
         || clusterMap == null || clusterMap.size() <= 1) return null;
     List<RegionPlan> plans = null;
     List<HRegionInfo> regions = clusterMap.get(masterServerName);
     if (regions != null) {
       Iterator<ServerName> keyIt = null;
       for (HRegionInfo region: regions) {
         if (shouldBeOnMaster(region)) continue;
 
         // Find a non-master regionserver to host the region
         if (keyIt == null || !keyIt.hasNext()) {
           keyIt = clusterMap.keySet().iterator();
         }
         ServerName dest = keyIt.next();
         if (masterServerName.equals(dest)) {
           if (!keyIt.hasNext()) {
             keyIt = clusterMap.keySet().iterator();
           }
           dest = keyIt.next();
         }
 
         // Move this region away from the master regionserver
         RegionPlan plan = new RegionPlan(region, masterServerName, dest);
         if (plans == null) {
           plans = new ArrayList<RegionPlan>();
         }
         plans.add(plan);
       }
     }
     for (Map.Entry<ServerName, List<HRegionInfo>> server: clusterMap.entrySet()) {
       if (masterServerName.equals(server.getKey())) continue;
       for (HRegionInfo region: server.getValue()) {
         if (!shouldBeOnMaster(region)) continue;
 
         // Move this region to the master regionserver
         RegionPlan plan = new RegionPlan(region, server.getKey(), masterServerName);
         if (plans == null) {
           plans = new ArrayList<RegionPlan>();
         }
         plans.add(plan);
       }
     }
     return plans;
   }
 
   /**
    * Assign the regions that should be on master regionserver.
    */
   protected Map<ServerName, List<HRegionInfo>> assignMasterRegions(
       Collection<HRegionInfo> regions, List<ServerName> servers) {
     if (servers == null || regions == null || regions.isEmpty()) {
       return null;
     }
     Map<ServerName, List<HRegionInfo>> assignments
       = new TreeMap<ServerName, List<HRegionInfo>>();
     if (masterServerName != null && servers.contains(masterServerName)) {
       assignments.put(masterServerName, new ArrayList<HRegionInfo>());
       for (HRegionInfo region: regions) {
         if (shouldBeOnMaster(region)) {
           assignments.get(masterServerName).add(region);
         }
       }
     }
     return assignments;
   }
 
   @Override
   public Configuration getConf() {
     return this.config;
   }
 
   @Override
   public synchronized void setClusterStatus(ClusterStatus st) {
     this.clusterStatus = st;
     regionFinder.setClusterStatus(st);
   }
 
   @Override
   public void setMasterServices(MasterServices masterServices) {
     masterServerName = masterServices.getServerName();
     this.services = masterServices;
     this.regionFinder.setServices(masterServices);
   }
 
   public void setRackManager(RackManager rackManager) {
     this.rackManager = rackManager;
   }
 
   protected boolean needsBalance(Cluster c) {
     ClusterLoadState cs = new ClusterLoadState(c.clusterState);
     if (cs.getNumServers() < MIN_SERVER_BALANCE) {
       if (LOG.isDebugEnabled()) {
         LOG.debug("Not running balancer because only " + cs.getNumServers()
             + " active regionserver(s)");
       }
       return false;
     }
     if(areSomeRegionReplicasColocated(c)) return true;
     // Check if we even need to do any load balancing
     // HBASE-3681 check sloppiness first
     float average = cs.getLoadAverage(); // for logging
     int floor = (int) Math.floor(average * (1 - slop));
     int ceiling = (int) Math.ceil(average * (1 + slop));
     if (!(cs.getMaxLoad() > ceiling || cs.getMinLoad() < floor)) {
       NavigableMap<ServerAndLoad, List<HRegionInfo>> serversByLoad = cs.getServersByLoad();
       if (LOG.isTraceEnabled()) {
         // If nothing to balance, then don't say anything unless trace-level logging.
         LOG.trace("Skipping load balancing because balanced cluster; " +
           "servers=" + cs.getNumServers() +
           " regions=" + cs.getNumRegions() + " average=" + average +
           " mostloaded=" + serversByLoad.lastKey().getLoad() +
           " leastloaded=" + serversByLoad.firstKey().getLoad());
       }
       return false;
     }
     return true;
   }
 
   /**
    * Subclasses should implement this to return true if the cluster has nodes that hosts
    * multiple replicas for the same region, or, if there are multiple racks and the same
    * rack hosts replicas of the same region
    * @param c Cluster information
    * @return whether region replicas are currently co-located
    */
   protected boolean areSomeRegionReplicasColocated(Cluster c) {
     return false;
   }
 
   /**
    * Generates a bulk assignment plan to be used on cluster startup using a
    * simple round-robin assignment.
    * <p>
    * Takes a list of all the regions and all the servers in the cluster and
    * returns a map of each server to the regions that it should be assigned.
    * <p>
    * Currently implemented as a round-robin assignment. Same invariant as load
    * balancing, all servers holding floor(avg) or ceiling(avg).
    *
    * TODO: Use block locations from HDFS to place regions with their blocks
    *
    * @param regions all regions
    * @param servers all servers
    * @return map of server to the regions it should take, or null if no
    *         assignment is possible (ie. no regions or no servers)
    */
   @Override
   public Map<ServerName, List<HRegionInfo>> roundRobinAssignment(List<HRegionInfo> regions,
       List<ServerName> servers) {
     metricsBalancer.incrMiscInvocations();
     Map<ServerName, List<HRegionInfo>> assignments = assignMasterRegions(regions, servers);
     if (assignments != null && !assignments.isEmpty()) {
       servers = new ArrayList<ServerName>(servers);
       // Guarantee not to put other regions on master
       servers.remove(masterServerName);
       List<HRegionInfo> masterRegions = assignments.get(masterServerName);
       if (!masterRegions.isEmpty()) {
         regions = new ArrayList<HRegionInfo>(regions);
         for (HRegionInfo region: masterRegions) {
           regions.remove(region);
         }
       }
     }
     if (regions == null || regions.isEmpty()) {
       return assignments;
     }
 
     int numServers = servers == null ? 0 : servers.size();
     if (numServers == 0) {
       LOG.warn("Wanted to do round robin assignment but no servers to assign to");
       return null;
     }
 
     // TODO: instead of retainAssignment() and roundRobinAssignment(), we should just run the
     // normal LB.balancerCluster() with unassignedRegions. We only need to have a candidate
     // generator for AssignRegionAction. The LB will ensure the regions are mostly local
     // and balanced. This should also run fast with fewer number of iterations.
 
     if (numServers == 1) { // Only one server, nothing fancy we can do here
       ServerName server = servers.get(0);
       assignments.put(server, new ArrayList<HRegionInfo>(regions));
       return assignments;
     }
 
     Cluster cluster = createCluster(servers, regions, false);
     List<HRegionInfo> unassignedRegions = new ArrayList<HRegionInfo>();
 
     roundRobinAssignment(cluster, regions, unassignedRegions,
       servers, assignments);
 
     List<HRegionInfo> lastFewRegions = new ArrayList<HRegionInfo>();
     // assign the remaining by going through the list and try to assign to servers one-by-one
     int serverIdx = RANDOM.nextInt(numServers);
     for (HRegionInfo region : unassignedRegions) {
       boolean assigned = false;
       for (int j = 0; j < numServers; j++) { // try all servers one by one
         ServerName serverName = servers.get((j + serverIdx) % numServers);
         if (!cluster.wouldLowerAvailability(region, serverName)) {
           List<HRegionInfo> serverRegions = assignments.get(serverName);
           if (serverRegions == null) {
             serverRegions = new ArrayList<HRegionInfo>();
             assignments.put(serverName, serverRegions);
           }
           serverRegions.add(region);
           cluster.doAssignRegion(region, serverName);
           serverIdx = (j + serverIdx + 1) % numServers; //remain from next server
           assigned = true;
           break;
         }
       }
       if (!assigned) {
         lastFewRegions.add(region);
       }
     }
     // just sprinkle the rest of the regions on random regionservers. The balanceCluster will
     // make it optimal later. we can end up with this if numReplicas > numServers.
     for (HRegionInfo region : lastFewRegions) {
       int i = RANDOM.nextInt(numServers);
       ServerName server = servers.get(i);
       List<HRegionInfo> serverRegions = assignments.get(server);
       if (serverRegions == null) {
         serverRegions = new ArrayList<HRegionInfo>();
         assignments.put(server, serverRegions);
       }
       serverRegions.add(region);
       cluster.doAssignRegion(region, server);
     }
     return assignments;
   }
 
   protected Cluster createCluster(List<ServerName> servers,
       Collection<HRegionInfo> regions, boolean forceRefresh) {
     if (forceRefresh) {
       regionFinder.refreshAndWait(regions);
     }
     // Get the snapshot of the current assignments for the regions in question, and then create
     // a cluster out of it. Note that we might have replicas already assigned to some servers
     // earlier. So we want to get the snapshot to see those assignments, but this will only contain
     // replicas of the regions that are passed (for performance).
     Map<ServerName, List<HRegionInfo>> clusterState = getRegionAssignmentsByServer(regions);
 
     for (ServerName server : servers) {
       if (!clusterState.containsKey(server)) {
         clusterState.put(server, EMPTY_REGION_LIST);
       }
     }
     return new Cluster(regions, clusterState, null, this.regionFinder,
       rackManager);
   }
 
   /**
    * Generates an immediate assignment plan to be used by a new master for
    * regions in transition that do not have an already known destination.
    *
    * Takes a list of regions that need immediate assignment and a list of all
    * available servers. Returns a map of regions to the server they should be
    * assigned to.
    *
    * This method will return quickly and does not do any intelligent balancing.
    * The goal is to make a fast decision not the best decision possible.
    *
    * Currently this is random.
    *
    * @param regions
    * @param servers
    * @return map of regions to the server it should be assigned to
    */
   @Override
   public Map<HRegionInfo, ServerName> immediateAssignment(List<HRegionInfo> regions,
       List<ServerName> servers) {
     metricsBalancer.incrMiscInvocations();
     if (servers == null || servers.isEmpty()) {
       LOG.warn("Wanted to do random assignment but no servers to assign to");
       return null;
     }
 
     Map<HRegionInfo, ServerName> assignments = new TreeMap<HRegionInfo, ServerName>();
     for (HRegionInfo region : regions) {
       assignments.put(region, randomAssignment(region, servers));
     }
     return assignments;
   }
 
   /**
    * Used to assign a single region to a random server.
    */
   @Override
   public ServerName randomAssignment(HRegionInfo regionInfo, List<ServerName> servers) {
     metricsBalancer.incrMiscInvocations();
     if (servers != null && servers.contains(masterServerName)) {
       if (shouldBeOnMaster(regionInfo)) {
         return masterServerName;
       }
       servers = new ArrayList<ServerName>(servers);
       // Guarantee not to put other regions on master
       servers.remove(masterServerName);
     }
 
     int numServers = servers == null ? 0 : servers.size();
     if (numServers == 0) {
       LOG.warn("Wanted to do retain assignment but no servers to assign to");
       return null;
     }
     if (numServers == 1) { // Only one server, nothing fancy we can do here
       return servers.get(0);
     }
 
     List<HRegionInfo> regions = Lists.newArrayList(regionInfo);
     Cluster cluster = createCluster(servers, regions, false);
     return randomAssignment(cluster, regionInfo, servers);
   }
 
   /**
    * Generates a bulk assignment startup plan, attempting to reuse the existing
    * assignment information from META, but adjusting for the specified list of
    * available/online servers available for assignment.
    * <p>
    * Takes a map of all regions to their existing assignment from META. Also
    * takes a list of online servers for regions to be assigned to. Attempts to
    * retain all assignment, so in some instances initial assignment will not be
    * completely balanced.
    * <p>
    * Any leftover regions without an existing server to be assigned to will be
    * assigned randomly to available servers.
    *
    * @param regions regions and existing assignment from meta
    * @param servers available servers
    * @return map of servers and regions to be assigned to them
    */
   @Override
   public Map<ServerName, List<HRegionInfo>> retainAssignment(Map<HRegionInfo, ServerName> regions,
       List<ServerName> servers) {
     // Update metrics
     metricsBalancer.incrMiscInvocations();
     Map<ServerName, List<HRegionInfo>> assignments
       = assignMasterRegions(regions.keySet(), servers);
     if (assignments != null && !assignments.isEmpty()) {
       servers = new ArrayList<ServerName>(servers);
       // Guarantee not to put other regions on master
       servers.remove(masterServerName);
       List<HRegionInfo> masterRegions = assignments.get(masterServerName);
       if (!masterRegions.isEmpty()) {
         regions = new HashMap<HRegionInfo, ServerName>(regions);
         for (HRegionInfo region: masterRegions) {
           regions.remove(region);
         }
       }
     }
     if (regions == null || regions.isEmpty()) {
       return assignments;
     }
 
     int numServers = servers == null ? 0 : servers.size();
     if (numServers == 0) {
       LOG.warn("Wanted to do retain assignment but no servers to assign to");
       return null;
     }
     if (numServers == 1) { // Only one server, nothing fancy we can do here
       ServerName server = servers.get(0);
       assignments.put(server, new ArrayList<HRegionInfo>(regions.keySet()));
       return assignments;
     }
 
     // Group all of the old assignments by their hostname.
     // We can't group directly by ServerName since the servers all have
     // new start-codes.
 
     // Group the servers by their hostname. It's possible we have multiple
     // servers on the same host on different ports.
     ArrayListMultimap<String, ServerName> serversByHostname = ArrayListMultimap.create();
     for (ServerName server : servers) {
       assignments.put(server, new ArrayList<HRegionInfo>());
       serversByHostname.put(server.getHostname(), server);
     }
 
     // Collection of the hostnames that used to have regions
     // assigned, but for which we no longer have any RS running
     // after the cluster restart.
     Set<String> oldHostsNoLongerPresent = Sets.newTreeSet();
 
     int numRandomAssignments = 0;
     int numRetainedAssigments = 0;
 
     Cluster cluster = createCluster(servers, regions.keySet(), true);
 
     for (Map.Entry<HRegionInfo, ServerName> entry : regions.entrySet()) {
       HRegionInfo region = entry.getKey();
       ServerName oldServerName = entry.getValue();
       List<ServerName> localServers = new ArrayList<ServerName>();
       if (oldServerName != null) {
         localServers = serversByHostname.get(oldServerName.getHostname());
       }
       if (localServers.isEmpty()) {
         // No servers on the new cluster match up with this hostname,
         // assign randomly.
         ServerName randomServer = randomAssignment(cluster, region, servers);
         assignments.get(randomServer).add(region);
         numRandomAssignments++;
         if (oldServerName != null) oldHostsNoLongerPresent.add(oldServerName.getHostname());
       } else if (localServers.size() == 1) {
         // the usual case - one new server on same host
         ServerName target = localServers.get(0);
         assignments.get(target).add(region);
         cluster.doAssignRegion(region, target);
         numRetainedAssigments++;
       } else {
         // multiple new servers in the cluster on this same host
         if (localServers.contains(oldServerName)) {
           assignments.get(oldServerName).add(region);
           cluster.doAssignRegion(region, oldServerName);
         } else {
           ServerName target = null;
           for (ServerName tmp: localServers) {
             if (tmp.getPort() == oldServerName.getPort()) {
               target = tmp;
               break;
             }
           }
           if (target == null) {
             target = randomAssignment(cluster, region, localServers);
           }
           assignments.get(target).add(region);
         }
         numRetainedAssigments++;
       }
     }
 
     String randomAssignMsg = "";
     if (numRandomAssignments > 0) {
       randomAssignMsg =
           numRandomAssignments + " regions were assigned "
               + "to random hosts, since the old hosts for these regions are no "
               + "longer present in the cluster. These hosts were:\n  "
               + Joiner.on("\n  ").join(oldHostsNoLongerPresent);
     }
 
     LOG.info("Reassigned " + regions.size() + " regions. " + numRetainedAssigments
         + " retained the pre-restart assignment. " + randomAssignMsg);
     return assignments;
   }
 
   @Override
   public void initialize() throws HBaseIOException{
   }
 
   @Override
   public void regionOnline(HRegionInfo regionInfo, ServerName sn) {
   }
 
   @Override
   public void regionOffline(HRegionInfo regionInfo) {
   }
 
   @Override
   public boolean isStopped() {
     return stopped;
   }
 
   @Override
   public void stop(String why) {
     LOG.info("Load Balancer stop requested: "+why);
     stopped = true;
   }
 
   /**
    * Used to assign a single region to a random server.
    */
   private ServerName randomAssignment(Cluster cluster, HRegionInfo regionInfo,
       List<ServerName> servers) {
     int numServers = servers.size(); // servers is not null, numServers > 1
     ServerName sn = null;
     final int maxIterations = numServers * 4;
     int iterations = 0;
 
     do {
       int i = RANDOM.nextInt(numServers);
       sn = servers.get(i);
     } while (cluster.wouldLowerAvailability(regionInfo, sn)
         && iterations++ < maxIterations);
     cluster.doAssignRegion(regionInfo, sn);
     return sn;
   }
 
   /**
    * Round robin a list of regions to a list of servers
    */
   private void roundRobinAssignment(Cluster cluster, List<HRegionInfo> regions,
       List<HRegionInfo> unassignedRegions, List<ServerName> servers,
       Map<ServerName, List<HRegionInfo>> assignments) {
 
     int numServers = servers.size();
     int numRegions = regions.size();
     int max = (int) Math.ceil((float) numRegions / numServers);
     int serverIdx = 0;
     if (numServers > 1) {
       serverIdx = RANDOM.nextInt(numServers);
     }
     int regionIdx = 0;
 
     for (int j = 0; j < numServers; j++) {
       ServerName server = servers.get((j + serverIdx) % numServers);
       List<HRegionInfo> serverRegions = new ArrayList<HRegionInfo>(max);
       for (int i = regionIdx; i < numRegions; i += numServers) {
         HRegionInfo region = regions.get(i % numRegions);
         if (cluster.wouldLowerAvailability(region, server)) {
           unassignedRegions.add(region);
         } else {
           serverRegions.add(region);
           cluster.doAssignRegion(region, server);
         }
       }
       assignments.put(server, serverRegions);
       regionIdx++;
     }
   }
 
   protected Map<ServerName, List<HRegionInfo>> getRegionAssignmentsByServer(
     Collection<HRegionInfo> regions) {
     if (this.services != null && this.services.getAssignmentManager() != null) {
       return this.services.getAssignmentManager().getSnapShotOfAssignment(regions);
     } else {
       return new HashMap<ServerName, List<HRegionInfo>>();
     }
   }
 
   @Override
   public void onConfigurationChange(Configuration conf) {
   }
 }