/**
    * 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 com.google.common.annotations.VisibleForTesting;
  import java.util.ArrayDeque;
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.Deque;
  import java.util.HashMap;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Map;
  import java.util.Map.Entry;
  import java.util.Random;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.ClusterStatus;
  import org.apache.hadoop.hbase.HBaseInterfaceAudience;
  import org.apache.hadoop.hbase.HRegionInfo;
  import org.apache.hadoop.hbase.RegionLoad;
  import org.apache.hadoop.hbase.ServerLoad;
  import org.apache.hadoop.hbase.ServerName;
  import org.apache.hadoop.hbase.master.MasterServices;
  import org.apache.hadoop.hbase.master.RegionPlan;
  import org.apache.hadoop.hbase.master.balancer.BaseLoadBalancer.Cluster.Action;
  import org.apache.hadoop.hbase.master.balancer.BaseLoadBalancer.Cluster.Action.Type;
  import org.apache.hadoop.hbase.master.balancer.BaseLoadBalancer.Cluster.AssignRegionAction;
  import org.apache.hadoop.hbase.master.balancer.BaseLoadBalancer.Cluster.MoveRegionAction;
  import org.apache.hadoop.hbase.master.balancer.BaseLoadBalancer.Cluster.SwapRegionsAction;
  import org.apache.hadoop.hbase.util.Bytes;
  import org.apache.hadoop.hbase.util.EnvironmentEdgeManager;
  
  /**
   * <p>This is a best effort load balancer. Given a Cost function F(C) =&gt; x It will
   * randomly try and mutate the cluster to Cprime. If F(Cprime) &lt; F(C) then the
   * new cluster state becomes the plan. It includes costs functions to compute the cost of:</p>
   * <ul>
   * <li>Region Load</li>
   * <li>Table Load</li>
   * <li>Data Locality</li>
   * <li>Memstore Sizes</li>
   * <li>Storefile Sizes</li>
   * </ul>
   *
   *
   * <p>Every cost function returns a number between 0 and 1 inclusive; where 0 is the lowest cost
   * best solution, and 1 is the highest possible cost and the worst solution.  The computed costs are
   * scaled by their respective multipliers:</p>
   *
   * <ul>
   *   <li>hbase.master.balancer.stochastic.regionLoadCost</li>
   *   <li>hbase.master.balancer.stochastic.moveCost</li>
   *   <li>hbase.master.balancer.stochastic.tableLoadCost</li>
   *   <li>hbase.master.balancer.stochastic.localityCost</li>
   *   <li>hbase.master.balancer.stochastic.memstoreSizeCost</li>
   *   <li>hbase.master.balancer.stochastic.storefileSizeCost</li>
   * </ul>
   *
   * <p>In addition to the above configurations, the balancer can be tuned by the following
   * configuration values:</p>
   * <ul>
   *   <li>hbase.master.balancer.stochastic.maxMoveRegions which
   *   controls what the max number of regions that can be moved in a single invocation of this
   *   balancer.</li>
   *   <li>hbase.master.balancer.stochastic.stepsPerRegion is the coefficient by which the number of
   *   regions is multiplied to try and get the number of times the balancer will
   *   mutate all servers.</li>
   *   <li>hbase.master.balancer.stochastic.maxSteps which controls the maximum number of times that
   *   the balancer will try and mutate all the servers. The balancer will use the minimum of this
   *   value and the above computation.</li>
   * </ul>
   *
   * <p>This balancer is best used with hbase.master.loadbalance.bytable set to false
   * so that the balancer gets the full picture of all loads on the cluster.</p>
   */
  @InterfaceAudience.LimitedPrivate(HBaseInterfaceAudience.CONFIG)
  @edu.umd.cs.findbugs.annotations.SuppressWarnings(value="IS2_INCONSISTENT_SYNC",
    justification="Complaint is about costFunctions not being synchronized; not end of the world")
  public class StochasticLoadBalancer extends BaseLoadBalancer {
  
   protected static final String STEPS_PER_REGION_KEY =
       "hbase.master.balancer.stochastic.stepsPerRegion";
   protected static final String MAX_STEPS_KEY =
       "hbase.master.balancer.stochastic.maxSteps";
   protected static final String MAX_RUNNING_TIME_KEY =
       "hbase.master.balancer.stochastic.maxRunningTime";
   protected static final String KEEP_REGION_LOADS =
       "hbase.master.balancer.stochastic.numRegionLoadsToRemember";
 
   private static final Random RANDOM = new Random(System.currentTimeMillis());
   private static final Log LOG = LogFactory.getLog(StochasticLoadBalancer.class);
 
   Map<String, Deque<RegionLoad>> loads = new HashMap<String, Deque<RegionLoad>>();
 
   // values are defaults
   private int maxSteps = 1000000;
   private int stepsPerRegion = 800;
   private long maxRunningTime = 30 * 1000 * 1; // 30 seconds.
   private int numRegionLoadsToRemember = 15;
 
   private CandidateGenerator[] candidateGenerators;
   private CostFromRegionLoadFunction[] regionLoadFunctions;
 
   private CostFunction[] costFunctions; // FindBugs: Wants this protected; IS2_INCONSISTENT_SYNC
 
   // Keep locality based picker and cost function to alert them
   // when new services are offered
   private LocalityBasedCandidateGenerator localityCandidateGenerator;
   private LocalityCostFunction localityCost;
   private RegionReplicaHostCostFunction regionReplicaHostCostFunction;
   private RegionReplicaRackCostFunction regionReplicaRackCostFunction;
 
   @Override
   public void onConfigurationChange(Configuration conf) {
     setConf(conf);
   }
 
   @Override
   public synchronized void setConf(Configuration conf) {
     super.setConf(conf);
     LOG.info("loading config");
 
     maxSteps = conf.getInt(MAX_STEPS_KEY, maxSteps);
 
     stepsPerRegion = conf.getInt(STEPS_PER_REGION_KEY, stepsPerRegion);
     maxRunningTime = conf.getLong(MAX_RUNNING_TIME_KEY, maxRunningTime);
 
     numRegionLoadsToRemember = conf.getInt(KEEP_REGION_LOADS, numRegionLoadsToRemember);
 
     if (localityCandidateGenerator == null) {
       localityCandidateGenerator = new LocalityBasedCandidateGenerator(services);
     }
     localityCost = new LocalityCostFunction(conf, services);
 
     if (candidateGenerators == null) {
       candidateGenerators = new CandidateGenerator[] {
           new RandomCandidateGenerator(),
           new LoadCandidateGenerator(),
           localityCandidateGenerator,
           new RegionReplicaRackCandidateGenerator(),
       };
     }
 
     regionLoadFunctions = new CostFromRegionLoadFunction[] {
       new ReadRequestCostFunction(conf),
       new WriteRequestCostFunction(conf),
       new MemstoreSizeCostFunction(conf),
       new StoreFileCostFunction(conf)
     };
 
     regionReplicaHostCostFunction = new RegionReplicaHostCostFunction(conf);
     regionReplicaRackCostFunction = new RegionReplicaRackCostFunction(conf);
 
     costFunctions = new CostFunction[]{
       new RegionCountSkewCostFunction(conf),
       new PrimaryRegionCountSkewCostFunction(conf),
       new MoveCostFunction(conf),
       localityCost,
       new TableSkewCostFunction(conf),
       regionReplicaHostCostFunction,
       regionReplicaRackCostFunction,
       regionLoadFunctions[0],
       regionLoadFunctions[1],
       regionLoadFunctions[2],
       regionLoadFunctions[3],
     };
   }
 
   @Override
   protected void setSlop(Configuration conf) {
     this.slop = conf.getFloat("hbase.regions.slop", 0.001F);
   }
 
   @Override
   public synchronized void setClusterStatus(ClusterStatus st) {
     super.setClusterStatus(st);
     updateRegionLoad();
     for(CostFromRegionLoadFunction cost : regionLoadFunctions) {
       cost.setClusterStatus(st);
     }
   }
 
   @Override
   public synchronized void setMasterServices(MasterServices masterServices) {
     super.setMasterServices(masterServices);
     this.localityCost.setServices(masterServices);
     this.localityCandidateGenerator.setServices(masterServices);
 
   }
 
   @Override
   protected synchronized boolean areSomeRegionReplicasColocated(Cluster c) {
     regionReplicaHostCostFunction.init(c);
     if (regionReplicaHostCostFunction.cost() > 0) return true;
     regionReplicaRackCostFunction.init(c);
     if (regionReplicaRackCostFunction.cost() > 0) return true;
     return false;
   }
 
   @VisibleForTesting
   Cluster.Action nextAction(Cluster cluster) {
     return candidateGenerators[(RANDOM.nextInt(candidateGenerators.length))]
             .generate(cluster);
   }
 
   /**
    * Given the cluster state this will try and approach an optimal balance. This
    * should always approach the optimal state given enough steps.
    */
   @Override
   public synchronized List<RegionPlan> balanceCluster(Map<ServerName,
     List<HRegionInfo>> clusterState) {
     List<RegionPlan> plans = balanceMasterRegions(clusterState);
     if (plans != null || clusterState == null || clusterState.size() <= 1) {
       return plans;
     }
     if (masterServerName != null && clusterState.containsKey(masterServerName)) {
       if (clusterState.size() <= 2) {
         return null;
       }
       clusterState = new HashMap<ServerName, List<HRegionInfo>>(clusterState);
       clusterState.remove(masterServerName);
     }
 
     // On clusters with lots of HFileLinks or lots of reference files,
     // instantiating the storefile infos can be quite expensive.
     // Allow turning this feature off if the locality cost is not going to
     // be used in any computations.
     RegionLocationFinder finder = null;
     if (this.localityCost != null && this.localityCost.getMultiplier() > 0) {
       finder = this.regionFinder;
     }
 
     //The clusterState that is given to this method contains the state
     //of all the regions in the table(s) (that's true today)
     // Keep track of servers to iterate through them.
     Cluster cluster = new Cluster(clusterState, loads, finder, rackManager);
     if (!needsBalance(cluster)) {
       return null;
     }
 
     long startTime = EnvironmentEdgeManager.currentTime();
 
     initCosts(cluster);
 
     double currentCost = computeCost(cluster, Double.MAX_VALUE);
 
     double initCost = currentCost;
     double newCost = currentCost;
 
     long computedMaxSteps = Math.min(this.maxSteps,
         ((long)cluster.numRegions * (long)this.stepsPerRegion * (long)cluster.numServers));
     // Perform a stochastic walk to see if we can get a good fit.
     long step;
 
     for (step = 0; step < computedMaxSteps; step++) {
       Cluster.Action action = nextAction(cluster);
 
       if (action.type == Type.NULL) {
         continue;
       }
 
       cluster.doAction(action);
       updateCostsWithAction(cluster, action);
 
       newCost = computeCost(cluster, currentCost);
 
       // Should this be kept?
       if (newCost < currentCost) {
         currentCost = newCost;
       } else {
         // Put things back the way they were before.
         // TODO: undo by remembering old values
         Action undoAction = action.undoAction();
         cluster.doAction(undoAction);
         updateCostsWithAction(cluster, undoAction);
       }
 
       if (EnvironmentEdgeManager.currentTime() - startTime >
           maxRunningTime) {
         break;
       }
     }
     long endTime = EnvironmentEdgeManager.currentTime();
 
     metricsBalancer.balanceCluster(endTime - startTime);
 
     if (initCost > currentCost) {
       plans = createRegionPlans(cluster);
       if (LOG.isDebugEnabled()) {
         LOG.debug("Finished computing new load balance plan.  Computation took "
             + (endTime - startTime) + "ms to try " + step
             + " different iterations.  Found a solution that moves "
             + plans.size() + " regions; Going from a computed cost of "
             + initCost + " to a new cost of " + currentCost);
       }
       return plans;
     }
     if (LOG.isDebugEnabled()) {
       LOG.debug("Could not find a better load balance plan.  Tried "
           + step + " different configurations in " + (endTime - startTime)
           + "ms, and did not find anything with a computed cost less than " + initCost);
     }
     return null;
   }
 
   /**
    * Create all of the RegionPlan's needed to move from the initial cluster state to the desired
    * state.
    *
    * @param cluster The state of the cluster
    * @return List of RegionPlan's that represent the moves needed to get to desired final state.
    */
   private List<RegionPlan> createRegionPlans(Cluster cluster) {
     List<RegionPlan> plans = new LinkedList<RegionPlan>();
     for (int regionIndex = 0;
          regionIndex < cluster.regionIndexToServerIndex.length; regionIndex++) {
       int initialServerIndex = cluster.initialRegionIndexToServerIndex[regionIndex];
       int newServerIndex = cluster.regionIndexToServerIndex[regionIndex];
 
       if (initialServerIndex != newServerIndex) {
         HRegionInfo region = cluster.regions[regionIndex];
         ServerName initialServer = cluster.servers[initialServerIndex];
         ServerName newServer = cluster.servers[newServerIndex];
 
         if (LOG.isTraceEnabled()) {
           LOG.trace("Moving Region " + region.getEncodedName() + " from server "
               + initialServer.getHostname() + " to " + newServer.getHostname());
         }
         RegionPlan rp = new RegionPlan(region, initialServer, newServer);
         plans.add(rp);
       }
     }
     return plans;
   }
 
   /**
    * Store the current region loads.
    */
   private synchronized void updateRegionLoad() {
     // We create a new hashmap so that regions that are no longer there are removed.
     // However we temporarily need the old loads so we can use them to keep the rolling average.
     Map<String, Deque<RegionLoad>> oldLoads = loads;
     loads = new HashMap<String, Deque<RegionLoad>>();
 
     for (ServerName sn : clusterStatus.getServers()) {
       ServerLoad sl = clusterStatus.getLoad(sn);
       if (sl == null) {
         continue;
       }
       for (Entry<byte[], RegionLoad> entry : sl.getRegionsLoad().entrySet()) {
         Deque<RegionLoad> rLoads = oldLoads.get(Bytes.toString(entry.getKey()));
         if (rLoads == null) {
           // There was nothing there
           rLoads = new ArrayDeque<RegionLoad>();
         } else if (rLoads.size() >= numRegionLoadsToRemember) {
           rLoads.remove();
         }
         rLoads.add(entry.getValue());
         loads.put(Bytes.toString(entry.getKey()), rLoads);
 
       }
     }
 
     for(CostFromRegionLoadFunction cost : regionLoadFunctions) {
       cost.setLoads(loads);
     }
   }
 
   protected void initCosts(Cluster cluster) {
     for (CostFunction c:costFunctions) {
       c.init(cluster);
     }
   }
 
   protected void updateCostsWithAction(Cluster cluster, Action action) {
     for (CostFunction c : costFunctions) {
       c.postAction(action);
     }
   }
 
   /**
    * This is the main cost function.  It will compute a cost associated with a proposed cluster
    * state.  All different costs will be combined with their multipliers to produce a double cost.
    *
    * @param cluster The state of the cluster
    * @param previousCost the previous cost. This is used as an early out.
    * @return a double of a cost associated with the proposed cluster state.  This cost is an
    *         aggregate of all individual cost functions.
    */
   protected double computeCost(Cluster cluster, double previousCost) {
     double total = 0;
 
     for (CostFunction c:costFunctions) {
       if (c.getMultiplier() <= 0) {
         continue;
       }
 
       total += c.getMultiplier() * c.cost();
 
       if (total > previousCost) {
         return total;
       }
     }
     return total;
   }
 
   /** Generates a candidate action to be applied to the cluster for cost function search */
   abstract static class CandidateGenerator {
     abstract Cluster.Action generate(Cluster cluster);
 
     /**
      * From a list of regions pick a random one. Null can be returned which
      * {@link StochasticLoadBalancer#balanceCluster(Map)} recognize as signal to try a region move
      * rather than swap.
      *
      * @param cluster        The state of the cluster
      * @param server         index of the server
      * @param chanceOfNoSwap Chance that this will decide to try a move rather
      *                       than a swap.
      * @return a random {@link HRegionInfo} or null if an asymmetrical move is
      *         suggested.
      */
     protected int pickRandomRegion(Cluster cluster, int server, double chanceOfNoSwap) {
       // Check to see if this is just a move.
       if (cluster.regionsPerServer[server].length == 0 || RANDOM.nextFloat() < chanceOfNoSwap) {
         // signal a move only.
         return -1;
       }
       int rand = RANDOM.nextInt(cluster.regionsPerServer[server].length);
       return cluster.regionsPerServer[server][rand];
 
     }
     protected int pickRandomServer(Cluster cluster) {
       if (cluster.numServers < 1) {
         return -1;
       }
 
       return RANDOM.nextInt(cluster.numServers);
     }
 
     protected int pickRandomRack(Cluster cluster) {
       if (cluster.numRacks < 1) {
         return -1;
       }
 
       return RANDOM.nextInt(cluster.numRacks);
     }
 
     protected int pickOtherRandomServer(Cluster cluster, int serverIndex) {
       if (cluster.numServers < 2) {
         return -1;
       }
       while (true) {
         int otherServerIndex = pickRandomServer(cluster);
         if (otherServerIndex != serverIndex) {
           return otherServerIndex;
         }
       }
     }
 
     protected int pickOtherRandomRack(Cluster cluster, int rackIndex) {
       if (cluster.numRacks < 2) {
         return -1;
       }
       while (true) {
         int otherRackIndex = pickRandomRack(cluster);
         if (otherRackIndex != rackIndex) {
           return otherRackIndex;
         }
       }
     }
 
     protected Cluster.Action pickRandomRegions(Cluster cluster,
                                                        int thisServer,
                                                        int otherServer) {
       if (thisServer < 0 || otherServer < 0) {
         return Cluster.NullAction;
       }
 
       // Decide who is most likely to need another region
       int thisRegionCount = cluster.getNumRegions(thisServer);
       int otherRegionCount = cluster.getNumRegions(otherServer);
 
       // Assign the chance based upon the above
       double thisChance = (thisRegionCount > otherRegionCount) ? 0 : 0.5;
       double otherChance = (thisRegionCount <= otherRegionCount) ? 0 : 0.5;
 
       int thisRegion = pickRandomRegion(cluster, thisServer, thisChance);
       int otherRegion = pickRandomRegion(cluster, otherServer, otherChance);
 
       return getAction(thisServer, thisRegion, otherServer, otherRegion);
     }
 
     protected Cluster.Action getAction (int fromServer, int fromRegion,
         int toServer, int toRegion) {
       if (fromServer < 0 || toServer < 0) {
         return Cluster.NullAction;
       }
       if (fromRegion > 0 && toRegion > 0) {
         return new Cluster.SwapRegionsAction(fromServer, fromRegion,
           toServer, toRegion);
       } else if (fromRegion > 0) {
         return new Cluster.MoveRegionAction(fromRegion, fromServer, toServer);
       } else if (toRegion > 0) {
         return new Cluster.MoveRegionAction(toRegion, toServer, fromServer);
       } else {
         return Cluster.NullAction;
       }
     }
   }
 
   static class RandomCandidateGenerator extends CandidateGenerator {
 
     @Override
     Cluster.Action generate(Cluster cluster) {
 
       int thisServer = pickRandomServer(cluster);
 
       // Pick the other server
       int otherServer = pickOtherRandomServer(cluster, thisServer);
 
       return pickRandomRegions(cluster, thisServer, otherServer);
     }
   }
 
   static class LoadCandidateGenerator extends CandidateGenerator {
 
     @Override
     Cluster.Action generate(Cluster cluster) {
       cluster.sortServersByRegionCount();
       int thisServer = pickMostLoadedServer(cluster, -1);
       int otherServer = pickLeastLoadedServer(cluster, thisServer);
 
       return pickRandomRegions(cluster, thisServer, otherServer);
     }
 
     private int pickLeastLoadedServer(final Cluster cluster, int thisServer) {
       Integer[] servers = cluster.serverIndicesSortedByRegionCount;
 
       int index = 0;
       while (servers[index] == null || servers[index] == thisServer) {
         index++;
         if (index == servers.length) {
           return -1;
         }
       }
       return servers[index];
     }
 
     private int pickMostLoadedServer(final Cluster cluster, int thisServer) {
       Integer[] servers = cluster.serverIndicesSortedByRegionCount;
 
       int index = servers.length - 1;
       while (servers[index] == null || servers[index] == thisServer) {
         index--;
         if (index < 0) {
           return -1;
         }
       }
       return servers[index];
     }
   }
 
   static class LocalityBasedCandidateGenerator extends CandidateGenerator {
 
     private MasterServices masterServices;
 
     LocalityBasedCandidateGenerator(MasterServices masterServices) {
       this.masterServices = masterServices;
     }
 
     @Override
     Cluster.Action generate(Cluster cluster) {
       if (this.masterServices == null) {
         int thisServer = pickRandomServer(cluster);
         // Pick the other server
         int otherServer = pickOtherRandomServer(cluster, thisServer);
         return pickRandomRegions(cluster, thisServer, otherServer);
       }
 
       cluster.calculateRegionServerLocalities();
       // Pick server with lowest locality
       int thisServer = pickLowestLocalityServer(cluster);
       int thisRegion;
       if (thisServer == -1) {
         LOG.warn("Could not pick lowest locality region server");
         return Cluster.NullAction;
       } else {
       // Pick lowest locality region on this server
         thisRegion = pickLowestLocalityRegionOnServer(cluster, thisServer);
       }
 
       if (thisRegion == -1) {
         return Cluster.NullAction;
       }
 
       // Pick the least loaded server with good locality for the region
       int otherServer = cluster.getLeastLoadedTopServerForRegion(thisRegion);
 
       if (otherServer == -1) {
         return Cluster.NullAction;
       }
 
       // Let the candidate region be moved to its highest locality server.
       int otherRegion = -1;
 
       return getAction(thisServer, thisRegion, otherServer, otherRegion);
     }
 
     private int pickLowestLocalityServer(Cluster cluster) {
       return cluster.getLowestLocalityRegionServer();
     }
 
     private int pickLowestLocalityRegionOnServer(Cluster cluster, int server) {
       return cluster.getLowestLocalityRegionOnServer(server);
     }
 
     void setServices(MasterServices services) {
       this.masterServices = services;
     }
   }
 
   /**
    * Generates candidates which moves the replicas out of the region server for
    * co-hosted region replicas
    */
   static class RegionReplicaCandidateGenerator extends CandidateGenerator {
 
     RandomCandidateGenerator randomGenerator = new RandomCandidateGenerator();
 
     /**
      * Randomly select one regionIndex out of all region replicas co-hosted in the same group
      * (a group is a server, host or rack)
      * @param primariesOfRegionsPerGroup either Cluster.primariesOfRegionsPerServer,
      * primariesOfRegionsPerHost or primariesOfRegionsPerRack
      * @param regionsPerGroup either Cluster.regionsPerServer, regionsPerHost or regionsPerRack
      * @param regionIndexToPrimaryIndex Cluster.regionsIndexToPrimaryIndex
      * @return a regionIndex for the selected primary or -1 if there is no co-locating
      */
     int selectCoHostedRegionPerGroup(int[] primariesOfRegionsPerGroup, int[] regionsPerGroup
         , int[] regionIndexToPrimaryIndex) {
       int currentPrimary = -1;
       int currentPrimaryIndex = -1;
       int selectedPrimaryIndex = -1;
       double currentLargestRandom = -1;
       // primariesOfRegionsPerGroup is a sorted array. Since it contains the primary region
       // ids for the regions hosted in server, a consecutive repetition means that replicas
       // are co-hosted
       for (int j = 0; j <= primariesOfRegionsPerGroup.length; j++) {
         int primary = j < primariesOfRegionsPerGroup.length
             ? primariesOfRegionsPerGroup[j] : -1;
         if (primary != currentPrimary) { // check for whether we see a new primary
           int numReplicas = j - currentPrimaryIndex;
           if (numReplicas > 1) { // means consecutive primaries, indicating co-location
             // decide to select this primary region id or not
             double currentRandom = RANDOM.nextDouble();
             // we don't know how many region replicas are co-hosted, we will randomly select one
             // using reservoir sampling (http://gregable.com/2007/10/reservoir-sampling.html)
             if (currentRandom > currentLargestRandom) {
               selectedPrimaryIndex = currentPrimary;
               currentLargestRandom = currentRandom;
             }
           }
           currentPrimary = primary;
           currentPrimaryIndex = j;
         }
       }
 
       // we have found the primary id for the region to move. Now find the actual regionIndex
       // with the given primary, prefer to move the secondary region.
       for (int j = 0; j < regionsPerGroup.length; j++) {
         int regionIndex = regionsPerGroup[j];
         if (selectedPrimaryIndex == regionIndexToPrimaryIndex[regionIndex]) {
           // always move the secondary, not the primary
           if (selectedPrimaryIndex != regionIndex) {
             return regionIndex;
           }
         }
       }
       return -1;
     }
 
     @Override
     Cluster.Action generate(Cluster cluster) {
       int serverIndex = pickRandomServer(cluster);
       if (cluster.numServers <= 1 || serverIndex == -1) {
         return Cluster.NullAction;
       }
 
       int regionIndex = selectCoHostedRegionPerGroup(
         cluster.primariesOfRegionsPerServer[serverIndex],
         cluster.regionsPerServer[serverIndex],
         cluster.regionIndexToPrimaryIndex);
 
       // if there are no pairs of region replicas co-hosted, default to random generator
       if (regionIndex == -1) {
         // default to randompicker
         return randomGenerator.generate(cluster);
       }
 
       int toServerIndex = pickOtherRandomServer(cluster, serverIndex);
       int toRegionIndex = pickRandomRegion(cluster, toServerIndex, 0.9f);
       return getAction (serverIndex, regionIndex, toServerIndex, toRegionIndex);
     }
   }
 
   /**
    * Generates candidates which moves the replicas out of the rack for
    * co-hosted region replicas in the same rack
    */
   static class RegionReplicaRackCandidateGenerator extends RegionReplicaCandidateGenerator {
     @Override
     Cluster.Action generate(Cluster cluster) {
       int rackIndex = pickRandomRack(cluster);
       if (cluster.numRacks <= 1 || rackIndex == -1) {
         return super.generate(cluster);
       }
 
       int regionIndex = selectCoHostedRegionPerGroup(
         cluster.primariesOfRegionsPerRack[rackIndex],
         cluster.regionsPerRack[rackIndex],
         cluster.regionIndexToPrimaryIndex);
 
       // if there are no pairs of region replicas co-hosted, default to random generator
       if (regionIndex == -1) {
         // default to randompicker
         return randomGenerator.generate(cluster);
       }
 
       int serverIndex = cluster.regionIndexToServerIndex[regionIndex];
       int toRackIndex = pickOtherRandomRack(cluster, rackIndex);
 
       int rand = RANDOM.nextInt(cluster.serversPerRack[toRackIndex].length);
       int toServerIndex = cluster.serversPerRack[toRackIndex][rand];
       int toRegionIndex = pickRandomRegion(cluster, toServerIndex, 0.9f);
       return getAction (serverIndex, regionIndex, toServerIndex, toRegionIndex);
     }
   }
 
   /**
    * Base class of StochasticLoadBalancer's Cost Functions.
    */
   abstract static class CostFunction {
 
     private float multiplier = 0;
 
     protected Cluster cluster;
 
     CostFunction(Configuration c) {
 
     }
 
     float getMultiplier() {
       return multiplier;
     }
 
     void setMultiplier(float m) {
       this.multiplier = m;
     }
 
     /** Called once per LB invocation to give the cost function
      * to initialize it's state, and perform any costly calculation.
      */
     void init(Cluster cluster) {
       this.cluster = cluster;
     }
 
     /** Called once per cluster Action to give the cost function
      * an opportunity to update it's state. postAction() is always
      * called at least once before cost() is called with the cluster
      * that this action is performed on. */
     void postAction(Action action) {
       switch (action.type) {
       case NULL: break;
       case ASSIGN_REGION:
         AssignRegionAction ar = (AssignRegionAction) action;
         regionMoved(ar.region, -1, ar.server);
         break;
       case MOVE_REGION:
         MoveRegionAction mra = (MoveRegionAction) action;
         regionMoved(mra.region, mra.fromServer, mra.toServer);
         break;
       case SWAP_REGIONS:
         SwapRegionsAction a = (SwapRegionsAction) action;
         regionMoved(a.fromRegion, a.fromServer, a.toServer);
         regionMoved(a.toRegion, a.toServer, a.fromServer);
         break;
       default:
         throw new RuntimeException("Uknown action:" + action.type);
       }
     }
 
     protected void regionMoved(int region, int oldServer, int newServer) {
     }
 
     abstract double cost();
 
     /**
      * Function to compute a scaled cost using {@link DescriptiveStatistics}. It
      * assumes that this is a zero sum set of costs.  It assumes that the worst case
      * possible is all of the elements in one region server and the rest having 0.
      *
      * @param stats the costs
      * @return a scaled set of costs.
      */
     protected double costFromArray(double[] stats) {
       double totalCost = 0;
       double total = getSum(stats);
 
       double count = stats.length;
       double mean = total/count;
 
       // Compute max as if all region servers had 0 and one had the sum of all costs.  This must be
       // a zero sum cost for this to make sense.
       double max = ((count - 1) * mean) + (total - mean);
 
       // It's possible that there aren't enough regions to go around
       double min;
       if (count > total) {
         min = ((count - total) * mean) + ((1 - mean) * total);
       } else {
         // Some will have 1 more than everything else.
         int numHigh = (int) (total - (Math.floor(mean) * count));
         int numLow = (int) (count - numHigh);
 
         min = (numHigh * (Math.ceil(mean) - mean)) + (numLow * (mean - Math.floor(mean)));
 
       }
       min = Math.max(0, min);
       for (int i=0; i<stats.length; i++) {
         double n = stats[i];
         double diff = Math.abs(mean - n);
         totalCost += diff;
       }
 
       double scaled =  scale(min, max, totalCost);
       return scaled;
     }
 
     private double getSum(double[] stats) {
       double total = 0;
       for(double s:stats) {
         total += s;
       }
       return total;
     }
 
     /**
      * Scale the value between 0 and 1.
      *
      * @param min   Min value
      * @param max   The Max value
      * @param value The value to be scaled.
      * @return The scaled value.
      */
     protected double scale(double min, double max, double value) {
       if (max <= min || value <= min) {
         return 0;
       }
       if ((max - min) == 0) return 0;
 
       return Math.max(0d, Math.min(1d, (value - min) / (max - min)));
     }
   }
 
   /**
    * Given the starting state of the regions and a potential ending state
    * compute cost based upon the number of regions that have moved.
    */
   static class MoveCostFunction extends CostFunction {
     private static final String MOVE_COST_KEY = "hbase.master.balancer.stochastic.moveCost";
     private static final String MAX_MOVES_PERCENT_KEY =
         "hbase.master.balancer.stochastic.maxMovePercent";
     private static final float DEFAULT_MOVE_COST = 100;
     private static final int DEFAULT_MAX_MOVES = 600;
     private static final float DEFAULT_MAX_MOVE_PERCENT = 0.25f;
 
     private final float maxMovesPercent;
 
     MoveCostFunction(Configuration conf) {
       super(conf);
 
       // Move cost multiplier should be the same cost or higher than the rest of the costs to ensure
       // that large benefits are need to overcome the cost of a move.
       this.setMultiplier(conf.getFloat(MOVE_COST_KEY, DEFAULT_MOVE_COST));
       // What percent of the number of regions a single run of the balancer can move.
       maxMovesPercent = conf.getFloat(MAX_MOVES_PERCENT_KEY, DEFAULT_MAX_MOVE_PERCENT);
     }
 
     @Override
     double cost() {
       // Try and size the max number of Moves, but always be prepared to move some.
       int maxMoves = Math.max((int) (cluster.numRegions * maxMovesPercent),
           DEFAULT_MAX_MOVES);
 
       double moveCost = cluster.numMovedRegions;
 
       // Don't let this single balance move more than the max moves.
       // This allows better scaling to accurately represent the actual cost of a move.
       if (moveCost > maxMoves) {
         return 1000000;   // return a number much greater than any of the other cost
       }
 
       return scale(0, cluster.numRegions, moveCost);
     }
   }
 
   /**
    * Compute the cost of a potential cluster state from skew in number of
    * regions on a cluster.
    */
   static class RegionCountSkewCostFunction extends CostFunction {
     private static final String REGION_COUNT_SKEW_COST_KEY =
         "hbase.master.balancer.stochastic.regionCountCost";
     private static final float DEFAULT_REGION_COUNT_SKEW_COST = 500;
 
     private double[] stats = null;
 
     RegionCountSkewCostFunction(Configuration conf) {
       super(conf);
       // Load multiplier should be the greatest as it is the most general way to balance data.
       this.setMultiplier(conf.getFloat(REGION_COUNT_SKEW_COST_KEY, DEFAULT_REGION_COUNT_SKEW_COST));
     }
 
     @Override
     double cost() {
       if (stats == null || stats.length != cluster.numServers) {
         stats = new double[cluster.numServers];
       }
 
       for (int i =0; i < cluster.numServers; i++) {
         stats[i] = cluster.regionsPerServer[i].length;
       }
 
       return costFromArray(stats);
     }
   }
 
   /**
    * Compute the cost of a potential cluster state from skew in number of
    * primary regions on a cluster.
    */
   static class PrimaryRegionCountSkewCostFunction extends CostFunction {
     private static final String PRIMARY_REGION_COUNT_SKEW_COST_KEY =
         "hbase.master.balancer.stochastic.primaryRegionCountCost";
     private static final float DEFAULT_PRIMARY_REGION_COUNT_SKEW_COST = 500;
 
     private double[] stats = null;
 
     PrimaryRegionCountSkewCostFunction(Configuration conf) {
       super(conf);
       // Load multiplier should be the greatest as primary regions serve majority of reads/writes.
       this.setMultiplier(conf.getFloat(PRIMARY_REGION_COUNT_SKEW_COST_KEY,
         DEFAULT_PRIMARY_REGION_COUNT_SKEW_COST));
     }
 
     @Override
     double cost() {
       if (!cluster.hasRegionReplicas) {
         return 0;
       }
       if (stats == null || stats.length != cluster.numServers) {
         stats = new double[cluster.numServers];
       }
 
       for (int i =0; i < cluster.numServers; i++) {
         stats[i] = 0;
         for (int regionIdx : cluster.regionsPerServer[i]) {
           if (regionIdx == cluster.regionIndexToPrimaryIndex[regionIdx]) {
             stats[i] ++;
           }
         }
       }
 
       return costFromArray(stats);
     }
   }
 
   /**
    * Compute the cost of a potential cluster configuration based upon how evenly
    * distributed tables are.
    */
   static class TableSkewCostFunction extends CostFunction {
 
     private static final String TABLE_SKEW_COST_KEY =
         "hbase.master.balancer.stochastic.tableSkewCost";
     private static final float DEFAULT_TABLE_SKEW_COST = 35;
 
     TableSkewCostFunction(Configuration conf) {
       super(conf);
       this.setMultiplier(conf.getFloat(TABLE_SKEW_COST_KEY, DEFAULT_TABLE_SKEW_COST));
     }
 
     @Override
     double cost() {
       double max = cluster.numRegions;
       double min = ((double) cluster.numRegions) / cluster.numServers;
       double value = 0;
 
       for (int i = 0; i < cluster.numMaxRegionsPerTable.length; i++) {
         value += cluster.numMaxRegionsPerTable[i];
       }
 
       return scale(min, max, value);
     }
   }
 
   /**
    * Compute a cost of a potential cluster configuration based upon where
    * {@link org.apache.hadoop.hbase.regionserver.StoreFile}s are located.
    */
   static class LocalityCostFunction extends CostFunction {
 
     private static final String LOCALITY_COST_KEY = "hbase.master.balancer.stochastic.localityCost";
     private static final float DEFAULT_LOCALITY_COST = 25;
 
     private MasterServices services;
 
     LocalityCostFunction(Configuration conf, MasterServices srv) {
       super(conf);
       this.setMultiplier(conf.getFloat(LOCALITY_COST_KEY, DEFAULT_LOCALITY_COST));
       this.services = srv;
     }
 
     void setServices(MasterServices srvc) {
       this.services = srvc;
     }
 
     @Override
     double cost() {
       double max = 0;
       double cost = 0;
 
       // If there's no master so there's no way anything else works.
       if (this.services == null) {
         return cost;
       }
 
       for (int i = 0; i < cluster.regionLocations.length; i++) {
         max += 1;
         int serverIndex = cluster.regionIndexToServerIndex[i];
         int[] regionLocations = cluster.regionLocations[i];
 
         // If we can't find where the data is getTopBlock returns null.
         // so count that as being the best possible.
         if (regionLocations == null) {
           continue;
         }
 
         int index = -1;
         for (int j = 0; j < regionLocations.length; j++) {
           if (regionLocations[j] >= 0 && regionLocations[j] == serverIndex) {
             index = j;
             break;
           }
         }
 
         if (index < 0) {
           cost += 1;
         } else {
           cost += (1 - cluster.getLocalityOfRegion(i, index));
         }
       }
       return scale(0, max, cost);
     }
   }
 
   /**
    * Base class the allows writing costs functions from rolling average of some
    * number from RegionLoad.
    */
   abstract static class CostFromRegionLoadFunction extends CostFunction {
 
     private ClusterStatus clusterStatus = null;
     private Map<String, Deque<RegionLoad>> loads = null;
     private double[] stats = null;
     CostFromRegionLoadFunction(Configuration conf) {
       super(conf);
     }
 
     void setClusterStatus(ClusterStatus status) {
       this.clusterStatus = status;
     }
 
     void setLoads(Map<String, Deque<RegionLoad>> l) {
       this.loads = l;
     }
 
     @Override
     double cost() {
       if (clusterStatus == null || loads == null) {
         return 0;
       }
 
       if (stats == null || stats.length != cluster.numServers) {
         stats = new double[cluster.numServers];
       }
 
       for (int i =0; i < stats.length; i++) {
         //Cost this server has from RegionLoad
         long cost = 0;
 
         // for every region on this server get the rl
         for(int regionIndex:cluster.regionsPerServer[i]) {
           Collection<RegionLoad> regionLoadList =  cluster.regionLoads[regionIndex];
 
           // Now if we found a region load get the type of cost that was requested.
           if (regionLoadList != null) {
             cost += getRegionLoadCost(regionLoadList);
           }
         }
 
         // Add the total cost to the stats.
         stats[i] = cost;
       }
 
       // Now return the scaled cost from data held in the stats object.
       return costFromArray(stats);
     }
 
     protected double getRegionLoadCost(Collection<RegionLoad> regionLoadList) {
       double cost = 0;
 
       for (RegionLoad rl : regionLoadList) {
         double toAdd = getCostFromRl(rl);
 
         if (cost == 0) {
           cost = toAdd;
         } else {
           cost = (.5 * cost) + (.5 * toAdd);
         }
       }
 
       return cost;
     }
 
     protected abstract double getCostFromRl(RegionLoad rl);
   }
 
   /**
    * Compute the cost of total number of read requests  The more unbalanced the higher the
    * computed cost will be.  This uses a rolling average of regionload.
    */
 
   static class ReadRequestCostFunction extends CostFromRegionLoadFunction {
 
     private static final String READ_REQUEST_COST_KEY =
         "hbase.master.balancer.stochastic.readRequestCost";
     private static final float DEFAULT_READ_REQUEST_COST = 5;
 
     ReadRequestCostFunction(Configuration conf) {
       super(conf);
       this.setMultiplier(conf.getFloat(READ_REQUEST_COST_KEY, DEFAULT_READ_REQUEST_COST));
     }
 
 
     @Override
     protected double getCostFromRl(RegionLoad rl) {
       return rl.getReadRequestsCount();
     }
   }
 
   /**
    * Compute the cost of total number of write requests.  The more unbalanced the higher the
    * computed cost will be.  This uses a rolling average of regionload.
    */
   static class WriteRequestCostFunction extends CostFromRegionLoadFunction {
 
     private static final String WRITE_REQUEST_COST_KEY =
         "hbase.master.balancer.stochastic.writeRequestCost";
     private static final float DEFAULT_WRITE_REQUEST_COST = 5;
 
     WriteRequestCostFunction(Configuration conf) {
       super(conf);
       this.setMultiplier(conf.getFloat(WRITE_REQUEST_COST_KEY, DEFAULT_WRITE_REQUEST_COST));
     }
 
     @Override
     protected double getCostFromRl(RegionLoad rl) {
       return rl.getWriteRequestsCount();
     }
   }
 
   /**
    * A cost function for region replicas. We give a very high cost to hosting
    * replicas of the same region in the same host. We do not prevent the case
    * though, since if numReplicas > numRegionServers, we still want to keep the
    * replica open.
    */
   static class RegionReplicaHostCostFunction extends CostFunction {
     private static final String REGION_REPLICA_HOST_COST_KEY =
         "hbase.master.balancer.stochastic.regionReplicaHostCostKey";
     private static final float DEFAULT_REGION_REPLICA_HOST_COST_KEY = 100000;
 
     long maxCost = 0;
     long[] costsPerGroup; // group is either server, host or rack
     int[][] primariesOfRegionsPerGroup;
 
     public RegionReplicaHostCostFunction(Configuration conf) {
       super(conf);
       this.setMultiplier(conf.getFloat(REGION_REPLICA_HOST_COST_KEY,
         DEFAULT_REGION_REPLICA_HOST_COST_KEY));
     }
 
     @Override
     void init(Cluster cluster) {
       super.init(cluster);
       // max cost is the case where every region replica is hosted together regardless of host
       maxCost = cluster.numHosts > 1 ? getMaxCost(cluster) : 0;
       costsPerGroup = new long[cluster.numHosts];
       primariesOfRegionsPerGroup = cluster.multiServersPerHost // either server based or host based
           ? cluster.primariesOfRegionsPerHost
           : cluster.primariesOfRegionsPerServer;
       for (int i = 0 ; i < primariesOfRegionsPerGroup.length; i++) {
         costsPerGroup[i] = costPerGroup(primariesOfRegionsPerGroup[i]);
       }
     }
 
     long getMaxCost(Cluster cluster) {
       if (!cluster.hasRegionReplicas) {
         return 0; // short circuit
       }
       // max cost is the case where every region replica is hosted together regardless of host
       int[] primariesOfRegions = new int[cluster.numRegions];
       System.arraycopy(cluster.regionIndexToPrimaryIndex, 0, primariesOfRegions, 0,
           cluster.regions.length);
 
       Arrays.sort(primariesOfRegions);
 
       // compute numReplicas from the sorted array
       return costPerGroup(primariesOfRegions);
     }
 
     @Override
     double cost() {
       if (maxCost <= 0) {
         return 0;
       }
 
       long totalCost = 0;
       for (int i = 0 ; i < costsPerGroup.length; i++) {
         totalCost += costsPerGroup[i];
       }
       return scale(0, maxCost, totalCost);
     }
 
     /**
      * For each primary region, it computes the total number of replicas in the array (numReplicas)
      * and returns a sum of numReplicas-1 squared. For example, if the server hosts
      * regions a, b, c, d, e, f where a and b are same replicas, and c,d,e are same replicas, it
      * returns (2-1) * (2-1) + (3-1) * (3-1) + (1-1) * (1-1).
      * @param primariesOfRegions a sorted array of primary regions ids for the regions hosted
      * @return a sum of numReplicas-1 squared for each primary region in the group.
      */
     protected long costPerGroup(int[] primariesOfRegions) {
       long cost = 0;
       int currentPrimary = -1;
       int currentPrimaryIndex = -1;
       // primariesOfRegions is a sorted array of primary ids of regions. Replicas of regions
       // sharing the same primary will have consecutive numbers in the array.
       for (int j = 0 ; j <= primariesOfRegions.length; j++) {
         int primary = j < primariesOfRegions.length ? primariesOfRegions[j] : -1;
         if (primary != currentPrimary) { // we see a new primary
           int numReplicas = j - currentPrimaryIndex;
           // square the cost
           if (numReplicas > 1) { // means consecutive primaries, indicating co-location
             cost += (numReplicas - 1) * (numReplicas - 1);
           }
           currentPrimary = primary;
           currentPrimaryIndex = j;
         }
       }
 
       return cost;
     }
 
     @Override
     protected void regionMoved(int region, int oldServer, int newServer) {
       if (maxCost <= 0) {
         return; // no need to compute
       }
       if (cluster.multiServersPerHost) {
         int oldHost = cluster.serverIndexToHostIndex[oldServer];
         int newHost = cluster.serverIndexToHostIndex[newServer];
         if (newHost != oldHost) {
           costsPerGroup[oldHost] = costPerGroup(cluster.primariesOfRegionsPerHost[oldHost]);
           costsPerGroup[newHost] = costPerGroup(cluster.primariesOfRegionsPerHost[newHost]);
         }
       } else {
         costsPerGroup[oldServer] = costPerGroup(cluster.primariesOfRegionsPerServer[oldServer]);
         costsPerGroup[newServer] = costPerGroup(cluster.primariesOfRegionsPerServer[newServer]);
       }
     }
   }
 
   /**
    * A cost function for region replicas for the rack distribution. We give a relatively high
    * cost to hosting replicas of the same region in the same rack. We do not prevent the case
    * though.
    */
   static class RegionReplicaRackCostFunction extends RegionReplicaHostCostFunction {
     private static final String REGION_REPLICA_RACK_COST_KEY =
         "hbase.master.balancer.stochastic.regionReplicaRackCostKey";
     private static final float DEFAULT_REGION_REPLICA_RACK_COST_KEY = 10000;
 
     public RegionReplicaRackCostFunction(Configuration conf) {
       super(conf);
       this.setMultiplier(conf.getFloat(REGION_REPLICA_RACK_COST_KEY, DEFAULT_REGION_REPLICA_RACK_COST_KEY));
     }
 
     @Override
     void init(Cluster cluster) {
       this.cluster = cluster;
       if (cluster.numRacks <= 1) {
         maxCost = 0;
         return; // disabled for 1 rack
       }
       // max cost is the case where every region replica is hosted together regardless of rack
       maxCost = getMaxCost(cluster);
       costsPerGroup = new long[cluster.numRacks];
       for (int i = 0 ; i < cluster.primariesOfRegionsPerRack.length; i++) {
         costsPerGroup[i] = costPerGroup(cluster.primariesOfRegionsPerRack[i]);
       }
     }
 
     @Override
     protected void regionMoved(int region, int oldServer, int newServer) {
       if (maxCost <= 0) {
         return; // no need to compute
       }
       int oldRack = cluster.serverIndexToRackIndex[oldServer];
       int newRack = cluster.serverIndexToRackIndex[newServer];
       if (newRack != oldRack) {
         costsPerGroup[oldRack] = costPerGroup(cluster.primariesOfRegionsPerRack[oldRack]);
         costsPerGroup[newRack] = costPerGroup(cluster.primariesOfRegionsPerRack[newRack]);
       }
     }
   }
 
   /**
    * Compute the cost of total memstore size.  The more unbalanced the higher the
    * computed cost will be.  This uses a rolling average of regionload.
    */
   static class MemstoreSizeCostFunction extends CostFromRegionLoadFunction {
 
     private static final String MEMSTORE_SIZE_COST_KEY =
         "hbase.master.balancer.stochastic.memstoreSizeCost";
     private static final float DEFAULT_MEMSTORE_SIZE_COST = 5;
 
     MemstoreSizeCostFunction(Configuration conf) {
       super(conf);
       this.setMultiplier(conf.getFloat(MEMSTORE_SIZE_COST_KEY, DEFAULT_MEMSTORE_SIZE_COST));
     }
 
     @Override
     protected double getCostFromRl(RegionLoad rl) {
       return rl.getMemStoreSizeMB();
     }
   }
   /**
    * Compute the cost of total open storefiles size.  The more unbalanced the higher the
    * computed cost will be.  This uses a rolling average of regionload.
    */
   static class StoreFileCostFunction extends CostFromRegionLoadFunction {
 
     private static final String STOREFILE_SIZE_COST_KEY =
         "hbase.master.balancer.stochastic.storefileSizeCost";
     private static final float DEFAULT_STOREFILE_SIZE_COST = 5;
 
     StoreFileCostFunction(Configuration conf) {
       super(conf);
       this.setMultiplier(conf.getFloat(STOREFILE_SIZE_COST_KEY, DEFAULT_STOREFILE_SIZE_COST));
     }
 
     @Override
     protected double getCostFromRl(RegionLoad rl) {
       return rl.getStorefileSizeMB();
     }
   }
 }