hbase-server/src/main/java/org/apache/hadoop/hbase/master/balancer/RegionCountSkewCostFunction.java

/*
 * 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 org.apache.hadoop.conf.Configuration;
import org.apache.yetus.audience.InterfaceAudience;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Compute the cost of a potential cluster state from skew in number of regions on a cluster.
 */
@InterfaceAudience.Private
class RegionCountSkewCostFunction extends CostFunction {

  private static final Logger LOG = LoggerFactory.getLogger(RegionCountSkewCostFunction.class);

  static final String REGION_COUNT_SKEW_COST_KEY =
    "hbase.master.balancer.stochastic.regionCountCost";
  static final float DEFAULT_REGION_COUNT_SKEW_COST = 500;

  private double[] stats = null;

  RegionCountSkewCostFunction(Configuration 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
  void init(BalancerClusterState cluster) {
    super.init(cluster);
    LOG.debug("{} sees a total of {} servers and {} regions.", getClass().getSimpleName(),
      cluster.numServers, cluster.numRegions);
    if (LOG.isTraceEnabled()) {
      for (int i = 0; i < cluster.numServers; i++) {
        LOG.trace("{} sees server '{}' has {} regions", getClass().getSimpleName(),
          cluster.servers[i], cluster.regionsPerServer[i].length);
      }
    }
  }

  @Override
  protected 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);
  }
}