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/**
 * 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.client.backoff;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hbase.HConstants;
import org.apache.hadoop.hbase.ServerName;
import org.apache.hadoop.hbase.classification.InterfaceAudience;
import org.apache.hadoop.hbase.classification.InterfaceStability;

import com.google.common.base.Preconditions;

/**
 * Simple exponential backoff policy on for the client that uses a  percent^4 times the
 * max backoff to generate the backoff time.
 */
@InterfaceAudience.Public
@InterfaceStability.Unstable
public class ExponentialClientBackoffPolicy implements ClientBackoffPolicy {

  private static final Log LOG = LogFactory.getLog(ExponentialClientBackoffPolicy.class);

  private static final long ONE_MINUTE = 60 * 1000;
  public static final long DEFAULT_MAX_BACKOFF = 5 * ONE_MINUTE;
  public static final String MAX_BACKOFF_KEY = "hbase.client.exponential-backoff.max";
  private long maxBackoff;
  private float heapOccupancyLowWatermark;
  private float heapOccupancyHighWatermark;

  public ExponentialClientBackoffPolicy(Configuration conf) {
    this.maxBackoff = conf.getLong(MAX_BACKOFF_KEY, DEFAULT_MAX_BACKOFF);
    this.heapOccupancyLowWatermark = conf.getFloat(HConstants.HEAP_OCCUPANCY_LOW_WATERMARK_KEY,
      HConstants.DEFAULT_HEAP_OCCUPANCY_LOW_WATERMARK);
    this.heapOccupancyHighWatermark = conf.getFloat(HConstants.HEAP_OCCUPANCY_HIGH_WATERMARK_KEY,
      HConstants.DEFAULT_HEAP_OCCUPANCY_HIGH_WATERMARK);
  }

  @Override
  public long getBackoffTime(ServerName serverName, byte[] region, ServerStatistics stats) {
    // no stats for the server yet, so don't backoff
    if (stats == null) {
      return 0;
    }

    ServerStatistics.RegionStatistics regionStats = stats.getStatsForRegion(region);
    // no stats for the region yet - don't backoff
    if (regionStats == null) {
      return 0;
    }

    // Factor in memstore load
    double percent = regionStats.getMemstoreLoadPercent() / 100.0;

    // Factor in heap occupancy
    float heapOccupancy = regionStats.getHeapOccupancyPercent() / 100.0f;

    // Factor in compaction pressure, 1.0 means heavy compaction pressure
    float compactionPressure = regionStats.getCompactionPressure() / 100.0f;
    if (heapOccupancy >= heapOccupancyLowWatermark) {
      // If we are higher than the high watermark, we are already applying max
      // backoff and cannot scale more (see scale() below)
      if (heapOccupancy > heapOccupancyHighWatermark) {
        heapOccupancy = heapOccupancyHighWatermark;
      }
      percent = Math.max(percent,
          scale(heapOccupancy, heapOccupancyLowWatermark, heapOccupancyHighWatermark,
              0.1, 1.0));
    }
    percent = Math.max(percent, compactionPressure);
    // square the percent as a value less than 1. Closer we move to 100 percent,
    // the percent moves to 1, but squaring causes the exponential curve
    double multiplier = Math.pow(percent, 4.0);
    if (multiplier > 1) {
      multiplier = 1;
    }
    return (long) (multiplier * maxBackoff);
  }

  /** Scale valueIn in the range [baseMin,baseMax] to the range [limitMin,limitMax] */
  private static double scale(double valueIn, double baseMin, double baseMax, double limitMin,
      double limitMax) {
    Preconditions.checkArgument(baseMin <= baseMax, "Illegal source range [%s,%s]",
        baseMin, baseMax);
    Preconditions.checkArgument(limitMin <= limitMax, "Illegal target range [%s,%s]",
        limitMin, limitMax);
    Preconditions.checkArgument(valueIn >= baseMin && valueIn <= baseMax,
        "Value %s must be within the range [%s,%s]", valueIn, baseMin, baseMax);
    return ((limitMax - limitMin) * (valueIn - baseMin) / (baseMax - baseMin)) + limitMin;
  }
}