content: /*
   * 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.cassandra.metrics;
  
  import java.util.concurrent.CopyOnWriteArrayList;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.locks.Lock;
  
  import com.codahale.metrics.Snapshot;
  import org.apache.cassandra.metrics.LogLinearHistogram.LogLinearSnapshot;
  import org.apache.cassandra.utils.Clock;
  
  import static org.apache.cassandra.metrics.CassandraReservoir.BucketStrategy.log_linear;
  
  public class ShardedHistogram extends OverrideHistogram
  {
      private static final long REFRESH_RATE = TimeUnit.SECONDS.toNanos(15);
  
      static class HistogramShard
      {
          final Lock lock;
          final LogLinearHistogram histogram;
  
          HistogramShard(Lock lock, LogLinearHistogram histogram)
          {
              this.lock = lock;
              this.histogram = histogram;
          }
  
          long total()
          {
              lock.lock();
              try
              {
                  return histogram.totalCount;
              }
              finally
              {
                  lock.unlock();
              }
          }
  
          public void updateSnapshot(LogLinearSnapshot snapshot)
          {
              lock.lock();
              try
              {
                  histogram.updateSnapshot(snapshot);
              }
              finally
              {
                  lock.unlock();
              }
          }
      }
  
      final CopyOnWriteArrayList<HistogramShard> shards = new CopyOnWriteArrayList<>();
      final long initialMaxValue;
  
      public ShardedHistogram()
      {
          this(1 << 16);
      }
  
      @Override
      public CassandraReservoir.BucketStrategy bucketStrategy()
      {
          return log_linear;
      }
  
      @Override
      public long[] bucketStarts(int length)
      {
          return LogLinearHistogram.bucketsWithLength(length);
      }
  
      public ShardedHistogram(long initialMaxValue)
      {
          this.initialMaxValue = initialMaxValue;
      }
  
      public LogLinearHistogram newShard(Lock guardedBy)
      {
          HistogramShard shard = new HistogramShard(guardedBy, new LogLinearHistogram(initialMaxValue));
          shards.add(shard);
          return shard.histogram;
      }
  
      private LogLinearSnapshot snapshot;
      private long snapshotAt;
  
      public synchronized void refresh()
      {
          refresh(Clock.Global.nanoTime());
      }
  
      private synchronized LogLinearSnapshot refresh(long now)
      {
          LogLinearSnapshot snapshot = LogLinearSnapshot.emptyForMax(initialMaxValue);
          for (HistogramShard shard : shards)
              shard.updateSnapshot(snapshot);
          this.snapshot = snapshot;
          this.snapshotAt = now;
          return snapshot;
      }
  
      private synchronized LogLinearSnapshot maybeRefresh()
      {
          if (shards.isEmpty())
              return snapshot = new LogLinearSnapshot(0);
  
          long now = Clock.Global.nanoTime();
          if (snapshot != null && snapshotAt + REFRESH_RATE >= now)
              return snapshot;
  
          return refresh(now);
      }
  
      @Override
      public synchronized long getCount()
      {
          return maybeRefresh().totalCount;
      }
  
      @Override
      public Snapshot getSnapshot()
      {
          return maybeRefresh();
      }
  }
  