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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.cassandra.db.memtable;
  
  import java.util.concurrent.atomic.AtomicReference;
  
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import org.apache.cassandra.db.commitlog.CommitLogPosition;
  import org.apache.cassandra.schema.TableMetadataRef;
  import org.apache.cassandra.utils.FBUtilities;
  import org.apache.cassandra.utils.MBeanWrapper;
  import org.github.jamm.Unmetered;
  
  import static org.apache.cassandra.config.CassandraRelevantProperties.MEMTABLE_SHARD_COUNT;
  
  public abstract class AbstractShardedMemtable extends AbstractAllocatorMemtable
  {
      private static final Logger logger = LoggerFactory.getLogger(AbstractShardedMemtable.class);
  
      public static final String SHARDS_OPTION = "shards";
      public static final String SHARDED_MEMTABLE_CONFIG_OBJECT_NAME = "org.apache.cassandra.db:type=ShardedMemtableConfig";
      static
      {
          MBeanWrapper.instance.registerMBean(new ShardedMemtableConfig(), SHARDED_MEMTABLE_CONFIG_OBJECT_NAME, MBeanWrapper.OnException.LOG);
      }
  
      // default shard count, used when a specific number of shards is not specified in the options
      private static volatile int defaultShardCount = MEMTABLE_SHARD_COUNT.getInt(FBUtilities.getAvailableProcessors());
  
      // The boundaries for the keyspace as they were calculated when the memtable is created.
      // The boundaries will be NONE for system keyspaces or if StorageService is not yet initialized.
      // The fact this is fixed for the duration of the memtable lifetime, guarantees we'll always pick the same shard
      // for a given key, even if we race with the StorageService initialization or with topology changes.
      @Unmetered
      protected final ShardBoundaries boundaries;
  
      AbstractShardedMemtable(AtomicReference<CommitLogPosition> commitLogLowerBound,
                              TableMetadataRef metadataRef,
                              Owner owner,
                              Integer shardCountOption)
      {
          super(commitLogLowerBound, metadataRef, owner);
          int shardCount = shardCountOption != null ? shardCountOption : defaultShardCount;
          this.boundaries = owner.localRangeSplits(shardCount);
      }
  
      private static class ShardedMemtableConfig implements ShardedMemtableConfigMXBean
      {
          @Override
          public void setDefaultShardCount(String shardCount)
          {
              if ("auto".equalsIgnoreCase(shardCount))
              {
                  defaultShardCount = FBUtilities.getAvailableProcessors();
              }
              else
              {
                  try
                  {
                      defaultShardCount = Integer.parseInt(shardCount);
                  }
                  catch (NumberFormatException ex)
                  {
                      logger.warn("Unable to parse {} as valid value for shard count", shardCount);
                      return;
                  }
              }
              logger.info("Requested setting shard count to {}; set to: {}", shardCount, defaultShardCount);
          }
  
          @Override
          public String getDefaultShardCount()
          {
              return Integer.toString(defaultShardCount);
          }
      }
  
      public static int getDefaultShardCount()
      {
          return defaultShardCount;
      }
  }
  