/**
    *
    * 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.regionserver;
  
  import java.io.IOException;
  import java.util.List;
  
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.KeyValue.KVComparator;
  import org.apache.hadoop.hbase.regionserver.compactions.CompactionContext;
  import org.apache.hadoop.hbase.regionserver.compactions.CompactionPolicy;
  import org.apache.hadoop.hbase.regionserver.compactions.Compactor;
  import org.apache.hadoop.hbase.util.ReflectionUtils;
  
  /**
   * StoreEngine is a factory that can create the objects necessary for HStore to operate.
   * Since not all compaction policies, compactors and store file managers are compatible,
   * they are tied together and replaced together via StoreEngine-s.
   */
  @InterfaceAudience.Private
  public abstract class StoreEngine<SF extends StoreFlusher,
      CP extends CompactionPolicy, C extends Compactor, SFM extends StoreFileManager> {
    protected SF storeFlusher;
    protected CP compactionPolicy;
    protected C compactor;
    protected SFM storeFileManager;
  
    /**
     * The name of the configuration parameter that specifies the class of
     * a store engine that is used to manage and compact HBase store files.
     */
    public static final String STORE_ENGINE_CLASS_KEY = "hbase.hstore.engine.class";
  
    private static final Class<? extends StoreEngine<?, ?, ?, ?>>
      DEFAULT_STORE_ENGINE_CLASS = DefaultStoreEngine.class;
  
    /**
     * @return Compaction policy to use.
     */
    public CompactionPolicy getCompactionPolicy() {
      return this.compactionPolicy;
    }
  
    /**
     * @return Compactor to use.
     */
    public Compactor getCompactor() {
      return this.compactor;
    }
  
    /**
     * @return Store file manager to use.
     */
    public StoreFileManager getStoreFileManager() {
      return this.storeFileManager;
    }
  
    /**
     * @return Store flusher to use.
     */
    public StoreFlusher getStoreFlusher() {
      return this.storeFlusher;
    }
  
    /**
     * @param filesCompacting Files currently compacting
     * @return whether a compaction selection is possible
     */
    public abstract boolean needsCompaction(List<StoreFile> filesCompacting);
  
    /**
     * Creates an instance of a compaction context specific to this engine.
     * Doesn't actually select or start a compaction. See CompactionContext class comment.
     * @return New CompactionContext object.
     */
    public abstract CompactionContext createCompaction() throws IOException;
  
    /**
     * Create the StoreEngine's components.
     */
    protected abstract void createComponents(
       Configuration conf, Store store, KVComparator kvComparator) throws IOException;
 
   private void createComponentsOnce(
       Configuration conf, Store store, KVComparator kvComparator) throws IOException {
     assert compactor == null && compactionPolicy == null
         && storeFileManager == null && storeFlusher == null;
     createComponents(conf, store, kvComparator);
     assert compactor != null && compactionPolicy != null
         && storeFileManager != null && storeFlusher != null;
   }
 
   /**
    * Create the StoreEngine configured for the given Store.
    * @param store The store. An unfortunate dependency needed due to it
    *              being passed to coprocessors via the compactor.
    * @param conf Store configuration.
    * @param kvComparator KVComparator for storeFileManager.
    * @return StoreEngine to use.
    */
   public static StoreEngine<?, ?, ?, ?> create(
       Store store, Configuration conf, KVComparator kvComparator) throws IOException {
     String className = conf.get(STORE_ENGINE_CLASS_KEY, DEFAULT_STORE_ENGINE_CLASS.getName());
     try {
       StoreEngine<?,?,?,?> se = ReflectionUtils.instantiateWithCustomCtor(
           className, new Class[] { }, new Object[] { });
       se.createComponentsOnce(conf, store, kvComparator);
       return se;
     } catch (Exception e) {
       throw new IOException("Unable to load configured store engine '" + className + "'", e);
     }
   }
 }