package org.apache.lucene.index;

/**
 * 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.
 */

import java.util.List;
import java.io.IOException;

/**
 * 
Expert: policy for deletion of stale {@link IndexCommitPoint index commits}. 
 * 
 * 
Implement this interface, and pass it to one
 * of the {@link IndexWriter} or {@link IndexReader}
 * constructors, to customize when older
 * {@link IndexCommitPoint point-in-time commits}
 * are deleted from the index directory.  The default deletion policy
 * is {@link KeepOnlyLastCommitDeletionPolicy}, which always
 * removes old commits as soon as a new commit is done (this
 * matches the behavior before 2.2).
 *
 * 
One expected use case for this (and the reason why it
 * was first created) is to work around problems with an
 * index directory accessed via filesystems like NFS because
 * NFS does not provide the "delete on last close" semantics
 * that Lucene's "point in time" search normally relies on.
 * By implementing a custom deletion policy, such as "a
 * commit is only removed once it has been stale for more
 * than X minutes", you can give your readers time to
 * refresh to the new commit before {@link IndexWriter}
 * removes the old commits.  Note that doing so will
 * increase the storage requirements of the index.  See LUCENE-710
 * for details.
 */

public interface IndexDeletionPolicy {

  /**
   * 
This is called once when a writer is first
   * instantiated to give the policy a chance to remove old
   * commit points.
   * 
   * 
The writer locates all index commits present in the 
   * index directory and calls this method.  The policy may 
   * choose to delete some of the commit points, doing so by
   * calling method {@link IndexCommitPoint#delete delete()} 
   * of {@link IndexCommitPoint}.
   * 
   * 
Note: the last CommitPoint is the most recent one,
   * i.e. the "front index state". Be careful not to delete it,
   * unless you know for sure what you are doing, and unless 
   * you can afford to lose the index content while doing that. 
   *
   * @param commits List of current 
   * {@link IndexCommitPoint point-in-time commits},
   *  sorted by age (the 0th one is the oldest commit).
   */
  public void onInit(List commits) throws IOException;

  /**
   * 
This is called each time the writer completed a commit.
   * This gives the policy a chance to remove old commit points
   * with each commit.
   *
   * 
The policy may now choose to delete old commit points 
   * by calling method {@link IndexCommitPoint#delete delete()} 
   * of {@link IndexCommitPoint}.
   * 
   * 
If writer has autoCommit = true then
   * this method will in general be called many times during
   * one instance of {@link IndexWriter}.  If
   * autoCommit = false then this method is
   * only called once when {@link IndexWriter#close} is
   * called, or not at all if the {@link IndexWriter#abort}
   * is called. 
   *
   * 
Note: the last CommitPoint is the most recent one,
   * i.e. the "front index state". Be careful not to delete it,
   * unless you know for sure what you are doing, and unless 
   * you can afford to lose the index content while doing that.
   *  
   * @param commits List of {@link IndexCommitPoint},
   *  sorted by age (the 0th one is the oldest commit).
   */
  public void onCommit(List commits) throws IOException;
}