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;
/**
* <p>Expert: policy for deletion of stale {@link IndexCommit index commits}.
*
* <p>Implement this interface, and pass it to one
* of the {@link IndexWriter} or {@link IndexReader}
* constructors, to customize when older
* {@link IndexCommit 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).</p>
*
* <p>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 <a
* target="top"
* href="http://issues.apache.org/jira/browse/LUCENE-710">LUCENE-710</a>
* for details.</p>
*/
public interface IndexDeletionPolicy {
/**
* <p>This is called once when a writer is first
* instantiated to give the policy a chance to remove old
* commit points.</p>
*
* <p>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 IndexCommit#delete delete()}
* of {@link IndexCommit}.</p>
*
* <p><u>Note:</u> 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 IndexCommit point-in-time commits},
* sorted by age (the 0th one is the oldest commit).
*/
public void onInit(List<? extends IndexCommit> commits) throws IOException;
/**
* <p>This is called each time the writer completed a commit.
* This gives the policy a chance to remove old commit points
* with each commit.</p>
*
* <p>The policy may now choose to delete old commit points
* by calling method {@link IndexCommit#delete delete()}
* of {@link IndexCommit}.</p>
*
* <p>This method is only called when {@link
* IndexWriter#commit} or {@link IndexWriter#close} is
* called, or possibly not at all if the {@link
* IndexWriter#rollback} is called.
*
* <p><u>Note:</u> 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 IndexCommit},
* sorted by age (the 0th one is the oldest commit).
*/
public void onCommit(List<? extends IndexCommit> commits) throws IOException;
}