IndexDeletionPolicy.cs

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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
 * 
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 * distributed under the License is distributed on an "AS IS" BASIS,
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using System;
using System.Collections.Generic;

namespace Lucene.Net.Index
{
    
    /// <summary> <p/>Expert: policy for deletion of stale <see cref="IndexCommit">index commits</see>. 
    /// 
    /// <p/>Implement this interface, and pass it to one
    /// of the <see cref="IndexWriter" /> or <see cref="IndexReader" />
    /// constructors, to customize when older
    /// <see cref="IndexCommit">point-in-time commits</see>
    /// are deleted from the index directory.  The default deletion policy
    /// is <see cref="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 <see cref="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/>
    /// </summary>
    
    public interface IndexDeletionPolicy
    {
        
        /// <summary> <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 <see cref="IndexCommit.Delete()" /> 
        /// of <see cref="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. 
        /// 
        /// </summary>
        /// <param name="commits">List of current 
        /// <see cref="IndexCommit">point-in-time commits</see>,
        /// sorted by age (the 0th one is the oldest commit).
        /// </param>
        void  OnInit<T>(IList<T> commits) where T : IndexCommit;

        /// <summary>
        /// <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 <see cref="IndexCommit.Delete()"/>
        /// of <see cref="IndexCommit" />.</p>
        /// 
        /// <p>This method is only called when <see cref="IndexWriter.Commit()"/>
        /// or <see cref="IndexWriter.Close()"/> is called, or possibly not at 
        /// all if the <see cref="IndexWriter.Rollback()"/> is called.</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.</p>
        /// </summary>
        /// <param name="commits">
        /// List of <see cref="IndexCommit" />, sorted by age (the 0th one is the oldest commit).
        /// </param>
        void  OnCommit<T>(IList<T> commits) where T : IndexCommit;
    }
}