SnapshotDeletionPolicy.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
 * 
 * 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.
 */

using System;
using System.Collections.Generic;
using Directory = Lucene.Net.Store.Directory;

namespace Lucene.Net.Index
{
    
    /// <summary>A <see cref="IndexDeletionPolicy" /> that wraps around any other
    /// <see cref="IndexDeletionPolicy" /> and adds the ability to hold and
    /// later release a single "snapshot" of an index.  While
    /// the snapshot is held, the <see cref="IndexWriter" /> will not
    /// remove any files associated with it even if the index is
    /// otherwise being actively, arbitrarily changed.  Because
    /// we wrap another arbitrary <see cref="IndexDeletionPolicy" />, this
    /// gives you the freedom to continue using whatever <see cref="IndexDeletionPolicy" />
    /// you would normally want to use with your
    /// index.  Note that you can re-use a single instance of
    /// SnapshotDeletionPolicy across multiple writers as long
    /// as they are against the same index Directory.  Any
    /// snapshot held when a writer is closed will "survive"
    /// when the next writer is opened.
    /// 
    /// <p/><b>WARNING</b>: This API is a new and experimental and
    /// may suddenly change.<p/> 
    /// </summary>
    
    public class SnapshotDeletionPolicy : IndexDeletionPolicy
    {
        
        private IndexCommit lastCommit;
        private IndexDeletionPolicy primary;
        private System.String snapshot;
        
        public SnapshotDeletionPolicy(IndexDeletionPolicy primary)
        {
            this.primary = primary;
        }
        
        public virtual void  OnInit<T>(IList<T> commits) where T : IndexCommit
        {
            lock (this)
            {
                primary.OnInit(WrapCommits(commits));
                lastCommit = commits[commits.Count - 1];
            }
        }
        
        public virtual void  OnCommit<T>(IList<T> commits) where T : IndexCommit
        {
            lock (this)
            {
                primary.OnCommit(WrapCommits(commits));
                lastCommit = commits[commits.Count - 1];
            }
        }
        
        /// <summary>Take a snapshot of the most recent commit to the
        /// index.  You must call release() to free this snapshot.
        /// Note that while the snapshot is held, the files it
        /// references will not be deleted, which will consume
        /// additional disk space in your index. If you take a
        /// snapshot at a particularly bad time (say just before
        /// you call optimize()) then in the worst case this could
        /// consume an extra 1X of your total index size, until
        /// you release the snapshot. 
        /// </summary>
        public virtual IndexCommit Snapshot()
        {
            lock (this)
            {
                if (lastCommit == null)
                {
                    throw new System.SystemException("no index commits to snapshot !");
                }

                if (snapshot == null)
                    snapshot = lastCommit.SegmentsFileName;
                else
                    throw new System.SystemException("snapshot is already set; please call release() first");
                return lastCommit;
            }
        }
        
        /// <summary>Release the currently held snapshot. </summary>
        public virtual void  Release()
        {
            lock (this)
            {
                if (snapshot != null)
                    snapshot = null;
                else
                    throw new System.SystemException("snapshot was not set; please call snapshot() first");
            }
        }
        
        private class MyCommitPoint : IndexCommit
        {
            private void  InitBlock(SnapshotDeletionPolicy enclosingInstance)
            {
                this.enclosingInstance = enclosingInstance;
            }
            private SnapshotDeletionPolicy enclosingInstance;
            public SnapshotDeletionPolicy Enclosing_Instance
            {
                get
                {
                    return enclosingInstance;
                }
                
            }
            internal IndexCommit cp;
            internal MyCommitPoint(SnapshotDeletionPolicy enclosingInstance, IndexCommit cp)
            {
                InitBlock(enclosingInstance);
                this.cp = cp;
            }

            public override string ToString()
            {
                return "SnapshotDeletionPolicy.SnapshotCommitPoint(" + cp + ")";
            }

            public override string SegmentsFileName
            {
                get { return cp.SegmentsFileName; }
            }

            public override ICollection<string> FileNames
            {
                get { return cp.FileNames; }
            }

            public override Directory Directory
            {
                get { return cp.Directory; }
            }

            public override void  Delete()
            {
                lock (Enclosing_Instance)
                {
                    // Suppress the delete request if this commit point is
                    // our current snapshot.
                    if (Enclosing_Instance.snapshot == null || !Enclosing_Instance.snapshot.Equals(SegmentsFileName))
                        cp.Delete();
                }
            }

            public override bool IsDeleted
            {
                get { return cp.IsDeleted; }
            }

            public override long Version
            {
                get { return cp.Version; }
            }

            public override long Generation
            {
                get { return cp.Generation; }
            }

            public override IDictionary<string, string> UserData
            {
                get { return cp.UserData; }
            }

            public override bool IsOptimized
            {
                get { return cp.IsOptimized; }
            }
        }
        
        private IList<IndexCommit> WrapCommits<T>(IList<T> commits) where T : IndexCommit
        {
            int count = commits.Count;
            var myCommits = new List<IndexCommit>(count);
            for (int i = 0; i < count; i++)
            {
                myCommits.Add(new MyCommitPoint(this, commits[i]));
            }
            return myCommits;
        }
    }
}