lucene/src/java/org/apache/lucene/index/SnapshotDeletionPolicy.java

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.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.Set;
import java.util.Map.Entry;
import java.io.IOException;

import org.apache.lucene.store.Directory;

/**
 * An {@link IndexDeletionPolicy} that wraps around any other
 * {@link IndexDeletionPolicy} and adds the ability to hold and later release
 * snapshots of an index. While a snapshot is held, the {@link IndexWriter} will
 * not remove any files associated with it even if the index is otherwise being
 * actively, arbitrarily changed. Because we wrap another arbitrary
 * {@link IndexDeletionPolicy}, this gives you the freedom to continue using
 * whatever {@link IndexDeletionPolicy} you would normally want to use with your
 * index.
 * 
 * <p>
 * This class maintains all snapshots in-memory, and so the information is not
 * persisted and not protected against system failures. If persistency is
 * important, you can use {@link PersistentSnapshotDeletionPolicy} (or your own
 * extension) and when creating a new instance of this deletion policy, pass the
 * persistent snapshots information to
 * {@link #SnapshotDeletionPolicy(IndexDeletionPolicy, Map)}.
 * 
 * @lucene.experimental
 */
public class SnapshotDeletionPolicy implements IndexDeletionPolicy {

  /** Holds a Snapshot's information. */
  private static class SnapshotInfo {
    String id;
    String segmentsFileName;
    IndexCommit commit;
    
    public SnapshotInfo(String id, String segmentsFileName, IndexCommit commit) {
      this.id = id;
      this.segmentsFileName = segmentsFileName;
      this.commit = commit;
    }
    
    @Override
    public String toString() {
      return id + " : " + segmentsFileName;
    }
  }
  
  protected class SnapshotCommitPoint extends IndexCommit {
    protected IndexCommit cp;

    protected SnapshotCommitPoint(IndexCommit cp) {
      this.cp = cp;
    }

    @Override
    public String toString() {
      return "SnapshotDeletionPolicy.SnapshotCommitPoint(" + cp + ")";
    }

    /**
     * Returns true if this segment can be deleted. The default implementation
     * returns false if this segment is currently held as snapshot.
     */
    protected boolean shouldDelete(String segmentsFileName) {
      return !segmentsFileToIDs.containsKey(segmentsFileName);
    }

    @Override
    public void delete() {
      synchronized (SnapshotDeletionPolicy.this) {
        // Suppress the delete request if this commit point is
        // currently snapshotted.
        if (shouldDelete(getSegmentsFileName())) {
          cp.delete();
        }
      }
    }

    @Override
    public Directory getDirectory() {
      return cp.getDirectory();
    }

    @Override
    public Collection<String> getFileNames() throws IOException {
      return cp.getFileNames();
    }

    @Override
    public long getGeneration() {
      return cp.getGeneration();
    }

    @Override
    public String getSegmentsFileName() {
      return cp.getSegmentsFileName();
    }

    @Override
    public Map<String, String> getUserData() throws IOException {
      return cp.getUserData();
    }

    @Override
    public long getVersion() {
      return cp.getVersion();
    }

    @Override
    public boolean isDeleted() {
      return cp.isDeleted();
    }

    @Override
    public boolean isOptimized() {
      return cp.isOptimized();
    }
  }

  /** Snapshots info */
  private Map<String, SnapshotInfo> idToSnapshot = new HashMap<String, SnapshotInfo>();

  // multiple IDs could point to the same commit point (segments file name)
  private Map<String, Set<String>> segmentsFileToIDs = new HashMap<String, Set<String>>();

  private IndexDeletionPolicy primary;
  protected IndexCommit lastCommit;

  public SnapshotDeletionPolicy(IndexDeletionPolicy primary) {
    this.primary = primary;
  }

  /**
   * {@link SnapshotDeletionPolicy} wraps another {@link IndexDeletionPolicy} to
   * enable flexible snapshotting.
   * 
   * @param primary
   *          the {@link IndexDeletionPolicy} that is used on non-snapshotted
   *          commits. Snapshotted commits, are not deleted until explicitly
   *          released via {@link #release(String)}
   * @param snapshotsInfo
   *          A mapping of snapshot ID to the segments filename that is being
   *          snapshotted. The expected input would be the output of
   *          {@link #getSnapshots()}. A null value signals that there are no
   *          initial snapshots to maintain.
   */
  public SnapshotDeletionPolicy(IndexDeletionPolicy primary,
      Map<String, String> snapshotsInfo) {
    this(primary);

    if (snapshotsInfo != null) {
      // Add the ID->segmentIDs here - the actual IndexCommits will be
      // reconciled on the call to onInit()
      for (Entry<String, String> e : snapshotsInfo.entrySet()) {
        registerSnapshotInfo(e.getKey(), e.getValue(), null);
      }
    }
  }

  /**
   * Checks if the given id is already used by another snapshot, and throws
   * {@link IllegalStateException} if it is.
   */
  protected void checkSnapshotted(String id) {
    if (isSnapshotted(id)) {
      throw new IllegalStateException("Snapshot ID " + id
          + " is already used - must be unique");
    }
  }

  /** Registers the given snapshot information. */
  protected void registerSnapshotInfo(String id, String segment, IndexCommit commit) {
    idToSnapshot.put(id, new SnapshotInfo(id, segment, commit));
    Set<String> ids = segmentsFileToIDs.get(segment);
    if (ids == null) {
      ids = new HashSet<String>();
      segmentsFileToIDs.put(segment, ids);
    }
    ids.add(id);
  }

  protected List<IndexCommit> wrapCommits(List<? extends IndexCommit> commits) {
    List<IndexCommit> wrappedCommits = new ArrayList<IndexCommit>(commits.size());
    for (IndexCommit ic : commits) {
      wrappedCommits.add(new SnapshotCommitPoint(ic));
    }
    return wrappedCommits;
  }

  /**
   * Get a snapshotted IndexCommit by ID. The IndexCommit can then be used to
   * open an IndexReader on a specific commit point, or rollback the index by
   * opening an IndexWriter with the IndexCommit specified in its
   * {@link IndexWriterConfig}.
   * 
   * @param id
   *          a unique identifier of the commit that was snapshotted.
   * @throws IllegalStateException
   *           if no snapshot exists by the specified ID.
   * @return The {@link IndexCommit} for this particular snapshot.
   */
  public synchronized IndexCommit getSnapshot(String id) {
    SnapshotInfo snapshotInfo = idToSnapshot.get(id);
    if (snapshotInfo == null) {
      throw new IllegalStateException("No snapshot exists by ID: " + id);
    }
    return snapshotInfo.commit;
  }

  /**
   * Get all the snapshots in a map of snapshot IDs to the segments they
   * 'cover.' This can be passed to
   * {@link #SnapshotDeletionPolicy(IndexDeletionPolicy, Map)} in order to
   * initialize snapshots at construction.
   */
  public synchronized Map<String, String> getSnapshots() {
    Map<String, String> snapshots = new HashMap<String, String>();
    for (Entry<String, SnapshotInfo> e : idToSnapshot.entrySet()) {
      snapshots.put(e.getKey(), e.getValue().segmentsFileName);
    }
    return snapshots;
  }

  /**
   * Returns true if the given ID is already used by a snapshot. You can call
   * this method before {@link #snapshot(String)} if you are not sure whether
   * the ID is already used or not.
   */
  public boolean isSnapshotted(String id) {
    return idToSnapshot.containsKey(id);
  }

  public synchronized void onCommit(List<? extends IndexCommit> commits)
      throws IOException {
    primary.onCommit(wrapCommits(commits));
    lastCommit = commits.get(commits.size() - 1);
  }

  public synchronized void onInit(List<? extends IndexCommit> commits)
      throws IOException {
    primary.onInit(wrapCommits(commits));
    lastCommit = commits.get(commits.size() - 1);

    /*
     * Assign snapshotted IndexCommits to their correct snapshot IDs as
     * specified in the constructor.
     */
    for (IndexCommit commit : commits) {
      Set<String> ids = segmentsFileToIDs.get(commit.getSegmentsFileName());
      if (ids != null) {
        for (String id : ids) {
          idToSnapshot.get(id).commit = commit;
        }
      }
    }

    /*
     * Second, see if there are any instances where a snapshot ID was specified
     * in the constructor but an IndexCommit doesn't exist. In this case, the ID
     * should be removed.
     * 
     * Note: This code is protective for extreme cases where IDs point to
     * non-existent segments. As the constructor should have received its
     * information via a call to getSnapshots(), the data should be well-formed.
     */
    // Find lost snapshots
    ArrayList<String> idsToRemove = null;
    for (Entry<String, SnapshotInfo> e : idToSnapshot.entrySet()) {
      if (e.getValue().commit == null) {
        if (idsToRemove == null) {
          idsToRemove = new ArrayList<String>();
        }
        idsToRemove.add(e.getKey());
      }
    }
    // Finally, remove those 'lost' snapshots.
    if (idsToRemove != null) {
      for (String id : idsToRemove) {
        SnapshotInfo info = idToSnapshot.remove(id);
        segmentsFileToIDs.remove(info.segmentsFileName);
      }
    }
  }

  /**
   * Release a snapshotted commit by ID.
   * 
   * @param id
   *          a unique identifier of the commit that is un-snapshotted.
   * @throws IllegalStateException
   *           if no snapshot exists by this ID.
   */
  public synchronized void release(String id) throws IOException {
    SnapshotInfo info = idToSnapshot.remove(id);
    if (info == null) {
      throw new IllegalStateException("Snapshot doesn't exist: " + id);
    }
    Set<String> ids = segmentsFileToIDs.get(info.segmentsFileName);
    if (ids != null) {
      ids.remove(id);
      if (ids.size() == 0) {
        segmentsFileToIDs.remove(info.segmentsFileName);
      }
    }
  }

  /**
   * Snapshots the last commit. Once a commit is 'snapshotted,' it is protected
   * from deletion (as long as this {@link IndexDeletionPolicy} is used). The
   * commit can be removed by calling {@link #release(String)} using the same ID
   * parameter followed by a call to {@link IndexWriter#deleteUnusedFiles()}.
   * <p>
   * <b>NOTE:</b> ID must be unique in the system. If the same ID is used twice,
   * an {@link IllegalStateException} is thrown.
   * <p>
   * <b>NOTE:</b> 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.
   * 
   * @param id
   *          a unique identifier of the commit that is being snapshotted.
   * @throws IllegalStateException
   *           if either there is no 'last commit' to snapshot, or if the
   *           parameter 'ID' refers to an already snapshotted commit.
   * @return the {@link IndexCommit} that was snapshotted.
   */
  public synchronized IndexCommit snapshot(String id) throws IOException {
    if (lastCommit == null) {
      // no commit exists. Really shouldn't happen, but might be if SDP is
      // accessed before onInit or onCommit were called.
      throw new IllegalStateException("No index commit to snapshot");
    }

    // Can't use the same snapshot ID twice...
    checkSnapshotted(id);

    registerSnapshotInfo(id, lastCommit.getSegmentsFileName(), lastCommit);
    return lastCommit;
  }

}