Package org.apache.lucene.index

Class SnapshotDeletionPolicy

  • Direct Known Subclasses:
    PersistentSnapshotDeletionPolicy
    public class SnapshotDeletionPolicy
extends IndexDeletionPolicy
    An IndexDeletionPolicy that wraps any other IndexDeletionPolicy and adds the ability to hold and later release snapshots of an index. While a snapshot is held, the IndexWriter will not remove any files associated with it even if the index is otherwise being actively, arbitrarily changed. Because we wrap another arbitrary IndexDeletionPolicy, this gives you the freedom to continue using whatever IndexDeletionPolicy you would normally want to use with your index.

    This class maintains all snapshots in-memory, and so the information is not persisted and not protected against system failures. If persistence is important, you can use PersistentSnapshotDeletionPolicy.

    WARNING: This API is experimental and might change in incompatible ways in the next release.

    • Field Detail

      • refCounts

        protected final Map<Long,​Integer> refCounts
        Records how many snapshots are held against each commit generation

      • indexCommits

        protected final Map<Long,​IndexCommit> indexCommits
        Used to map gen to IndexCommit.

    • Method Detail

      • onCommit

        public void onCommit​(List<? extends IndexCommit> commits)
              throws IOException
        Description copied from class: IndexDeletionPolicy
        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 delete() of IndexCommit.

        This method is only called when IndexWriter.commit() or IndexWriter.close() is called, or possibly not at all if the IndexWriter.rollback() 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.

        Specified by:
        onCommit in class IndexDeletionPolicy
        Parameters:
        commits - List of IndexCommit, sorted by age (the 0th one is the oldest commit).
        Throws:
        IOException

      • onInit

        public void onInit​(List<? extends IndexCommit> commits)
            throws IOException
        Description copied from class: 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 delete() of IndexCommit.

        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.

        Specified by:
        onInit in class IndexDeletionPolicy
        Parameters:
        commits - List of current point-in-time commits, sorted by age (the 0th one is the oldest commit). Note that for a new index this method is invoked with an empty list.
        Throws:
        IOException

      • releaseGen

        protected void releaseGen​(long gen)
                   throws IOException
        Release a snapshot by generation.
        Throws:
        IOException

      • snapshot

        public IndexCommit snapshot()
                     throws IOException
        Snapshots the last commit and returns it. Once a commit is 'snapshotted,' it is protected from deletion (as long as this IndexDeletionPolicy is used). The snapshot can be removed by calling release(IndexCommit) followed by a call to IndexWriter.deleteUnusedFiles().

        NOTE: 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 forceMerge) then in the worst case this could consume an extra 1X of your total index size, until you release the snapshot.

        Returns:
        the IndexCommit that was snapshotted.
        Throws:
        IllegalStateException - if this index does not have any commits yet
        IOException

      • getSnapshots

        public List<IndexCommit> getSnapshots()
        Returns all IndexCommits held by at least one snapshot.

      • getSnapshotCount

        public int getSnapshotCount()
        Returns the total number of snapshots currently held.

      • getIndexCommit

        public IndexCommit getIndexCommit​(long gen)
        Retrieve an IndexCommit from its generation; returns null if this IndexCommit is not currently snapshotted