lucene/backward-codecs/src/java/org/apache/lucene/codecs/lucene50/Lucene50StoredFieldsFormat.java

/*
 * 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.
 */
package org.apache.lucene.codecs.lucene50;


import java.io.IOException;
import java.util.Objects;

import org.apache.lucene.codecs.StoredFieldsFormat;
import org.apache.lucene.codecs.StoredFieldsReader;
import org.apache.lucene.codecs.StoredFieldsWriter;
import org.apache.lucene.codecs.compressing.CompressingStoredFieldsFormat;
import org.apache.lucene.codecs.compressing.CompressionMode;
import org.apache.lucene.index.FieldInfos;
import org.apache.lucene.index.SegmentInfo;
import org.apache.lucene.index.StoredFieldVisitor;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.IOContext;
import org.apache.lucene.util.packed.DirectMonotonicWriter;

/**
 * Lucene 5.0 stored fields format.
 *
 * <p><b>Principle</b>
 * <p>This {@link StoredFieldsFormat} compresses blocks of documents in
 * order to improve the compression ratio compared to document-level
 * compression. It uses the <a href="http://code.google.com/p/lz4/">LZ4</a>
 * compression algorithm by default in 16KB blocks, which is fast to compress 
 * and very fast to decompress data. Although the default compression method 
 * that is used ({@link Mode#BEST_SPEED BEST_SPEED}) focuses more on speed than on 
 * compression ratio, it should provide interesting compression ratios
 * for redundant inputs (such as log files, HTML or plain text). For higher
 * compression, you can choose ({@link Mode#BEST_COMPRESSION BEST_COMPRESSION}), which uses 
 * the <a href="http://en.wikipedia.org/wiki/DEFLATE">DEFLATE</a> algorithm with 60KB blocks 
 * for a better ratio at the expense of slower performance. 
 * These two options can be configured like this:
 * <pre class="prettyprint">
 *   // the default: for high performance
 *   indexWriterConfig.setCodec(new Lucene54Codec(Mode.BEST_SPEED));
 *   // instead for higher performance (but slower):
 *   // indexWriterConfig.setCodec(new Lucene54Codec(Mode.BEST_COMPRESSION));
 * </pre>
 * <p><b>File formats</b>
 * <p>Stored fields are represented by three files:
 * <ol>
 * <li><a name="field_data"></a>
 * <p>A fields data file (extension <tt>.fdt</tt>). This file stores a compact
 * representation of documents in compressed blocks of 16KB or more. When
 * writing a segment, documents are appended to an in-memory <tt>byte[]</tt>
 * buffer. When its size reaches 16KB or more, some metadata about the documents
 * is flushed to disk, immediately followed by a compressed representation of
 * the buffer using the
 * <a href="https://github.com/lz4/lz4">LZ4</a>
 * <a href="http://fastcompression.blogspot.fr/2011/05/lz4-explained.html">compression format</a>.</p>
 * <p>Notes
 * <ul>
 * <li>If documents are larger than 16KB then chunks will likely contain only
 * one document. However, documents can never spread across several chunks (all
 * fields of a single document are in the same chunk).</li>
 * <li>When at least one document in a chunk is large enough so that the chunk
 * is larger than 32KB, the chunk will actually be compressed in several LZ4
 * blocks of 16KB. This allows {@link StoredFieldVisitor}s which are only
 * interested in the first fields of a document to not have to decompress 10MB
 * of data if the document is 10MB, but only 16KB.</li>
 * <li>Given that the original lengths are written in the metadata of the chunk,
 * the decompressor can leverage this information to stop decoding as soon as
 * enough data has been decompressed.</li>
 * <li>In case documents are incompressible, the overhead of the compression format
 * is less than 0.5%.</li>
 * </ul>
 * </li>
 * <li><a name="field_index"></a>
 * <p>A fields index file (extension <tt>.fdx</tt>). This file stores two
 * {@link DirectMonotonicWriter monotonic arrays}, one for the first doc IDs of
 * each block of compressed documents, and another one for the corresponding
 * offsets on disk. At search time, the array containing doc IDs is
 * binary-searched in order to find the block that contains the expected doc ID,
 * and the associated offset on disk is retrieved from the second array.</p>
 * <li><a name="field_meta"></a>
 * <p>A fields meta file (extension <tt>.fdm</tt>). This file stores metadata
 * about the monotonic arrays stored in the index file.</p>
 * </li>
 * </ol>
 * <p><b>Known limitations</b>
 * <p>This {@link StoredFieldsFormat} does not support individual documents
 * larger than (<tt>2<sup>31</sup> - 2<sup>14</sup></tt>) bytes.
 * @lucene.experimental
 */
public class Lucene50StoredFieldsFormat extends StoredFieldsFormat {
  
  /** Configuration option for stored fields. */
  public static enum Mode {
    /** Trade compression ratio for retrieval speed. */
    BEST_SPEED,
    /** Trade retrieval speed for compression ratio. */
    BEST_COMPRESSION
  }
  
  /** Attribute key for compression mode. */
  public static final String MODE_KEY = Lucene50StoredFieldsFormat.class.getSimpleName() + ".mode";
  
  final Mode mode;
  
  /** Stored fields format with default options */
  public Lucene50StoredFieldsFormat() {
    this(Mode.BEST_SPEED);
  }
  
  /** Stored fields format with specified mode */
  public Lucene50StoredFieldsFormat(Mode mode) {
    this.mode = Objects.requireNonNull(mode);
  }

  @Override
  public final StoredFieldsReader fieldsReader(Directory directory, SegmentInfo si, FieldInfos fn, IOContext context) throws IOException {
    String value = si.getAttribute(MODE_KEY);
    if (value == null) {
      throw new IllegalStateException("missing value for " + MODE_KEY + " for segment: " + si.name);
    }
    Mode mode = Mode.valueOf(value);
    return impl(mode).fieldsReader(directory, si, fn, context);
  }

  @Override
  public StoredFieldsWriter fieldsWriter(Directory directory, SegmentInfo si, IOContext context) throws IOException {
    throw new UnsupportedOperationException("Old codecs may only be used for reading");
  }
  
  StoredFieldsFormat impl(Mode mode) {
    switch (mode) {
      case BEST_SPEED: 
        return new CompressingStoredFieldsFormat("Lucene50StoredFieldsFastData", CompressionMode.FAST, 1 << 14, 128, 10);
      case BEST_COMPRESSION: 
        return new CompressingStoredFieldsFormat("Lucene50StoredFieldsHighData", CompressionMode.HIGH_COMPRESSION, 61440, 512, 10);
      default: throw new AssertionError();
    }
  }
}