org.apache.lucene.codecs.lucene87

Class Lucene87StoredFieldsFormat

java.lang.Object

org.apache.lucene.codecs.StoredFieldsFormat

org.apache.lucene.codecs.lucene87.Lucene87StoredFieldsFormat

public class Lucene87StoredFieldsFormat
extends StoredFieldsFormat

Lucene 8.7 stored fields format.

Principle

This StoredFieldsFormat compresses blocks of documents in order to improve the compression ratio compared to document-level compression. It uses the LZ4 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 (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 (BEST_COMPRESSION), which uses the DEFLATE algorithm with 48kB blocks and shared dictionaries for a better ratio at the expense of slower performance. These two options can be configured like this:

   // the default: for high performance
   indexWriterConfig.setCodec(new Lucene87Codec(Mode.BEST_SPEED));
   // instead for higher performance (but slower):
   // indexWriterConfig.setCodec(new Lucene87Codec(Mode.BEST_COMPRESSION));
 

File formats

Stored fields are represented by three files:

1. A fields data file (extension .fdt). 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 byte[] 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 LZ4 compression format.

   Notes

   • 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 StoredFieldVisitors 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.
   • 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.
   • In case documents are incompressible, the overhead of the compression format is less than 0.5%.

2. A fields index file (extension .fdx). This file stores two 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.

3. A fields meta file (extension .fdm). This file stores metadata about the monotonic arrays stored in the index file.

Known limitations

This StoredFieldsFormat does not support individual documents larger than (231 - 214) bytes.

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

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	Lucene87StoredFieldsFormat.Mode Configuration option for stored fields.

Field Summary

Fields

Modifier and Type	Field and Description
static CompressionMode	BEST_COMPRESSION_MODE Compression mode for Lucene87StoredFieldsFormat.Mode.BEST_COMPRESSION
static CompressionMode	BEST_SPEED_MODE Compression mode for Lucene87StoredFieldsFormat.Mode.BEST_SPEED
static String	MODE_KEY Attribute key for compression mode.

Constructor Summary

Constructors

Constructor and Description
Lucene87StoredFieldsFormat() Stored fields format with default options
Lucene87StoredFieldsFormat(Lucene87StoredFieldsFormat.Mode mode) Stored fields format with specified mode

Method Summary

Modifier and Type	Method and Description
StoredFieldsReader	fieldsReader(Directory directory, SegmentInfo si, FieldInfos fn, IOContext context) Returns a StoredFieldsReader to load stored fields.
StoredFieldsWriter	fieldsWriter(Directory directory, SegmentInfo si, IOContext context) Returns a StoredFieldsWriter to write stored fields.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

MODE_KEY

public static final String MODE_KEY

Attribute key for compression mode.

BEST_COMPRESSION_MODE

public static final CompressionMode BEST_COMPRESSION_MODE

Compression mode for Lucene87StoredFieldsFormat.Mode.BEST_COMPRESSION

BEST_SPEED_MODE

public static final CompressionMode BEST_SPEED_MODE

Compression mode for Lucene87StoredFieldsFormat.Mode.BEST_SPEED

Constructor Detail

Lucene87StoredFieldsFormat

public Lucene87StoredFieldsFormat()

Stored fields format with default options

Lucene87StoredFieldsFormat

public Lucene87StoredFieldsFormat(Lucene87StoredFieldsFormat.Mode mode)

Stored fields format with specified mode

Method Detail

fieldsReader

public StoredFieldsReader fieldsReader(Directory directory,
                                       SegmentInfo si,
                                       FieldInfos fn,
                                       IOContext context)
                                throws IOException

Description copied from class: StoredFieldsFormat

Returns a StoredFieldsReader to load stored fields.

Specified by:

fieldsReader in class StoredFieldsFormat

Throws:

IOException

fieldsWriter

public StoredFieldsWriter fieldsWriter(Directory directory,
                                       SegmentInfo si,
                                       IOContext context)
                                throws IOException

Description copied from class: StoredFieldsFormat

Returns a StoredFieldsWriter to write stored fields.

Specified by:

fieldsWriter in class StoredFieldsFormat

Throws:

IOException