org.apache.lucene/lucene-core/8.7.0 : org/apache/lucene/codecs/lucene86/package-info.java

org.apache.lucene.codecs.lucene86
https://lucene.apache.org/lucene-parent/lucene-core: Apache Lucene Java Core (The Apache Software Foundation)
Apache License, Version 2.0
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Lucene 8.6 file format.
Apache Lucene - Index File Formats


Introduction
Definitions
  
  Inverted Indexing
  Types of Fields
  Segments
  Document Numbers
  

Index Structure Overview
File Naming
Summary of File Extensions
  
  Lock File
  History
  Limitations
  




Introduction

This document defines the index file formats used in this version of Lucene.
If you are using a different version of Lucene, please consult the copy of
docs/ that was distributed with
the version you are using.
This document attempts to provide a high-level definition of the Apache
Lucene file formats.


Definitions

The fundamental concepts in Lucene are index, document, field and term.
An index contains a sequence of documents.

A document is a sequence of fields.
A field is a named sequence of terms.
A term is a sequence of bytes.

The same sequence of bytes in two different fields is considered a different
term. Thus terms are represented as a pair: the string naming the field, and the
bytes within the field.

Inverted Indexing
The index stores statistics about terms in order to make term-based search
more efficient. Lucene's index falls into the family of indexes known as an
inverted index. This is because it can list, for a term, the documents
that contain it. This is the inverse of the natural relationship, in which
documents list terms.

Types of Fields
In Lucene, fields may be stored, in which case their text is stored
in the index literally, in a non-inverted manner. Fields that are inverted are
called indexed. A field may be both stored and indexed.
The text of a field may be tokenized into terms to be indexed, or the
text of a field may be used literally as a term to be indexed. Most fields are
tokenized, but sometimes it is useful for certain identifier fields to be
indexed literally.
See the Field java docs for more information on Fields.

Segments
Lucene indexes may be composed of multiple sub-indexes, or segments.
Each segment is a fully independent index, which could be searched separately.
Indexes evolve by:

Creating new segments for newly added documents.
Merging existing segments.

Searches may involve multiple segments and/or multiple indexes, each index
potentially composed of a set of segments.

Document Numbers
Internally, Lucene refers to documents by an integer document number.
The first document added to an index is numbered zero, and each subsequent
document added gets a number one greater than the previous.
Note that a document's number may change, so caution should be taken when
storing these numbers outside of Lucene. In particular, numbers may change in
the following situations:


The numbers stored in each segment are unique only within the segment, and
must be converted before they can be used in a larger context. The standard
technique is to allocate each segment a range of values, based on the range of
numbers used in that segment. To convert a document number from a segment to an
external value, the segment's base document number is added. To convert
an external value back to a segment-specific value, the segment is identified
by the range that the external value is in, and the segment's base value is
subtracted. For example two five document segments might be combined, so that
the first segment has a base value of zero, and the second of five. Document
three from the second segment would have an external value of eight.


When documents are deleted, gaps are created in the numbering. These are
eventually removed as the index evolves through merging. Deleted documents are
dropped when segments are merged. A freshly-merged segment thus has no gaps in
its numbering.




Index Structure Overview

Each segment index maintains the following:

 Segment info. This contains metadata about a segment, such as the number of documents, what files it uses, and information about how the segment is sorted 
 Field names. This contains the set of field names used in the index. 

Stored Field values.
This contains, for each document, a list of attribute-value pairs, where the attributes
are field names. These are used to store auxiliary information about the document, such as
its title, url, or an identifier to access a database. The set of stored fields are what is
returned for each hit when searching. This is keyed by document number.

 Term dictionary. A dictionary containing all of the terms used in all of the indexed fields of all of the documents. The dictionary also contains the number of documents which contain the term, and pointers to the term's frequency and proximity data. 
 Term Frequency data. For each term in the dictionary, the numbers of all the documents that contain that term, and the frequency of the term in that document, unless frequencies are omitted (IndexOptions.DOCS) 
 Term Proximity data. For each term in the dictionary, the positions that the term occurs in each document. Note that this will not exist if all fields in all documents omit position data. 
 Normalization factors. For each field in each document, a value is stored that is multiplied into the score for hits on that field. 
 Term Vectors. For each field in each document, the term vector (sometimes called document vector) may be stored. A term vector consists of term text and term frequency. To add Term Vectors to your index see the Field constructors 
 Per-document values. Like stored values, these are also keyed by document number, but are generally intended to be loaded into main memory for fast access. Whereas stored values are generally intended for summary results from searches, per-document values are useful for things like scoring factors. 
 Live documents. An optional file indicating which documents are live. 
 Point values. Optional pair of files, recording dimensionally indexed fields, to enable fast numeric range filtering and large numeric values like BigInteger and BigDecimal (1D) and geographic shape intersection (2D, 3D). 

Details on each of these are provided in their linked pages.


File Naming

All files belonging to a segment have the same name with varying extensions.
The extensions correspond to the different file formats described below. When
using the Compound File format (default for small segments) these files (except
for the Segment info file, the Lock file, and Deleted documents file) are collapsed
into a single .cfs file (see below for details)
Typically, all segments in an index are stored in a single directory,
although this is not required.
File names are never re-used. That is, when any file is saved
to the Directory it is given a never before used filename. This is achieved
using a simple generations approach. For example, the first segments file is
segments_1, then segments_2, etc. The generation is a sequential long integer
represented in alpha-numeric (base 36) form.


Summary of File Extensions

The following table summarizes the names and extensions of the files in
Lucene:

lucene filenames by extension

Name
Extension
Brief Description


Segments File
segments_N
Stores information about a commit point


Lock File
write.lock
The Write lock prevents multiple IndexWriters from writing to the same
file.


Segment Info
.si
Stores metadata about a segment


Compound File
.cfs, .cfe
An optional "virtual" file consisting of all the other index files for
systems that frequently run out of file handles.


Fields
.fnm
Stores information about the fields


Field Index
.fdx
Contains pointers to field data


Field Data
.fdt
The stored fields for documents


Term Dictionary
.tim
The term dictionary, stores term info


Term Index
.tip
The index into the Term Dictionary


Frequencies
.doc
Contains the list of docs which contain each term along with frequency


Positions
.pos
Stores position information about where a term occurs in the index


Payloads
.pay
Stores additional per-position metadata information such as character offsets and user payloads


Norms
.nvd, .nvm
Encodes length and boost factors for docs and fields


Per-Document Values
.dvd, .dvm
Encodes additional scoring factors or other per-document information.


Term Vector Index
.tvx
Stores offset into the document data file


Term Vector Data
.tvd
Contains term vector data.


Live Documents
.liv
Info about what documents are live


Point values
.dii, .dim
Holds indexed points, if any




Lock File
The write lock, which is stored in the index directory by default, is named
"write.lock". If the lock directory is different from the index directory then
the write lock will be named "XXXX-write.lock" where XXXX is a unique prefix
derived from the full path to the index directory. When this file is present, a
writer is currently modifying the index (adding or removing documents). This
lock file ensures that only one writer is modifying the index at a time.

History
Compatibility notes are provided in this document, describing how file
formats have changed from prior versions:

In version 2.1, the file format was changed to allow lock-less commits (ie,
no more commit lock). The change is fully backwards compatible: you can open a
pre-2.1 index for searching or adding/deleting of docs. When the new segments
file is saved (committed), it will be written in the new file format (meaning
no specific "upgrade" process is needed). But note that once a commit has
occurred, pre-2.1 Lucene will not be able to read the index.
In version 2.3, the file format was changed to allow segments to share a
single set of doc store (vectors & stored fields) files. This allows for
faster indexing in certain cases. The change is fully backwards compatible (in
the same way as the lock-less commits change in 2.1).
In version 2.4, Strings are now written as true UTF-8 byte sequence, not
Java's modified UTF-8. See 
LUCENE-510 for details.
In version 2.9, an optional opaque Map<String,String> CommitUserData
may be passed to IndexWriter's commit methods (and later retrieved), which is
recorded in the segments_N file. See 
LUCENE-1382 for details. Also,
diagnostics were added to each segment written recording details about why it
was written (due to flush, merge; which OS/JRE was used; etc.). See issue
LUCENE-1654 for details.
In version 3.0, compressed fields are no longer written to the index (they
can still be read, but on merge the new segment will write them, uncompressed).
See issue LUCENE-1960
for details.
In version 3.1, segments records the code version that created them. See
LUCENE-2720 for details.
Additionally segments track explicitly whether or not they have term vectors.
See LUCENE-2811
for details.
In version 3.2, numeric fields are written as natively to stored fields
file, previously they were stored in text format only.
In version 3.4, fields can omit position data while still indexing term
frequencies.
In version 4.0, the format of the inverted index became extensible via the Codec api. Fast per-document storage (DocValues) was introduced. Normalization factors need no longer be a single byte, they can be any NumericDocValues. Terms need not be unicode strings, they can be any byte sequence. Term offsets can optionally be indexed into the postings lists. Payloads can be stored in the term vectors.
In version 4.1, the format of the postings list changed to use either
of FOR compression or variable-byte encoding, depending upon the frequency
of the term. Terms appearing only once were changed to inline directly into
the term dictionary. Stored fields are compressed by default. 
In version 4.2, term vectors are compressed by default. DocValues has
a new multi-valued type (SortedSet), that can be used for faceting/grouping/joining
on multi-valued fields.
In version 4.5, DocValues were extended to explicitly represent missing values.
In version 4.6, FieldInfos were extended to support per-field DocValues generation, to
allow updating NumericDocValues fields.
In version 4.8, checksum footers were added to the end of each index file
for improved data integrity. Specifically, the last 8 bytes of every index file
contain the zlib-crc32 checksum of the file.
In version 4.9, DocValues has a new multi-valued numeric type (SortedNumeric)
that is suitable for faceting/sorting/analytics.
In version 5.4, DocValues have been improved to store more information on disk:
addresses for binary fields and ord indexes for multi-valued fields.
In version 6.0, Points were added, for multi-dimensional range/distance search.
In version 6.2, new Segment info format that reads/writes the index sort, to support index sorting.
In version 7.0, DocValues have been improved to better support sparse doc values
thanks to an iterator API.
In version 8.0, postings have been enhanced to record, for each block of
doc ids, the (term freq, normalization factor) pairs that may trigger the
maximum score of the block. This information is recorded alongside skip data
in order to be able to skip blocks of doc ids if they may not produce high
enough scores.
Additionally doc values and norms has been extended with jump-tables to make access O(1)
instead of O(n), where n is the number of elements to skip when advancing in the data.
In version 8.4, postings, positions, offsets and payload lengths have move to a more
performant encoding that is vectorized.
In version 8.6, index sort serialization is delegated to the sorts themselves, to
allow user-defined sorts to be used


Limitations

Lucene uses a Java int to refer to
document numbers, and the index file format uses an Int32
on-disk to store document numbers. This is a limitation
of both the index file format and the current implementation. Eventually these
should be replaced with either UInt64 values, or better yet, VInt values which have no limit.

/**
 * Lucene 8.6 file format.
 *
 * <h2>Apache Lucene - Index File Formats</h2>
 * <div>
 * <ul>
 * <li><a href="#Introduction">Introduction</a></li>
 * <li><a href="#Definitions">Definitions</a>
 *   <ul>
 *   <li><a href="#Inverted_Indexing">Inverted Indexing</a></li>
 *   <li><a href="#Types_of_Fields">Types of Fields</a></li>
 *   <li><a href="#Segments">Segments</a></li>
 *   <li><a href="#Document_Numbers">Document Numbers</a></li>
 *   </ul>
 * </li>
 * <li><a href="#Overview">Index Structure Overview</a></li>
 * <li><a href="#File_Naming">File Naming</a></li>
 * <li><a href="#file-names">Summary of File Extensions</a>
 *   <ul>
 *   <li><a href="#Lock_File">Lock File</a></li>
 *   <li><a href="#History">History</a></li>
 *   <li><a href="#Limitations">Limitations</a></li>
 *   </ul>
 * </li>
 * </ul>
 * </div>
 * <a name="Introduction"></a>
 * <h3>Introduction</h3>
 * <div>
 * <p>This document defines the index file formats used in this version of Lucene.
 * If you are using a different version of Lucene, please consult the copy of
 * <code>docs/</code> that was distributed with
 * the version you are using.</p>
 * <p>This document attempts to provide a high-level definition of the Apache
 * Lucene file formats.</p>
 * </div>
 * <a name="Definitions"></a>
 * <h3>Definitions</h3>
 * <div>
 * <p>The fundamental concepts in Lucene are index, document, field and term.</p>
 * <p>An index contains a sequence of documents.</p>
 * <ul>
 * <li>A document is a sequence of fields.</li>
 * <li>A field is a named sequence of terms.</li>
 * <li>A term is a sequence of bytes.</li>
 * </ul>
 * <p>The same sequence of bytes in two different fields is considered a different
 * term. Thus terms are represented as a pair: the string naming the field, and the
 * bytes within the field.</p>
 * <a name="Inverted_Indexing"></a>
 * <h4>Inverted Indexing</h4>
 * <p>The index stores statistics about terms in order to make term-based search
 * more efficient. Lucene's index falls into the family of indexes known as an
 * <i>inverted index.</i> This is because it can list, for a term, the documents
 * that contain it. This is the inverse of the natural relationship, in which
 * documents list terms.</p>
 * <a name="Types_of_Fields"></a>
 * <h4>Types of Fields</h4>
 * <p>In Lucene, fields may be <i>stored</i>, in which case their text is stored
 * in the index literally, in a non-inverted manner. Fields that are inverted are
 * called <i>indexed</i>. A field may be both stored and indexed.</p>
 * <p>The text of a field may be <i>tokenized</i> into terms to be indexed, or the
 * text of a field may be used literally as a term to be indexed. Most fields are
 * tokenized, but sometimes it is useful for certain identifier fields to be
 * indexed literally.</p>
 * <p>See the {@link org.apache.lucene.document.Field Field}
 * java docs for more information on Fields.</p>
 * <a name="Segments"></a>
 * <h4>Segments</h4>
 * <p>Lucene indexes may be composed of multiple sub-indexes, or <i>segments</i>.
 * Each segment is a fully independent index, which could be searched separately.
 * Indexes evolve by:</p>
 * <ol>
 * <li>Creating new segments for newly added documents.</li>
 * <li>Merging existing segments.</li>
 * </ol>
 * <p>Searches may involve multiple segments and/or multiple indexes, each index
 * potentially composed of a set of segments.</p>
 * <a name="Document_Numbers"></a>
 * <h4>Document Numbers</h4>
 * <p>Internally, Lucene refers to documents by an integer <i>document number</i>.
 * The first document added to an index is numbered zero, and each subsequent
 * document added gets a number one greater than the previous.</p>
 * <p>Note that a document's number may change, so caution should be taken when
 * storing these numbers outside of Lucene. In particular, numbers may change in
 * the following situations:</p>
 * <ul>
 * <li>
 * <p>The numbers stored in each segment are unique only within the segment, and
 * must be converted before they can be used in a larger context. The standard
 * technique is to allocate each segment a range of values, based on the range of
 * numbers used in that segment. To convert a document number from a segment to an
 * external value, the segment's <i>base</i> document number is added. To convert
 * an external value back to a segment-specific value, the segment is identified
 * by the range that the external value is in, and the segment's base value is
 * subtracted. For example two five document segments might be combined, so that
 * the first segment has a base value of zero, and the second of five. Document
 * three from the second segment would have an external value of eight.</p>
 * </li>
 * <li>
 * <p>When documents are deleted, gaps are created in the numbering. These are
 * eventually removed as the index evolves through merging. Deleted documents are
 * dropped when segments are merged. A freshly-merged segment thus has no gaps in
 * its numbering.</p>
 * </li>
 * </ul>
 * </div>
 * <a name="Overview"></a>
 * <h3>Index Structure Overview</h3>
 * <div>
 * <p>Each segment index maintains the following:</p>
 * <ul>
 * <li>
 * {@link org.apache.lucene.codecs.lucene86.Lucene86SegmentInfoFormat Segment info}.
 *    This contains metadata about a segment, such as the number of documents,
 *    what files it uses, and information about how the segment is sorted
 * </li>
 * <li>
 * {@link org.apache.lucene.codecs.lucene60.Lucene60FieldInfosFormat Field names}.
 *    This contains the set of field names used in the index.
 * </li>
 * <li>
 * Stored Field values.
 * This contains, for each document, a list of attribute-value pairs, where the attributes
 * are field names. These are used to store auxiliary information about the document, such as
 * its title, url, or an identifier to access a database. The set of stored fields are what is
 * returned for each hit when searching. This is keyed by document number.
 * </li>
 * <li>
 * {@link org.apache.lucene.codecs.lucene84.Lucene84PostingsFormat Term dictionary}.
 * A dictionary containing all of the terms used in all of the
 * indexed fields of all of the documents. The dictionary also contains the number
 * of documents which contain the term, and pointers to the term's frequency and
 * proximity data.
 * </li>
 * <li>
 * {@link org.apache.lucene.codecs.lucene84.Lucene84PostingsFormat Term Frequency data}.
 * For each term in the dictionary, the numbers of all the
 * documents that contain that term, and the frequency of the term in that
 * document, unless frequencies are omitted ({@link org.apache.lucene.index.IndexOptions#DOCS IndexOptions.DOCS})
 * </li>
 * <li>
 * {@link org.apache.lucene.codecs.lucene84.Lucene84PostingsFormat Term Proximity data}.
 * For each term in the dictionary, the positions that the
 * term occurs in each document. Note that this will not exist if all fields in
 * all documents omit position data.
 * </li>
 * <li>
 * {@link org.apache.lucene.codecs.lucene80.Lucene80NormsFormat Normalization factors}.
 * For each field in each document, a value is stored
 * that is multiplied into the score for hits on that field.
 * </li>
 * <li>
 * {@link org.apache.lucene.codecs.lucene50.Lucene50TermVectorsFormat Term Vectors}.
 * For each field in each document, the term vector (sometimes
 * called document vector) may be stored. A term vector consists of term text and
 * term frequency. To add Term Vectors to your index see the
 * {@link org.apache.lucene.document.Field Field} constructors
 * </li>
 * <li>
 * {@link org.apache.lucene.codecs.lucene80.Lucene80DocValuesFormat Per-document values}.
 * Like stored values, these are also keyed by document
 * number, but are generally intended to be loaded into main memory for fast
 * access. Whereas stored values are generally intended for summary results from
 * searches, per-document values are useful for things like scoring factors.
 * </li>
 * <li>
 * {@link org.apache.lucene.codecs.lucene50.Lucene50LiveDocsFormat Live documents}.
 * An optional file indicating which documents are live.
 * </li>
 * <li>
 * {@link org.apache.lucene.codecs.lucene86.Lucene86PointsFormat Point values}.
 * Optional pair of files, recording dimensionally indexed fields, to enable fast
 * numeric range filtering and large numeric values like BigInteger and BigDecimal (1D)
 * and geographic shape intersection (2D, 3D).
 * </li>
 * </ul>
 * <p>Details on each of these are provided in their linked pages.</p>
 * </div>
 * <a name="File_Naming"></a>
 * <h3>File Naming</h3>
 * <div>
 * <p>All files belonging to a segment have the same name with varying extensions.
 * The extensions correspond to the different file formats described below. When
 * using the Compound File format (default for small segments) these files (except
 * for the Segment info file, the Lock file, and Deleted documents file) are collapsed
 * into a single .cfs file (see below for details)</p>
 * <p>Typically, all segments in an index are stored in a single directory,
 * although this is not required.</p>
 * <p>File names are never re-used. That is, when any file is saved
 * to the Directory it is given a never before used filename. This is achieved
 * using a simple generations approach. For example, the first segments file is
 * segments_1, then segments_2, etc. The generation is a sequential long integer
 * represented in alpha-numeric (base 36) form.</p>
 * </div>
 * <a name="file-names"></a>
 * <h3>Summary of File Extensions</h3>
 * <div>
 * <p>The following table summarizes the names and extensions of the files in
 * Lucene:</p>
 * <table class="padding4" style="border-spacing: 1px; border-collapse: separate">
 * <caption>lucene filenames by extension</caption>
 * <tr>
 * <th>Name</th>
 * <th>Extension</th>
 * <th>Brief Description</th>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.index.SegmentInfos Segments File}</td>
 * <td>segments_N</td>
 * <td>Stores information about a commit point</td>
 * </tr>
 * <tr>
 * <td><a href="#Lock_File">Lock File</a></td>
 * <td>write.lock</td>
 * <td>The Write lock prevents multiple IndexWriters from writing to the same
 * file.</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene86.Lucene86SegmentInfoFormat Segment Info}</td>
 * <td>.si</td>
 * <td>Stores metadata about a segment</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene50.Lucene50CompoundFormat Compound File}</td>
 * <td>.cfs, .cfe</td>
 * <td>An optional "virtual" file consisting of all the other index files for
 * systems that frequently run out of file handles.</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene60.Lucene60FieldInfosFormat Fields}</td>
 * <td>.fnm</td>
 * <td>Stores information about the fields</td>
 * </tr>
 * <tr>
 * <td>Field Index</td>
 * <td>.fdx</td>
 * <td>Contains pointers to field data</td>
 * </tr>
 * <tr>
 * <td>Field Data</td>
 * <td>.fdt</td>
 * <td>The stored fields for documents</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene84.Lucene84PostingsFormat Term Dictionary}</td>
 * <td>.tim</td>
 * <td>The term dictionary, stores term info</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene84.Lucene84PostingsFormat Term Index}</td>
 * <td>.tip</td>
 * <td>The index into the Term Dictionary</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene84.Lucene84PostingsFormat Frequencies}</td>
 * <td>.doc</td>
 * <td>Contains the list of docs which contain each term along with frequency</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene84.Lucene84PostingsFormat Positions}</td>
 * <td>.pos</td>
 * <td>Stores position information about where a term occurs in the index</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene84.Lucene84PostingsFormat Payloads}</td>
 * <td>.pay</td>
 * <td>Stores additional per-position metadata information such as character offsets and user payloads</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene80.Lucene80NormsFormat Norms}</td>
 * <td>.nvd, .nvm</td>
 * <td>Encodes length and boost factors for docs and fields</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene80.Lucene80DocValuesFormat Per-Document Values}</td>
 * <td>.dvd, .dvm</td>
 * <td>Encodes additional scoring factors or other per-document information.</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene50.Lucene50TermVectorsFormat Term Vector Index}</td>
 * <td>.tvx</td>
 * <td>Stores offset into the document data file</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene50.Lucene50TermVectorsFormat Term Vector Data}</td>
 * <td>.tvd</td>
 * <td>Contains term vector data.</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene50.Lucene50LiveDocsFormat Live Documents}</td>
 * <td>.liv</td>
 * <td>Info about what documents are live</td>
 * </tr>
 * <tr>
 * <td>{@link org.apache.lucene.codecs.lucene86.Lucene86PointsFormat Point values}</td>
 * <td>.dii, .dim</td>
 * <td>Holds indexed points, if any</td>
 * </tr>
 * </table>
 * </div>
 * <a name="Lock_File"></a>
 * <h3>Lock File</h3>
 * The write lock, which is stored in the index directory by default, is named
 * "write.lock". If the lock directory is different from the index directory then
 * the write lock will be named "XXXX-write.lock" where XXXX is a unique prefix
 * derived from the full path to the index directory. When this file is present, a
 * writer is currently modifying the index (adding or removing documents). This
 * lock file ensures that only one writer is modifying the index at a time.
 * <a name="History"></a>
 * <h3>History</h3>
 * <p>Compatibility notes are provided in this document, describing how file
 * formats have changed from prior versions:</p>
 * <ul>
 * <li>In version 2.1, the file format was changed to allow lock-less commits (ie,
 * no more commit lock). The change is fully backwards compatible: you can open a
 * pre-2.1 index for searching or adding/deleting of docs. When the new segments
 * file is saved (committed), it will be written in the new file format (meaning
 * no specific "upgrade" process is needed). But note that once a commit has
 * occurred, pre-2.1 Lucene will not be able to read the index.</li>
 * <li>In version 2.3, the file format was changed to allow segments to share a
 * single set of doc store (vectors &amp; stored fields) files. This allows for
 * faster indexing in certain cases. The change is fully backwards compatible (in
 * the same way as the lock-less commits change in 2.1).</li>
 * <li>In version 2.4, Strings are now written as true UTF-8 byte sequence, not
 * Java's modified UTF-8. See <a href="http://issues.apache.org/jira/browse/LUCENE-510">
 * LUCENE-510</a> for details.</li>
 * <li>In version 2.9, an optional opaque Map&lt;String,String&gt; CommitUserData
 * may be passed to IndexWriter's commit methods (and later retrieved), which is
 * recorded in the segments_N file. See <a href="http://issues.apache.org/jira/browse/LUCENE-1382">
 * LUCENE-1382</a> for details. Also,
 * diagnostics were added to each segment written recording details about why it
 * was written (due to flush, merge; which OS/JRE was used; etc.). See issue
 * <a href="http://issues.apache.org/jira/browse/LUCENE-1654">LUCENE-1654</a> for details.</li>
 * <li>In version 3.0, compressed fields are no longer written to the index (they
 * can still be read, but on merge the new segment will write them, uncompressed).
 * See issue <a href="http://issues.apache.org/jira/browse/LUCENE-1960">LUCENE-1960</a>
 * for details.</li>
 * <li>In version 3.1, segments records the code version that created them. See
 * <a href="http://issues.apache.org/jira/browse/LUCENE-2720">LUCENE-2720</a> for details.
 * Additionally segments track explicitly whether or not they have term vectors.
 * See <a href="http://issues.apache.org/jira/browse/LUCENE-2811">LUCENE-2811</a>
 * for details.</li>
 * <li>In version 3.2, numeric fields are written as natively to stored fields
 * file, previously they were stored in text format only.</li>
 * <li>In version 3.4, fields can omit position data while still indexing term
 * frequencies.</li>
 * <li>In version 4.0, the format of the inverted index became extensible via
 * the {@link org.apache.lucene.codecs.Codec Codec} api. Fast per-document storage
 * ({@code DocValues}) was introduced. Normalization factors need no longer be a
 * single byte, they can be any {@link org.apache.lucene.index.NumericDocValues NumericDocValues}.
 * Terms need not be unicode strings, they can be any byte sequence. Term offsets
 * can optionally be indexed into the postings lists. Payloads can be stored in the
 * term vectors.</li>
 * <li>In version 4.1, the format of the postings list changed to use either
 * of FOR compression or variable-byte encoding, depending upon the frequency
 * of the term. Terms appearing only once were changed to inline directly into
 * the term dictionary. Stored fields are compressed by default. </li>
 * <li>In version 4.2, term vectors are compressed by default. DocValues has
 * a new multi-valued type (SortedSet), that can be used for faceting/grouping/joining
 * on multi-valued fields.</li>
 * <li>In version 4.5, DocValues were extended to explicitly represent missing values.</li>
 * <li>In version 4.6, FieldInfos were extended to support per-field DocValues generation, to
 * allow updating NumericDocValues fields.</li>
 * <li>In version 4.8, checksum footers were added to the end of each index file
 * for improved data integrity. Specifically, the last 8 bytes of every index file
 * contain the zlib-crc32 checksum of the file.</li>
 * <li>In version 4.9, DocValues has a new multi-valued numeric type (SortedNumeric)
 * that is suitable for faceting/sorting/analytics.
 * <li>In version 5.4, DocValues have been improved to store more information on disk:
 * addresses for binary fields and ord indexes for multi-valued fields.
 * <li>In version 6.0, Points were added, for multi-dimensional range/distance search.
 * <li>In version 6.2, new Segment info format that reads/writes the index sort, to support index sorting.
 * <li>In version 7.0, DocValues have been improved to better support sparse doc values
 * thanks to an iterator API.</li>
 * <li>In version 8.0, postings have been enhanced to record, for each block of
 * doc ids, the (term freq, normalization factor) pairs that may trigger the
 * maximum score of the block. This information is recorded alongside skip data
 * in order to be able to skip blocks of doc ids if they may not produce high
 * enough scores.
 * Additionally doc values and norms has been extended with jump-tables to make access O(1)
 * instead of O(n), where n is the number of elements to skip when advancing in the data.</li>
 * <li>In version 8.4, postings, positions, offsets and payload lengths have move to a more
 * performant encoding that is vectorized.</li>
 * <li>In version 8.6, index sort serialization is delegated to the sorts themselves, to
 * allow user-defined sorts to be used</li>
 * </ul>
 * <a name="Limitations"></a>
 * <h3>Limitations</h3>
 * <div>
 * <p>Lucene uses a Java <code>int</code> to refer to
 * document numbers, and the index file format uses an <code>Int32</code>
 * on-disk to store document numbers. This is a limitation
 * of both the index file format and the current implementation. Eventually these
 * should be replaced with either <code>UInt64</code> values, or
 * better yet, {@link org.apache.lucene.store.DataOutput#writeVInt VInt} values which have no limit.</p>
 * </div>
 */
package org.apache.lucene.codecs.lucene86;
Name	Extension	Brief Description
`Segments File`	segments_N	Stores information about a commit point
Lock File	write.lock	The Write lock prevents multiple IndexWriters from writing to the same file.
`Segment Info`	.si	Stores metadata about a segment
`Compound File`	.cfs, .cfe	An optional "virtual" file consisting of all the other index files for systems that frequently run out of file handles.
`Fields`	.fnm	Stores information about the fields
Field Index	.fdx	Contains pointers to field data
Field Data	.fdt	The stored fields for documents
`Term Dictionary`	.tim	The term dictionary, stores term info
`Term Index`	.tip	The index into the Term Dictionary
`Frequencies`	.doc	Contains the list of docs which contain each term along with frequency
`Positions`	.pos	Stores position information about where a term occurs in the index
`Payloads`	.pay	Stores additional per-position metadata information such as character offsets and user payloads
`Norms`	.nvd, .nvm	Encodes length and boost factors for docs and fields
`Per-Document Values`	.dvd, .dvm	Encodes additional scoring factors or other per-document information.
`Term Vector Index`	.tvx	Stores offset into the document data file
`Term Vector Data`	.tvd	Contains term vector data.
`Live Documents`	.liv	Info about what documents are live
`Point values`	.dii, .dim	Holds indexed points, if any
/

org.apache.lucene/ lucene-core/ 8.7.0/ org/apache/lucene/codecs/lucene86/package-info.java

Apache Lucene - Index File Formats

Introduction

Definitions

Inverted Indexing

Types of Fields

Segments

Document Numbers

Index Structure Overview

File Naming

Summary of File Extensions

Lock File

History

Limitations
