Package org.apache.lucene.codecs.lucene42

Lucene 4.2 file format.


Class Summary
Lucene42Codec Implements the Lucene 4.2 index format, with configurable per-field postings and docvalues formats.
Lucene42DocValuesFormat Lucene 4.2 DocValues format.
Lucene42FieldInfosFormat Lucene 4.2 Field Infos format.
Lucene42NormsFormat Lucene 4.2 score normalization format.
Lucene42TermVectorsFormat Lucene 4.2 term vectors format.

Package org.apache.lucene.codecs.lucene42 Description

Lucene 4.2 file format.

Apache Lucene - Index File Formats


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.

Apache Lucene is written in Java, but several efforts are underway to write versions of Lucene in other programming languages. If these versions are to remain compatible with Apache Lucene, then a language-independent definition of the Lucene index format is required. This document thus attempts to provide a complete and independent definition of the Apache Lucene file formats.

As Lucene evolves, this document should evolve. Versions of Lucene in different programming languages should endeavor to agree on file formats, and generate new versions of this document.


The fundamental concepts in Lucene are index, document, field and term.

An index contains a sequence of documents.

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.


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:

  1. Creating new segments for newly added documents.
  2. 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:

Index Structure Overview

Each segment index maintains the following:

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 in 1.4 and greater) 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.

As of version 2.1 (lock-less commits), file names are never re-used (there is one exception, "segments.gen", see below). 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:

Name Extension Brief Description
Segments File segments.gen, 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 Documents .tvd Contains information about each document that has term vectors
Term Vector Fields .tvf The field level info about term vectors
Deleted Documents .del Info about what files are deleted

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.


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


When referring to term numbers, Lucene's current implementation uses a Java int to hold the term index, which means the maximum number of unique terms in any single index segment is ~2.1 billion times the term index interval (default 128) = ~274 billion. This is technically not a limitation of the index file format, just of Lucene's current implementation.

Similarly, 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.

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