org.apache.lucene.codecs.uniformsplit

Class UniformSplitTermsWriter

java.lang.Object

org.apache.lucene.codecs.FieldsConsumer

org.apache.lucene.codecs.uniformsplit.UniformSplitTermsWriter

All Implemented Interfaces:

Closeable, AutoCloseable

Direct Known Subclasses:

STUniformSplitTermsWriter

public class UniformSplitTermsWriter
extends FieldsConsumer

A block-based terms index and dictionary that assigns terms to nearly uniform length blocks. This technique is called Uniform Split.

The block construction is driven by two parameters, targetNumBlockLines and deltaNumLines. Each block size (number of terms) is targetNumBlockLines+-deltaNumLines. The algorithm computes the minimal distinguishing prefix (MDP) between each term and its previous term (alphabetically ordered). Then it selects in the neighborhood of the targetNumBlockLines, and within the deltaNumLines, the term with the minimal MDP. This term becomes the first term of the next block and its MDP is the block key. This block key is added to the terms dictionary trie.

We call dictionary the trie structure in memory, and block file the disk file containing the block lines, with one term and its corresponding term state details per line.

When seeking a term, the dictionary seeks the floor leaf of the trie for the searched term and jumps to the corresponding file pointer in the block file. There, the block terms are scanned for the exact searched term.

The terms inside a block do not need to share a prefix. Only the block key is used to find the block from the dictionary trie. And the block key is selected because it is the locally smallest MDP. This makes the dictionary trie very compact.

An interesting property of the Uniform Split technique is the very linear balance between memory usage and lookup performance. By decreasing the target block size, the block scan becomes faster, and since there are more blocks, the dictionary trie memory usage increases. Additionally, small blocks are faster to read from disk. A good sweet spot for the target block size is 32 with delta of 3 (10%) (default values). This can be tuned in the constructor.

There are additional optimizations:

• Each block has a header that allows the lookup to jump directly to the middle term with a fast comparison. This reduces the linear scan by 2 for a small disk size increase.
• Each block term is incrementally encoded according to its previous term. This both reduces the disk size and speeds up the block scan.
• All term line details (the terms states) are written after all terms. This allows faster term scan without needing to decode the term states.
• All file pointers are base-encoded. Their value is relative to the block base file pointer (not to the previous file pointer), this allows to read the term state of any term independently.

Blocks can be compressed or encrypted with an optional BlockEncoder provided in the constructor.

The block file contains all the term blocks for each field sequentially. It also contains the fields metadata at the end of the file.

The dictionary file contains the trie (FST bytes) for each field sequentially.

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

Field Summary

Fields

Modifier and Type	Field and Description
protected BlockEncoder	blockEncoder
protected IndexOutput	blockOutput
static int	DEFAULT_DELTA_NUM_LINES Default value for the maximum allowed delta variation of the block size (delta of the number of terms per block).
static int	DEFAULT_TARGET_NUM_BLOCK_LINES Default value for the target block size (number of terms per block).
protected int	deltaNumLines
protected IndexOutput	dictionaryOutput
protected FieldInfos	fieldInfos
protected FieldMetadata.Serializer	fieldMetadataWriter
protected static int	MAX_NUM_BLOCK_LINES Upper limit of the block size (maximum number of terms per block).
protected int	maxDoc
protected PostingsWriterBase	postingsWriter
protected int	targetNumBlockLines

Constructor Summary

Constructors

Modifier	Constructor and Description
	UniformSplitTermsWriter(PostingsWriterBase postingsWriter, SegmentWriteState state, BlockEncoder blockEncoder)
	UniformSplitTermsWriter(PostingsWriterBase postingsWriter, SegmentWriteState state, int targetNumBlockLines, int deltaNumLines, BlockEncoder blockEncoder)
protected	UniformSplitTermsWriter(PostingsWriterBase postingsWriter, SegmentWriteState state, int targetNumBlockLines, int deltaNumLines, BlockEncoder blockEncoder, FieldMetadata.Serializer fieldMetadataWriter, String codecName, int versionCurrent, String termsBlocksExtension, String dictionaryExtension)

Method Summary

Modifier and Type	Method and Description
void	close()
protected static void	validateSettings(int targetNumBlockLines, int deltaNumLines) Validates the constructor settings.
void	write(Fields fields, NormsProducer normsProducer)
protected void	writeDictionary(IndexDictionary.Builder dictionaryBuilder) Writes the dictionary index (FST) to disk.
protected void	writeEncodedFieldsMetadata(ByteBuffersDataOutput fieldsOutput)
protected void	writeFieldsMetadata(int fieldsNumber, ByteBuffersDataOutput fieldsOutput)
protected int	writeFieldTerms(BlockWriter blockWriter, DataOutput fieldsOutput, TermsEnum termsEnum, FieldInfo fieldInfo, NormsProducer normsProducer)
protected BlockTermState	writePostingLine(TermsEnum termsEnum, FieldMetadata fieldMetadata, NormsProducer normsProducer) Writes the posting values for the current term in the given TermsEnum and updates the FieldMetadata stats.
protected void	writeUnencodedFieldsMetadata(ByteBuffersDataOutput fieldsOutput)

Methods inherited from class org.apache.lucene.codecs.FieldsConsumer

merge

Methods inherited from class java.lang.Object

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

Field Detail

DEFAULT_TARGET_NUM_BLOCK_LINES

public static final int DEFAULT_TARGET_NUM_BLOCK_LINES

Default value for the target block size (number of terms per block).

See Also:

Constant Field Values

DEFAULT_DELTA_NUM_LINES

public static final int DEFAULT_DELTA_NUM_LINES

Default value for the maximum allowed delta variation of the block size (delta of the number of terms per block). The block size will be [target block size]+-[allowed delta].

See Also:

Constant Field Values

MAX_NUM_BLOCK_LINES

protected static final int MAX_NUM_BLOCK_LINES

Upper limit of the block size (maximum number of terms per block).

See Also:

Constant Field Values

fieldInfos

protected final FieldInfos fieldInfos

postingsWriter

protected final PostingsWriterBase postingsWriter

maxDoc

protected final int maxDoc

targetNumBlockLines

protected final int targetNumBlockLines

deltaNumLines

protected final int deltaNumLines

blockEncoder

protected final BlockEncoder blockEncoder

fieldMetadataWriter

protected final FieldMetadata.Serializer fieldMetadataWriter

blockOutput

protected final IndexOutput blockOutput

dictionaryOutput

protected final IndexOutput dictionaryOutput

Constructor Detail

UniformSplitTermsWriter

public UniformSplitTermsWriter(PostingsWriterBase postingsWriter,
                               SegmentWriteState state,
                               BlockEncoder blockEncoder)
                        throws IOException

Parameters:

blockEncoder - Optional block encoder, may be null if none. It can be used for compression or encryption.

Throws:

IOException

UniformSplitTermsWriter

public UniformSplitTermsWriter(PostingsWriterBase postingsWriter,
                               SegmentWriteState state,
                               int targetNumBlockLines,
                               int deltaNumLines,
                               BlockEncoder blockEncoder)
                        throws IOException

Parameters:

blockEncoder - Optional block encoder, may be null if none. It can be used for compression or encryption.

Throws:

IOException

UniformSplitTermsWriter

protected UniformSplitTermsWriter(PostingsWriterBase postingsWriter,
                                  SegmentWriteState state,
                                  int targetNumBlockLines,
                                  int deltaNumLines,
                                  BlockEncoder blockEncoder,
                                  FieldMetadata.Serializer fieldMetadataWriter,
                                  String codecName,
                                  int versionCurrent,
                                  String termsBlocksExtension,
                                  String dictionaryExtension)
                           throws IOException

Parameters:

targetNumBlockLines - Target number of lines per block. Must be strictly greater than 0. The parameters can be pre-validated with validateSettings(int, int). There is one term per block line, with its corresponding details (TermState).

deltaNumLines - Maximum allowed delta variation of the number of lines per block. Must be greater than or equal to 0 and strictly less than targetNumBlockLines. The block size will be targetNumBlockLines+-deltaNumLines. The block size must always be less than or equal to MAX_NUM_BLOCK_LINES.

blockEncoder - Optional block encoder, may be null if none. It can be used for compression or encryption.

Throws:

IOException

Method Detail

validateSettings

protected static void validateSettings(int targetNumBlockLines,
                                       int deltaNumLines)

Validates the constructor settings.

Parameters:

targetNumBlockLines - Target number of lines per block. Must be strictly greater than 0.

deltaNumLines - Maximum allowed delta variation of the number of lines per block. Must be greater than or equal to 0 and strictly less than targetNumBlockLines. Additionally, targetNumBlockLines + deltaNumLines must be less than or equal to MAX_NUM_BLOCK_LINES.

write

public void write(Fields fields,
                  NormsProducer normsProducer)
           throws IOException

Specified by:

write in class FieldsConsumer

Throws:

IOException

writeFieldsMetadata

protected void writeFieldsMetadata(int fieldsNumber,
                                   ByteBuffersDataOutput fieldsOutput)
                            throws IOException

Throws:

IOException

writeUnencodedFieldsMetadata

protected void writeUnencodedFieldsMetadata(ByteBuffersDataOutput fieldsOutput)
                                     throws IOException

Throws:

IOException

writeEncodedFieldsMetadata

protected void writeEncodedFieldsMetadata(ByteBuffersDataOutput fieldsOutput)
                                   throws IOException

Throws:

IOException

writeFieldTerms

protected int writeFieldTerms(BlockWriter blockWriter,
                              DataOutput fieldsOutput,
                              TermsEnum termsEnum,
                              FieldInfo fieldInfo,
                              NormsProducer normsProducer)
                       throws IOException

Returns:

1 if the field was written; 0 otherwise.

Throws:

IOException

writePostingLine

protected BlockTermState writePostingLine(TermsEnum termsEnum,
                                          FieldMetadata fieldMetadata,
                                          NormsProducer normsProducer)
                                   throws IOException

Writes the posting values for the current term in the given TermsEnum and updates the FieldMetadata stats.

Returns:

the written BlockTermState; or null if none.

Throws:

IOException

writeDictionary

protected void writeDictionary(IndexDictionary.Builder dictionaryBuilder)
                        throws IOException

Writes the dictionary index (FST) to disk.

Throws:

IOException

close

public void close()
           throws IOException

Specified by:

close in interface Closeable

Specified by:

close in interface AutoCloseable

Specified by:

close in class FieldsConsumer

Throws:

IOException