Package 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 Details

    • 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:

    • 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:

    • 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:

    • 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 Details

    • 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 Details