Package org.apache.lucene.index.memory

Class MemoryIndex

java.lang.Object
org.apache.lucene.index.memory.MemoryIndex
public class MemoryIndex extends Object
High-performance single-document main memory Apache Lucene fulltext search index.

Overview

This class is a replacement/substitute for a large subset of RAMDirectory functionality. It is designed to enable maximum efficiency for on-the-fly matchmaking combining structured and fuzzy fulltext search in realtime streaming applications such as Nux XQuery based XML message queues, publish-subscribe systems for Blogs/newsfeeds, text chat, data acquisition and distribution systems, application level routers, firewalls, classifiers, etc. Rather than targeting fulltext search of infrequent queries over huge persistent data archives (historic search), this class targets fulltext search of huge numbers of queries over comparatively small transient realtime data (prospective search). For example as in 
 float score = search(String text, Query query)

Each instance can hold at most one Lucene "document", with a document containing zero or more "fields", each field having a name and a fulltext value. The fulltext value is tokenized (split and transformed) into zero or more index terms (aka words) on addField(), according to the policy implemented by an Analyzer. For example, Lucene analyzers can split on whitespace, normalize to lower case for case insensitivity, ignore common terms with little discriminatory value such as "he", "in", "and" (stop words), reduce the terms to their natural linguistic root form such as "fishing" being reduced to "fish" (stemming), resolve synonyms/inflexions/thesauri (upon indexing and/or querying), etc. For details, see Lucene Analyzer Intro.

Arbitrary Lucene queries can be run against this class - see Lucene Query Syntax as well as Query Parser Rules. Note that a Lucene query selects on the field names and associated (indexed) tokenized terms, not on the original fulltext(s) - the latter are not stored but rather thrown away immediately after tokenization.

For some interesting background information on search technology, see Bob Wyman's Prospective Search, Jim Gray's A Call to Arms - Custom subscriptions, and Tim Bray's On Search, the Series.

Example Usage

 Analyzer analyzer = new SimpleAnalyzer(version);
 MemoryIndex index = new MemoryIndex();
 index.addField("content", "Readings about Salmons and other select Alaska fishing Manuals", analyzer);
 index.addField("author", "Tales of James", analyzer);
 QueryParser parser = new QueryParser(version, "content", analyzer);
 float score = index.search(parser.parse("+author:james +salmon~ +fish* manual~"));
 if (score > 0.0f) {
     System.out.println("it's a match");
 } else {
     System.out.println("no match found");
 }
 System.out.println("indexData=" + index.toString());

Example XQuery Usage

 (: An XQuery that finds all books authored by James that have something to do with "salmon fishing manuals", sorted by relevance :)
 declare namespace lucene = "java:nux.xom.pool.FullTextUtil";
 declare variable $query := "+salmon~ +fish* manual~"; (: any arbitrary Lucene query can go here :)
 
 for $book in /books/book[author="James" and lucene:match(abstract, $query) > 0.0]
 let $score := lucene:match($book/abstract, $query)
 order by $score descending
 return $book

No thread safety guarantees

An instance can be queried multiple times with the same or different queries, but an instance is not thread-safe. If desired use idioms such as: 
 MemoryIndex index = ...
 synchronized (index) {
    // read and/or write index (i.e. add fields and/or query)
 }

Performance Notes

Internally there's a new data structure geared towards efficient indexing and searching, plus the necessary support code to seamlessly plug into the Lucene framework.

This class performs very well for very small texts (e.g. 10 chars) as well as for large texts (e.g. 10 MB) and everything in between. Typically, it is about 10-100 times faster than RAMDirectory. Note that RAMDirectory has particularly large efficiency overheads for small to medium sized texts, both in time and space. Indexing a field with N tokens takes O(N) in the best case, and O(N logN) in the worst case. Memory consumption is probably larger than for RAMDirectory.

Example throughput of many simple term queries over a single MemoryIndex: ~500000 queries/sec on a MacBook Pro, jdk 1.5.0_06, server VM. As always, your mileage may vary.

If you're curious about the whereabouts of bottlenecks, run java 1.5 with the non-perturbing '-server -agentlib:hprof=cpu=samples,depth=10' flags, then study the trace log and correlate its hotspot trailer with its call stack headers (see hprof tracing ).

  • Constructor Summary

    Constructors
    Constructor
    Description
    MemoryIndex()
    Constructs an empty instance.
    MemoryIndex(boolean storeOffsets)
    Constructs an empty instance that can optionally store the start and end character offset of each token term in the text.

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    addField(String fieldName, String text, Analyzer analyzer)
    Convenience method; Tokenizes the given field text and adds the resulting terms to the index; Equivalent to adding an indexed non-keyword Lucene Field that is tokenized, not stored, termVectorStored with positions (or termVectorStored with positions and offsets),
    void
    addField(String fieldName, TokenStream stream)
    Equivalent to addField(fieldName, stream, 1.0f).
    void
    addField(String fieldName, TokenStream stream, float boost)
    Iterates over the given token stream and adds the resulting terms to the index; Equivalent to adding a tokenized, indexed, termVectorStored, unstored, Lucene Field.
    void
    addField(String fieldName, TokenStream stream, float boost, int positionIncrementGap)
    Iterates over the given token stream and adds the resulting terms to the index; Equivalent to adding a tokenized, indexed, termVectorStored, unstored, Lucene Field.
    void
    addField(String fieldName, TokenStream stream, float boost, int positionIncrementGap, int offsetGap)
    Iterates over the given token stream and adds the resulting terms to the index; Equivalent to adding a tokenized, indexed, termVectorStored, unstored, Lucene Field.
    IndexSearcher
    createSearcher()
    Creates and returns a searcher that can be used to execute arbitrary Lucene queries and to collect the resulting query results as hits.
    long
    getMemorySize()
    Returns a reasonable approximation of the main memory [bytes] consumed by this instance.
    <T> TokenStream
    keywordTokenStream(Collection<T> keywords)
    Convenience method; Creates and returns a token stream that generates a token for each keyword in the given collection, "as is", without any transforming text analysis.
    void
    reset()
    Resets the MemoryIndex to its initial state and recycles all internal buffers.
    float
    search(Query query)
    Convenience method that efficiently returns the relevance score by matching this index against the given Lucene query expression.
    String
    toString()
    Returns a String representation of the index data for debugging purposes.

    Methods inherited from class java.lang.Object

    equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

  • Constructor Details

    • MemoryIndex

      public MemoryIndex()
      Constructs an empty instance.

    • MemoryIndex

      public MemoryIndex(boolean storeOffsets)
      Constructs an empty instance that can optionally store the start and end character offset of each token term in the text. This can be useful for highlighting of hit locations with the Lucene highlighter package. Protected until the highlighter package matures, so that this can actually be meaningfully integrated.
      Parameters:
      storeOffsets - whether or not to store the start and end character offset of each token term in the text

  • Method Details

    • addField

      public void addField(String fieldName, String text, Analyzer analyzer)
      Convenience method; Tokenizes the given field text and adds the resulting terms to the index; Equivalent to adding an indexed non-keyword Lucene Field that is tokenized, not stored, termVectorStored with positions (or termVectorStored with positions and offsets),
      Parameters:
      fieldName - a name to be associated with the text
      text - the text to tokenize and index.
      analyzer - the analyzer to use for tokenization

    • keywordTokenStream

      public <T> TokenStream keywordTokenStream(Collection<T> keywords)
      Convenience method; Creates and returns a token stream that generates a token for each keyword in the given collection, "as is", without any transforming text analysis. The resulting token stream can be fed into addField(String, TokenStream), perhaps wrapped into another TokenFilter, as desired.
      Parameters:
      keywords - the keywords to generate tokens for
      Returns:
      the corresponding token stream

    • addField

      public void addField(String fieldName, TokenStream stream)
      Equivalent to addField(fieldName, stream, 1.0f).
      Parameters:
      fieldName - a name to be associated with the text
      stream - the token stream to retrieve tokens from

    • addField

      public void addField(String fieldName, TokenStream stream, float boost)
      Iterates over the given token stream and adds the resulting terms to the index; Equivalent to adding a tokenized, indexed, termVectorStored, unstored, Lucene Field. Finally closes the token stream. Note that untokenized keywords can be added with this method via keywordTokenStream(Collection), the Lucene KeywordTokenizer or similar utilities.
      Parameters:
      fieldName - a name to be associated with the text
      stream - the token stream to retrieve tokens from.
      boost - the boost factor for hits for this field
      See Also:

    • addField

      public void addField(String fieldName, TokenStream stream, float boost, int positionIncrementGap)
      Iterates over the given token stream and adds the resulting terms to the index; Equivalent to adding a tokenized, indexed, termVectorStored, unstored, Lucene Field. Finally closes the token stream. Note that untokenized keywords can be added with this method via keywordTokenStream(Collection), the Lucene KeywordTokenizer or similar utilities.
      Parameters:
      fieldName - a name to be associated with the text
      stream - the token stream to retrieve tokens from.
      boost - the boost factor for hits for this field
      positionIncrementGap - the position increment gap if fields with the same name are added more than once
      See Also:

    • addField

      public void addField(String fieldName, TokenStream stream, float boost, int positionIncrementGap, int offsetGap)
      Iterates over the given token stream and adds the resulting terms to the index; Equivalent to adding a tokenized, indexed, termVectorStored, unstored, Lucene Field. Finally closes the token stream. Note that untokenized keywords can be added with this method via keywordTokenStream(Collection), the Lucene KeywordTokenizer or similar utilities.
      Parameters:
      fieldName - a name to be associated with the text
      stream - the token stream to retrieve tokens from.
      boost - the boost factor for hits for this field
      positionIncrementGap - the position increment gap if fields with the same name are added more than once
      offsetGap - the offset gap if fields with the same name are added more than once
      See Also:

    • createSearcher

      public IndexSearcher createSearcher()
      Creates and returns a searcher that can be used to execute arbitrary Lucene queries and to collect the resulting query results as hits.
      Returns:
      a searcher

    • search

      public float search(Query query)
      Convenience method that efficiently returns the relevance score by matching this index against the given Lucene query expression.
      Parameters:
      query - an arbitrary Lucene query to run against this index
      Returns:
      the relevance score of the matchmaking; A number in the range [0.0 .. 1.0], with 0.0 indicating no match. The higher the number the better the match.

    • getMemorySize

      public long getMemorySize()
      Returns a reasonable approximation of the main memory [bytes] consumed by this instance. Useful for smart memory sensititive caches/pools.
      Returns:
      the main memory consumption

    • toString

      public String toString()
      Returns a String representation of the index data for debugging purposes.
      Overrides:
      toString in class Object
      Returns:
      the string representation

    • reset

      public void reset()
      Resets the MemoryIndex to its initial state and recycles all internal buffers.