The Lucene Search Engine

Kira Radinsky

Based on the material from: Thomas Paul and Steven J. Owens

What is Lucene?

• Doug Cutting’s grandmother’s middle name

• A open source set of Java Classses– Search Engine/Document Classifier/Indexer

– Developed by Doug Cutting (1996)• Xerox/Apple/Excite/Nutch/Yahoo/Cloudera

• Hadoop founder, Board of directors of the Apache Software

• Jakarta Apache Product. Strong open source community support.

• High-performance, full-featured text search engine library

• Easy to use yet powerful API

Use the Source, Luke

• Document• Field

– Represents a section of a Document: name for the section + the actual data.

• Analyzer– Abstract class (to provide interface)– Document -> tokens (for later indexing)– StandardAnalyzer class.

• IndexWriter– Creates and maintains indexes.

• IndexSearcher– Searches through an index.

• QueryParser– Builds a parser that can search through an index.

• Query– Abstract class that contains the search criteria created by the QueryParser.

• Hits– Contains the Document objects that are returned by running the Query object against the index.

Indexing a Document

Document from an article

private Document createDocument(String article, String author,String title, String topic,String url, Date dateWritten)

{

Document document = new Document();document.add(Field.Text("author", author));document.add(Field.Text("title", title));document.add(Field.Text("topic", topic));document.add(Field.UnIndexed("url", url));document.add(Field.Keyword("date", dateWritten));document.add(Field.UnStored("article", article));return document;

}

The Field Object

Factory Method Tokenized Indexed Stored Use for

Field.Text(String name, String value)

Yes Yes Yescontents you want stored

Field.Text(String name, Reader value)

Yes Yes Nocontents you don't want stored

Field.Keyword(String name, String value)

No Yes Yesvalues you don't want broken down

Field.UnIndexed(String name, String value)

No No Yesvalues you don't want indexed

Field.UnStored(String name, String value)

Yes Yes Novalues you don't want stored

Store a Document in the index

String indexDirectory = "lucene-index";

private void indexDocument(Document document)

throws Exception

{

Analyzer analyzer = new StandardAnalyzer();

IndexWriter writer = new IndexWriter(

indexDirectory, analyzer, false

);

writer.addDocument(document);

writer.optimize();

writer.close();

}

Analyzers and Tokenizers

SimpleAnalyzer SimpleAnalyzer seems to just use a Tokenizer that converts all of the input to lower case.

StopAnalyzer StopAnalyzer includes the lower-case filter, and also has a filter that drops out any "stop words", words like articles (a, an, the, etc) that occur so commonly in english that they might as well be noise for searching purposes. StopAnalyzer comes with a set of stop words, but you can instantiate it with your own array of stop words.

StandardAnalyzer StandardAnalyzer does both lower-case and stop-word filtering, and in addition tries to do some basic clean-up of words, for example taking out apostrophes ( ' ) and removing periods from acronyms (i.e. "T.L.A." becomes "TLA").

Lucene Sandbox Here you can find analyzers in your own language

Adding to an Index

public void indexArticle(String article, String author,String title, String topic,String url, Date dateWritten)

throws Exception {

Document document = createDocument(

article, author, title, topic, url, dateWritten

);indexDocument(document);

}

Searching the Index

Searching

IndexSearcher is = new IndexSearcher(indexDirectory);

Analyzer analyzer = new StandardAnalyzer();

QueryParser parser = new QueryParser("article", analyzer);

Query query = parser.parse(searchCriteria);

Hits hits = is.search(query);

Extracting Document objects

for (int i=0; i<hits.length(); i++)

{

Document doc = hits.doc(i);

// display the articles that were found to the user

}

Search Criteria

Supports several searches: AND OR and NOT,

fuzzy, proximity searches, wildcard searches, and

range searches

– author:Henry relativity AND "quantum physics“

– "string theory" NOT Einstein

– "Galileo Kepler"~5

– author:Johnson date:[01/01/2004 TO 01/31/2004]

Thread Safety

• Indexing and searching are not only thread safe, but process safe. What this means is that:– Multiple index searchers can read the lucene index

files at the same time.– An index writer or reader can edit the lucene index

files while searches are ongoing– Multiple index writers or readers can try to edit the

lucene index files at the same time (it's important for the index writer/reader to be closed so it will release the file lock).

• The query parser is not thread safe, • The index writer however, is thread safe,