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DotLucene: Full-Text Search for Your Intranet or Website using 37 Lines of Code

, 6 Nov 2012
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An introduction to Lucene.Net, the open source full-text search engine.

Update 

November 6, 2012: The project is now working with Lucene.Net 3.0 and .NET Framework 4.0. Includes Visual Studio 2010 solution.  

Lucene.Net: Excellent Full-Text Search Engine

Can there be a full-text search coded on 37 lines? Well, I am going to cheat a bit and use Lucene.Net for the dirty work. Lucene.Net is a .NET port of Jakarta Lucene search engine. Here is a quick list of its features:

  • It can be used in ASP.NET, Win Forms or console applications.
  • Very good performance.
  • Ranked search results.
  • Search query highlighting in results.
  • Searches structured and unstructured data.
  • Metadata searching (query by date, search custom fields...).
  • Index size approximately 30% of the indexed text.
  • Can also store full indexed documents.
  • Pure managed .NET.
  • Very friendly licensing (Apache Software License 2.0).
  • Localizable (support for Brazilian, Czech, Chinese, Dutch, English, French, Japanese, Korean and Russian included).
  • Extensible (source code included).

Warning 

Don't take the line count too seriously. I will show you that the core functionality doesn't take more than 37 lines of code, but to make it a real application you will need to spend some more time on it...

Demo Project 

We will build a simple demo project that shows how to:

  • index HTML files found in a specified directory (including subdirectories).
  • search the index using a ASP.NET application.
  • highlight the query words in the search results.

But Lucene.Net has more potential. In real-world application, you would probably want to:

  • Add the new documents to the index when they appear in the directory. You don't need to rebuild the whole index.
  • Include other file types. Lucene.Net can index any file type which you are able to convert to plain text.

Why Not to Use Microsoft Indexing Server?

If you are happy with the Indexing Server, no problem. However, Lucene.Net has many advantages:

  • Lucene.Net is a single assembly of 100% managed code. It has no external dependencies. 
  • You can use it to index any type of data (e-mails, XML, HTML files, etc.) from any source (database, web, etc.). That's because you need to supply plain text to the indexer. Loading and parsing the source is up to you.
  • Allows you to specify the attributes ("fields") that should be included in the index. You can search using these fields (e.g. by author, date, keywords).
  • It is an open source.
  • It is easily extensible.

Line 1: Creating the Index

The following line of code creates a new index stored on disk. directory is a path to the directory where the index will be stored. 

IndexWriter writer = new IndexWriter(FSDirectory.Open(directory), new StandardAnalyzer(Version.LUCENE_30), true, IndexWriter.MaxFieldLength.LIMITED); 

In this example, we create the index from scratch. This is not necessary, you can also open an existing index and add documents to it. You can also update existing documents by deleting it and adding a new version.

Lines 2 - 12: Adding documents 

For each HTML document, we will add two fields into the index: 

  • text field that contains the text of the HTML file (with stripped tags). The text itself won't be stored in the index.
  • path field that contains the file path. It will be indexed and stored in full in the index.
public void AddHtmlDocument(string path)
{
    Document doc = new Document();

    string rawText;
    using (StreamReader sr = 
       new StreamReader(path, System.Text.Encoding.Default))
    {
        rawText = parseHtml(sr.ReadToEnd());
    }
    
    doc.Add(new Field("text", rawText, Field.Store.YES, Field.Index.ANALYZED));
    doc.Add(new Field("path", path, Field.Store.YES, Field.Index.NOT_ANALYZED));
    writer.AddDocument(doc);
}  

Lines 13 - 14: Optimizing and Saving the Index

After adding the documents, you need to close the indexer. Optimization will improve search performance.

writer.Optimize();
writer.Close(); 

Line 15: Opening the Index for Searching

Before doing any search, you need to open the index. directory is the path to the directory where the index was stored. 

IndexSearcher searcher = new IndexSearcher(FSDirectory.Open(indexDirectory));  

Lines 16 - 27: Searching 

Now we can parse the query (text is the default field to search for).

var parser = new QueryParser(Version.LUCENE_30, "text", analyzer);
Query query = parser.Parse(this.Query); 
TopDocs hits = searcher.Search(query, 200);  

Variable hits is a collection of result documents. We will go through it and store the results in a DataTable.

DataTable dt = new DataTable();
dt.Columns.Add("path", typeof(string));
dt.Columns.Add("sample", typeof(string));

for (int i = 0; i < hits.TotalHits; i++) 
{
    // get the document from index
    Document doc = searcher.Doc(hits.ScoreDocs[i].Doc);

    // get the document filename
    // we can't get the text from the index 
    //because we didn't store it there
    DataRow row = dt.NewRow();
    row["path"] = doc.Get("path");

    dt.Rows.Add(row);
}  

Lines 28 - 37: Query Highlighting 

Let's create a highlighter. We will use bold font for highlighting (<B>phrase</B>). 

IFormatter formatter = new SimpleHTMLFormatter("<span style=\"font-weight:bold;\">", "</span>");
SimpleFragmenter fragmenter = new SimpleFragmenter(80);
QueryScorer scorer = new QueryScorer(query);
Highlighter highlighter = new Highlighter(formatter, scorer);
highlighter.TextFragmenter = fragmenter; 

During the result fetching, we will load the relevant part of the original text.

for (int i = 0; i < hits.TotalHits; i++) 
{
    // ...
    TokenStream stream = analyzer.TokenStream("", new StringReader(doc.Get("text")));
    row["sample"] = highlighter.GetBestFragments(stream, doc.Get("text"), 2, "...");    string plainText;
    // ...
}  

Resources

License

This article, along with any associated source code and files, is licensed under The Apache License, Version 2.0

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About the Author

Dan Letecky

Czech Republic Czech Republic
My open-source event calendar/scheduling AJAX controls:
 
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GeneralExposing "DotLucene: Full-Text Search for Your Intranet or Website using 37 Lines of Code". as a web services Pinmemberkbsnet6-Dec-08 1:01 

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