Searching in Distributed Lucene

Searches are performed after indexing the data. In order to search the data, NCacheDirectory is passed to the IndexReader. Multiple Index Readers can be opened against an Index either through an IndexWriter or a NCacheDirectory. The instance of the IndexReader is passed to the IndexSearcher. The IndexSearcher is responsible for searching the data according to the given queries. Lucene provides a wide range of queries, and Distributed Lucene supports all the Lucene queries.

Lucene.Net.Search.TopDocs.TotalHits represents the number of top results achieved as a result of the searching performed. In the case of Distributed Lucene, this value is converted from integer to long. Similarly, the values of Lucene.Net.Search.ScoreDocs.Doc, Lucene.Net.Index.IndexReader.NumDoc and Lucene.Net.Index.IndexReader.MaxDocs are also converted from integer to long.

In Distributed Lucene, if you delete a Document and call commit, you'll still be able to read the contents of the deleted Document through an instance of an IndexReader, provided that this IndexReader was opened before you made the commit call (after deletion). If you dispose this IndexReader after your commit call and then reopen it, you will not be able to read the contents of the deleted Document.

You can retrieve results even when the cluster is partial. NCache Distributed Lucene provides AllowPartialResults flag which can be set to TRUE for retrieving partial data. By default, its value is FALSE.

Note

You should call Dispose at the end of every reader instance, otherwise it lives in-memory and causes memory leakages.

Prerequisites

• .NET/.NET Core

• Make sure that you have created and started a Lucene cache through the NCache Web Manager or PowerShell.
• Make sure that Client Notifications are enabled.
• Install the Lucene.Net.NCache or Lucene.Net.NCache.Professional NuGet package in your application by executing either one the following command in the Package Manager Console:

Enterprise:

Install-Package Lucene.Net.NCache

Professional:

Install-Package Lucene.Net.NCache.Professional

• Make sure that the cache is already running.
• Make sure that your application is not using any native Lucene DLL/Reference.
• To ensure the operation is fail-safe, it is recommended to handle any potential exceptions within your application, as explained in Handling Failures.
• To handle any unseen exceptions, refer to the Troubleshooting section.

Searching Data

The code sample below demonstrates how you can search your indexed documents with Distributed Lucene.

• .NET/.NET Core

IndexReader reader = null;
IndexSearcher indexSearcher = null;
try
{
    // Open a new reader instance
    // The 'applyAllDeletes' is set to true which means all enqueued deletes will be applied on the writer
    reader = indexWriter.GetReader(true);
    // OR
    //  reader = DirectoryReader.Open(indexWriter, true);

    // OR
    //  reader = DirectoryReader.Open(ncacheDirectory, 1);

    // Allow partial results in case of partial connectivity with cluster
    reader.AllowPartialResutls = true;

    // A searcher is opened to perform searching
    indexSearcher = new IndexSearcher(reader);

    // Specify the searchTerm and the fieldName
    string searchTerm = "Beverages";
    string fieldName = "Category";

    // Specify lucene version and analyzer type 
    LuceneVersion version = LuceneVersion.LUCENE_48;
    Analyzer analyzer = new WhitespaceAnalyzer(version);

    // Create a query parser and parse the query with the parser 
    QueryParser parser = new QueryParser(version, fieldName, analyzer);
    Query query = parser.Parse(searchTerm);

    // Returns the top 10000 hits from the result set
    ScoreDoc[] docsFound = indexSearcher.Search(query, 10000).ScoreDocs;

    // Please make sure that no operations should be performed after calling this

}
catch (Exception ex)
{
    // Handle Lucene exceptions
}
finally
{
    if (indexSearcher != null) indexSearcher.Dispose();
    if (reader != null) reader.Dispose();
}

Note

You should call Dispose at the end of every reader instance.

Additional Resources

NCache provides a sample application for searching documents in Distributed Lucene on GitHub.

See Also

Distributed Lucene Overview
Distributed Lucene Geo-Spatial API
Distributed Lucene Facets
Distributed Lucene Indexing