/// <summary>
        /// Initializes a new instance of the <see cref="LuceneIndex" /> class.
        /// </summary>
        /// <param name="indexPath">The path to the directory that will contain the Lucene index files.</param>
        /// <param name="schema">The schema.</param>
        /// <exception cref="System.ArgumentNullException"></exception>
        public LuceneIndex(string indexPath, Schema schema)
        {
            if (String.IsNullOrWhiteSpace(indexPath))
                throw new ArgumentNullException(nameof(indexPath)); 
            if (schema == null)
                throw new ArgumentNullException(nameof(schema));

            IndexPath = indexPath;
            Schema = schema;

            if (System.IO.Directory.Exists(IndexPath))
            {
                if (Schema.IsDefault())
                    throw new InvalidOperationException($"There is an existing index on '{IndexPath}'.");
            }                
            else
            {
                System.IO.Directory.CreateDirectory(IndexPath);
            }                        

            _indexDirectory = new MMapDirectory(Paths.get(IndexPath));

            var taxonomyIndexPath = System.IO.Path.Combine(IndexPath, "taxonomy");
            if (!System.IO.Directory.Exists(taxonomyIndexPath))            
                System.IO.Directory.CreateDirectory(taxonomyIndexPath);

            _taxonomyDirectory = new MMapDirectory(Paths.get(taxonomyIndexPath));         
                           
            _compositeAnalyzer = new CompositeAnalyzer(Schema);            

            _ramBufferSizeMB = Double.Parse(ConfigurationManager.AppSettings["IndexWriter.RAMBufferSizeMB"] ?? "128");

            var config = new IndexWriterConfig(_compositeAnalyzer)                            
                            .SetOpenMode(IndexWriterConfigOpenMode.CREATE_OR_APPEND)
                            .SetRAMBufferSizeMB(_ramBufferSizeMB)
                            .SetCommitOnClose(true);                            
            
            _indexWriter = new IndexWriter(_indexDirectory, config);
            _taxonomyWriter = new DirectoryTaxonomyWriter(_taxonomyDirectory, IndexWriterConfigOpenMode.CREATE_OR_APPEND);

            _searcherTaxonomyManager = new SearcherTaxonomyManager(_indexWriter, true, null, _taxonomyWriter);            
            _facetBuilder = new LuceneFacetBuilder(_taxonomyWriter);                        

            _refreshIntervalSeconds = Double.Parse(ConfigurationManager.AppSettings["IndexSearcher.RefreshIntervalSeconds"] ?? "0.5");    
            _commitIntervalSeconds = Double.Parse(ConfigurationManager.AppSettings["IndexWriter.CommitIntervalSeconds"] ?? "60");
           
            _writeAllowedFlag = new ManualResetEventSlim(true);

            _refreshTimer = new Timer(o => Refresh(), null, TimeSpan.FromSeconds(_refreshIntervalSeconds), TimeSpan.FromSeconds(_refreshIntervalSeconds));
            _commitTimer = new Timer(o => Commit(), null, TimeSpan.FromSeconds(_commitIntervalSeconds), TimeSpan.FromSeconds(_commitIntervalSeconds));

        }