public DirectoryTaxonomyWriter(Directory directory, OpenMode openMode,
      TaxonomyWriterCache cache) throws IOException {

    dir = directory;
    IndexWriterConfig config = createIndexWriterConfig(openMode);
    indexWriter = openIndexWriter(dir, config);

    // verify (to some extent) that merge policy in effect would preserve category docids 
    assert !(indexWriter.getConfig().getMergePolicy() instanceof TieredMergePolicy) : 
      "for preserving category docids, merging none-adjacent segments is not allowed";
    
    // after we opened the writer, and the index is locked, it's safe to check
    // the commit data and read the index epoch
    openMode = config.getOpenMode();
    if (!DirectoryReader.indexExists(directory)) {
      indexEpoch = 1;
    } else {
      String epochStr = null;
      Map<String, String> commitData = readCommitData(directory);
      if (commitData != null) {
        epochStr = commitData.get(INDEX_EPOCH);
      }
      // no commit data, or no epoch in it means an old taxonomy, so set its epoch to 1, for lack
      // of a better value.
      indexEpoch = epochStr == null ? 1 : Long.parseLong(epochStr, 16);
    }
    
    if (openMode == OpenMode.CREATE) {
      ++indexEpoch;
    }
    
    FieldType ft = new FieldType(TextField.TYPE_NOT_STORED);
    ft.setOmitNorms(true);
    parentStreamField = new Field(Consts.FIELD_PAYLOADS, parentStream, ft);
    fullPathField = new StringField(Consts.FULL, "", Field.Store.YES);

    nextID = indexWriter.maxDoc();

    if (cache == null) {
      cache = defaultTaxonomyWriterCache();
    }
    this.cache = cache;

    if (nextID == 0) {
      cacheIsComplete = true;
      // Make sure that the taxonomy always contain the root category
      // with category id 0.
      addCategory(new FacetLabel());
    } else {
      // There are some categories on the disk, which we have not yet
      // read into the cache, and therefore the cache is incomplete.
      // We choose not to read all the categories into the cache now,
      // to avoid terrible performance when a taxonomy index is opened
      // to add just a single category. We will do it later, after we
      // notice a few cache misses.
      cacheIsComplete = false;
    }

}