Index: modules/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java
===================================================================
--- modules/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java	(revision 1206586)
+++ modules/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java	(working copy)
@@ -233,7 +233,10 @@
    * etc.<br>
    * <b>NOTE:</b> the instance this method returns will be closed upon calling
    * to {@link #close()}.
-   * 
+   * <br><b>NOTE:</b> internal docids of this index must not be altered.
+   * For that, categories are never deleted from the taxonomy index.
+   * In addition, the merge policy in effect must not merge segments out of order.
+   *  
    * @param directory
    *          the {@link Directory} on top of which an {@link IndexWriter}
    *          should be opened.
@@ -245,10 +248,20 @@
     // Make sure we use a MergePolicy which merges segments in-order and thus
     // keeps the doc IDs ordered as well (this is crucial for the taxonomy
     // index).
-    IndexWriterConfig config = new IndexWriterConfig(Version.LUCENE_40,
+    IndexWriterConfig config = createIndexWriterConfig(openMode);
+    return new IndexWriter(directory, config);
+  }
+
+  /**
+   * Create the {@link IndexWriterConfig} that would be used for opening the internal index writer. 
+   * <br>The merge policy in effect must not merge segments out of order. See
+   * comment in {@link #openIndexWriter(Directory, OpenMode) openIndexWriter()} for the logic behind this.
+   * @param openMode see {@link OpenMode}
+   */
+  protected IndexWriterConfig createIndexWriterConfig(OpenMode openMode) {
+    return new IndexWriterConfig(Version.LUCENE_40,
         new KeywordAnalyzer()).setOpenMode(openMode).setMergePolicy(
         new LogByteSizeMergePolicy());
-    return new IndexWriter(directory, config);
   }
 
   // Currently overridden by a unit test that verifies that every index we open