assertTrue(extraFieldNames.contains("custom_field"));
        assertFalse(extraFieldNames.contains("_id"), "Reserved field _id should be excluded");
        assertFalse(extraFieldNames.contains("_version"), "Reserved field _version should be excluded");
        assertFalse(extraFieldNames.contains("_seq_no"), "Reserved field _seq_no should be excluded");

                    return "doc_id";
                case FessConfig.INDEX_FIELD_ID:
                    return "id";
                case FessConfig.INDEX_FIELD_VERSION:
                    return "_version";
                case FessConfig.INDEX_FIELD_seq_no:
                    return "_seq_no";
                case FessConfig.INDEX_FIELD_primary_term:
                    return "_primary_term";

        }

        @Override
        public String getIndexFieldId() {
            return "_id";
        }

        @Override
        public String getIndexFieldVersion() {
            return "_version";
        }

        @Override
        public String getIndexFieldSeqNo() {
            return "_seq_no";
        }

        @Override
        public String getIndexFieldPrimaryTerm() {

index.field.url=url
# Field name for document ID in the index.
index.field.doc_id=doc_id
# Field name for internal ID in the index.
index.field.id=_id
# Field name for version in the index.
index.field.version=_version
# Field name for sequence number in the index.
index.field.seq_no=_seq_no
# Field name for primary term in the index.
index.field.primary_term=_primary_term
# Field name for language in the index.

     */
    String getIndexFieldId();

    /**
     * Get the value for the key 'index.field.version'. <br>
     * The value is, e.g. _version <br>
     * comment: Field name for version in the index.
     * @return The value of found property. (NotNull: if not found, exception but basically no way)
     */
    String getIndexFieldVersion();

    /**

<?xml version="1.0" encoding="UTF-8"?>
<metadata>
  <groupId>org.codelibs.fess</groupId>
  <artifactId>fess-crawler-db-mysql</artifactId>
  <versioning>
    <latest>1.0.12</latest>
    <release>1.0.12</release>
    <versions>
      <version>1.0.0</version>
      <version>1.0.1</version>
      <version>1.0.2</version>
      <version>1.0.3</version>
      <version>1.0.4</version>
      <version>1.0.5</version>
      <version>1.0.6</version>

<?xml version="1.0" encoding="UTF-8"?>
<metadata>
  <groupId>org.codelibs.fess</groupId>
  <artifactId>fess-crawler-playwright</artifactId>
  <versioning>
    <latest>14.14.0</latest>
    <release>14.14.0</release>
    <versions>
      <version>14.5.0</version>
      <version>14.6.0</version>
      <version>14.6.1</version>
      <version>14.6.2</version>
      <version>14.6.3</version>
      <version>14.7.0</version>

<?xml version="1.0" encoding="UTF-8"?>
<metadata>
  <groupId>org.codelibs.fess</groupId>
  <artifactId>fess-crawler-db-h2</artifactId>
  <versioning>
    <latest>1.0.12</latest>
    <release>1.0.12</release>
    <versions>
      <version>1.0.0</version>
      <version>1.0.1</version>
      <version>1.0.2</version>
      <version>1.0.3</version>
      <version>1.0.4</version>
      <version>1.0.5</version>
      <version>1.0.6</version>

      <version>2.4.0</version>
      <version>2.4.1</version>
      <version>2.4.2</version>
      <version>2.4.3</version>
      <version>2.4.4</version>
      <version>2.4.5</version>
      <version>2.5.0</version>
      <version>2.5.1</version>
      <version>2.5.2</version>
      <version>2.5.3</version>
      <version>2.6.0</version>
      <version>2.6.1</version>
      <version>2.6.2</version>

      <version>2.3.4</version>
      <version>2.3.5</version>
      <version>2.3.6</version>
      <version>2.4.0</version>
      <version>2.4.1</version>
      <version>2.4.2</version>
      <version>2.4.3</version>
      <version>2.4.4</version>
      <version>2.4.5</version>
      <version>2.5.0</version>
      <version>2.5.1</version>
      <version>2.5.2</version>
      <version>2.5.3</version>