@Override
  public boolean isIdentityField(String fieldName) {
    PdxField field = blobType.getPdxField(fieldName);
    return field != null && field.isIdentityField();
  }
@param idx of the variable length field
@return the offset to the variable length field
geode-core/src/main/java/org/apache/geode/pdx/internal/PdxReaderImpl.java