private void startPrimaryKeyIndex(Attributes atts) {
    IndexCreationData icd = (IndexCreationData) stack.peek();
    // icd.setIndexType(PRIMARY_KEY);
    boolean throwExcep = false;
    if (atts.getLength() == 1) {
      String field = atts.getValue(FIELD);
      if (field == null) {
        throwExcep = true;
      } else {
        icd.setIndexData(IndexType.PRIMARY_KEY, null, field, null);
      }
    } else {
      throwExcep = true;
    }
    if (throwExcep) {
      throw new InternalGemFireException(
          "CacheXmlParser::startPrimaryKeyIndex:Primary-Key Index creation field is null.");
    }
  }
When a <code>primary-key</code> element is encounter, we pop the IndexCreationData object from
the Stack. Set the required parameters in the IndexCreationData object & set it in
RegionCreation object.
geode-core/src/main/java/org/apache/geode/internal/cache/xmlcache/CacheXmlParser.java