@Override
  protected void appendFields(StringBuilder buff) {
    super.appendFields(buff);
    buff.append("; key=").append(key).append("; callback arg=").append(cbArg)
        .append("; context=").append(context);
  }
Empty constructor to satisfy {@link DataSerializer} requirements
geode-core/src/main/java/org/apache/geode/internal/cache/partitioned/GetMessage.java