需要添加 <propagate-configuration/> 到 sub-workflow action 中

<action name="main-node">
  <sub-workflow>
     <app-path>/user/${wf:user()}/${examplesRoot}/apps/map-reduce/workflow.xml</app-path>
     <propagate-configuration/>
  </sub-workflow>
  <ok to="end" />
  <error to="fail" />
</action>