title: How to sort the union datastream of flink without watermark
body: <p>The flink flow has multi data stream, then I merge those data stream with org.apache.flink.streaming.api.datastream.DataStream#union method.
  Then, I got the problem, the datastream is disordered and I can not set window to sort the data in data stream.</p>
  
  <p><a href="https://stackoverflow.com/questions/47576408/sorting-union-of-streams-to-identify-user-sessions-in-apache-flink">Sorting union of streams to identify user sessions in Apache Flink</a></p>
  
  <p>I got the the answer, but the com.liam.learn.flink.example.union.UnionStreamDemo.SortFunction#onTimer 
  never been invoked. </p>
  
  <p>Environment Info: flink version 1.7.0</p>
  
  <p>In general, I hope to sort the union datastream witout watermark.</p>