title: Flink: Window evaluation
body: <p>I am using <code>window(SlidingEventTimeWindows.of(Time.seconds(120), Time.seconds(1)))</code> sliding window for real_time streaming of data. I am also using the interface <code>AssignerWithPeriodicWatermarks</code> to extract the timestamps and assign watermarks. I am applying the custom method <code>apply()</code> on windowed stream. </p>
  
  <p>In this scenario, what will trigger the window evaluation(<code>apply()</code> execution), is it happened after every 1 sec. which is equal to slide of window or is it after encountering the watermark in the stream?  </p>