Watermarks in Event Time windowing

[Taher Koitawala]

Hi All,
          Can someone show a good example of how watermarks need to be
generated when using EventTime windows?  What happens if the watermark is
same as the timestamp?  How does the watermark help in the window to be
triggered and what if watermarks are kept behind the currentTimestamps in
the assignTimestampsAndWatermarks function.