Re: How watermark is generated in sql DDL statement

Posted by Benchao Li on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/How-watermark-is-generated-in-sql-DDL-statement-tp34391p34397.html

WatermarkAssignerOperator is an inner mechanism for generating watermarks.

The "Bounded Out of Orderness" is just one kind of the watermark expressions, which 

is most commonly used.

The main logic of WatermarkAssignerOperator is:

- keep currentWatermark and lastWatermark

- when each element comes in

  - get watermark from this element, using the watermark expression 

  - if the watermark > currentWatermark, then currentWatermark is updated

  - if currentWatermark - lastWatermark > watermarkInterval

    - emit watermark to downstream, and update lastWatermark

lec ssmi <[hidden email]> 于2020年4月17日周五 下午4:50写道：

Maybe you are all right. I was  more confused  .

As the cwiki said, flink could use BoundedOutOfOrderTimestamps , 

but I have heard about WatermarkAssignerOperator from Blink developers. 

Benchao Li <[hidden email]> 于2020年4月17日周五 下午4:33写道：

Hi lec ssmi,

It's a good question. In blink planner, we use code gen to handle watermark expression.

And in `WatermarkAssignerOperator` we calculate current watermark when each element comes in.

If the watermark - lastEmitedWatermark > watermark interval, we will emit the new watermark.

So it's neither `PeriodicWatermark` nor `PunctuatedWatermark`. 

lec ssmi <[hidden email]> 于2020年4月17日周五 下午3:12写道：

Hi:

   In sql API , the declaration of watermark is realized by ddl statement . But which way is it implemented?

   PeriodicWatermark   or   PunctuatedWatermark？

  There seems to be  no explanation on the official website.

   

  Thanks.

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Benchao Li
School of Electronics Engineering and Computer Science, Peking University
Tel:[phone]
Email: [hidden email]; [hidden email]

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