Fw:A question about flink watermark illustration in official documents

[罗昊]

Recently I read flink official documents for something about watermarks。

url：https://ci.apache.org/projects/flink/flink-docs-release-1.9/dev/event_time.html

there are two pictures illustrating flink watermark mechanism, which puzzle me mush:

The first picture is easy to understand, But in the second, I wonder how do we get w(11) and w(17)?

As we know, we can define how to generate watermark in the flink job, in other words, watermarks are generated by certain rules. So what are the rules that the watermarks are generated in the second pic.

I look up for almost  all offficial documents of different version flink and they use the same pictures.

It puzzled me much。Is there any explaination？

waiting for your answers ，Thx!

 

 

[Matthias]

Hi 罗昊,

the 2nd picture is meant to visualize the issue of out-of-orderness in general. I'd say it's not referring to a specific strategy. 

But one way to interpret the image is using the BoundedOutOfOrderness strategy for watermark generation [1]: You can define an upper bound B for the out-of-orderness. The watermark generator assumes that there's a delay of B, i.e. for an event with timestamp T, no events older than {@code T - B} will follow any more. The delayed watermarks you see in image 2 could be achieved using this bounded out-of-orderness strategy. 

The usage of watermark strategies is also addressed in the docs [2].

I hope this helps.

Matthias

[1] https://github.com/apache/flink/blob/c6997c97c575d334679915c328792b8a3067cfb5/flink-core/src/main/java/org/apache/flink/api/common/eventtime/BoundedOutOfOrdernessWatermarks.java#L37

[2] https://ci.apache.org/projects/flink/flink-docs-stable/dev/event_timestamps_watermarks.html#generating-watermarks

On Tue, Mar 30, 2021 at 6:26 AM 罗昊 <[hidden email]> wrote:

Recently I read flink official documents for something about watermarks。

url：https://ci.apache.org/projects/flink/flink-docs-release-1.9/dev/event_time.html

there are two pictures illustrating flink watermark mechanism, which puzzle me mush:

The first picture is easy to understand, But in the second, I wonder how do we get w(11) and w(17)?

As we know, we can define how to generate watermark in the flink job, in other words, watermarks are generated by certain rules. So what are the rules that the watermarks are generated in the second pic.

I look up for almost  all offficial documents of different version flink and they use the same pictures.

It puzzled me much。Is there any explaination？

waiting for your answers ，Thx!