So in flink you can use a few different bundled in strategies for emitting watermarks, or you can define your own. Here's the documentation on that https://ci.apache.org/projects/flink/flink-docs-release-1.4/...

For example, you can use the bundled MaxOutOfOrdernessTimestampExtractor to emit watermarks that lag behind the maximum timestamp seen so far by some constant amount (say, 50 ms). You could customize this behaviour by implementing the required interface, to emit a watermark, say, whenever we pass an other calendar day. You can also assign strictly-ascending watermarks which you could say is equivalent to maxOutOfOrderness = 0.

You can also choose to emit watermarks based on the content of the stream (see Punctuated Watermarks https://ci.apache.org/projects/flink/flink-docs-release-1.4/...)