I am new to Flink and Stream.
I am about to perform an interval join between two streams. I got those stream data from kinesis. And I saw examples of setting a watermark for the interval join. I am wondering if this is necessary? Where will the flink try to get the timestamp for interval join?

BTW: From the flink doc it mentions that: For example, when using Kafka or Kinesis you would get timestamps directly from the Kafka/Kinesis records. Does this mean Flink can get a timestamp without declaring a watermark for streams?

Also, any good idea to unit test interval join?

I asked Copliot for this and it mentions that a watermark is needed.