Let's say we have an EventTimeSlidingWindow with an EventTime trigger based on some watermark. If the watermark is generated very infrequently, say every five minutes, and the window sizes are say one minute, then will five window results get fired at the same time when the watermark progresses? i.e. in my output stream would I have the same timestamp for all of their output, which is the time when the watermark generator produced the watermark?
Not exactly. Flink registers a timer with a timestamp when the window will fire. That timestamp is the event which has caused the window to open. As the watermark advances it will cause all the timers to fire (more or less at once) with the timestamps registered previously. So you are right with the assumption that all the windows will fire but having timestamps when the window (timer) was registered.
But be aware that if you "only" create a watermark every 5 minutes the minimum delay of the window will be that 5 minutes. But still the timestamps will be correct.