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If you look at the values, you can see that they are not overlapping. This should be rather clear, because the count e.g. can not be 1 and 2 at the same time. The metadata show, that every 500 (in this case milliseconds) a new bid is send to the system and a new aggregation is build.
Another important think to note is, that a new aggregation will be calculated, when a new event arrives a the system. For this example it take 8 seconds before the first value is created. For streams with a low data rate the AssureHeartbeat operator can be used to produce values on a regular base.