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The groupby clause has the form of a list of node.X or event.Y property specifications (e.g. node.customer or node.group), which are used to group events into buckets for counting: only events that share the same values for all the listed grouping properties will be counted together. For example, the groupby clause [ 'node.customer', 'event.priority' ] would cause this suppression rule to be applied independently for all combinations of customer and event priority. The clause given in the example block above will suppress 2-8 node configuration events for any individual node within 120 seconds; without the groupby repeat node configuration events would be suppressed regardless of where they happened.
The common node properties are listed here, and the standard event properties are documented on this page.