is there a guide on how Reduce works

[tshepang]

I think I don't understand Reduce, because given the following, I expected to have an event whenever .count reached 10 events (assuming sum merge strategy would happen magically on that field):

inputs = ["activity"]
type = "reduce"
ends_when = "to_float!(.count) == 10"
group_by = [
  "name",
  "node",
]

This gets fed events of this shape

{
  "count": 1.0,
  "name": "some-name",
  "node": "some-node",
  "timestamp": "2022-08-02T13:40:22.815Z"
}

What I get instead is just an assortment of events with varying .count values.

[tobz]

Beyond the examples on the documentation page, there's not much else.

However, apropos to your particular example: the ends_when condition is used to determine if an incoming event is one that should cause the reducer to emit the reduced event before the expiry period. In your example, it looks like you're trying to figure out when the accumulated .count of your reduced events reaches 10, and to flush the reducer state for that grouping, but the ends_when condition only operates on the events coming in.

You could accomplish that style of reduction/flushing with the stateful lua transform, but reduce currently only supports the aforementioned behaviors (terminal event condition, expiry period) for flushing a reduced event.