github.com/m3db/m3@v1.5.1-0.20231129193456-75a402aa583b/src/metrics/transformation/unary_multi.go

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package transformation

import (
	"math"
	"time"
)

// transformReset returns the provided datapoint and a zero datapoint one second later.
//
// This transform is useful for force resetting a counter value in Prometheus. When running the M3Aggregator in HA, both
// the follower and leader are computing aggregate counters, but they started counting at different times. If these
// counters are emitted as monotonic cumulative counters, during failover the counter decreases if the new leader
// started counting later. Prometheus assumes any decrease in a counter is due a counter reset, which leads to strange
// display results since the counter did not actually reset.
//
// This transform gets around this issue by explicitly not accumulating results, like Add, and force resets the counter
// with a zero value so PromQL properly graphs the delta as the rate value.
//
// This does have the downside of an extra 0 datapoint per resolution period. The storage cost is more than just the
// extra 0 value since the value is stored 1 second after the actual datapoint. This degrades the timestamp encoding
// since the timestamps are no longer at a fixed interval. In practice we see a 3x increase in storage for these
// aggregated counters.
//
// Currently only a single extra datapoint per aggregation is supported. If multiple transforms in an aggregation emit
// an additional datapoint, only the last one is used.
func transformReset() UnaryMultiOutputTransform {
	return UnaryMultiOutputTransformFn(func(dp Datapoint, resolution time.Duration) (Datapoint, Datapoint) {
		// Add the reset datapoint to be half the resolution period to ensure equal spacing between datapoints.
		// We take the max with 1 to ensure there's at least a 1 nanosecond gap.
		resetWindow := int64(math.Max(float64(resolution.Nanoseconds()/2), 1))

		return dp, Datapoint{Value: 0, TimeNanos: dp.TimeNanos + resetWindow*int64(time.Nanosecond)}
	})
}