Component(s)

processor/cumulativetodelta

What happened?

Description

The cumulativetodeltaprocessor correctly drops sum and exponential histogram datapoints when a counter reset is detected (the new value is lower than the previous one (restart)). However, when a regular histogram is reset, the processor does not drop the point. Instead, it forwards the new datapoint with the full cumulative bucket counts and sum but tagged as AggregationTemporalityDelta.

This produces a corrupted delta: a one-time spike that includes both pre-reset and post-reset events. Downstream consumers will over count.

Where

processor/cumulativetodeltaprocessor/internal/tracking/tracker.go, in Convert for pmetric.MetricTypeHistogram:

delta := value.Clone()

// Calculate deltas unless histogram count was reset
if valid && delta.Count >= prevValue.Count {
    delta.Count -= prevValue.Count
    delta.Sum -= prevValue.Sum
    for index, prevBucket := range prevValue.BucketCounts {
        delta.BucketCounts[index] -= prevBucket
    }
}

out.HistogramValue = &delta  // when reset, this is the full cumulative value, not a delta

When delta.Count < prevValue.Count the subtraction is skipped but the point is still returned with valid=true, so the processor emits it as a delta.

Steps to Reproduce

Send the following sequence to the processor (one cumulative histogram metric, three points):

Expected Result

Match the sum / exponential histogram paths: on reset, mark the point invalid so the processor drops it (keep in memory waiting next datapoint to compute the delta, if >=).

Actual Result

I attached a diff file with a unit test to reproduce that bug

cumulativetodelta-histo-reset-bug.patch

Collector version

main

Environment information

Environment

N/A — bug is in the processor logic itself.

OpenTelemetry Collector configuration

processors:
  cumulativetodelta:

Log output

N/A

Additional context

No response

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