Deprecate the unused configuration DimensionsCacheSize

[Frapschen]

Component(s)

connector/spanmetrics

Describe the issue you're reporting

The DimensionsCacheSize is the limit of the metricKeyToDimensions cache size:

opentelemetry-collector-contrib/connector/spanmetricsconnector/connector.go

Line 116 in 154ba2b

metricKeyToDimensionsCache, err := cache.NewCache[metrics.Key, pcommon.Map](cfg.DimensionsCacheSize)

It's use to keep the attributes build from span and try to improve collectors memory usage.

As the code of spanconnector iterates, spanconnector uses ResourceMetrics to maintain the intermediate state of metrics:

opentelemetry-collector-contrib/connector/spanmetricsconnector/connector.go

Lines 88 to 95 in 154ba2b

	type resourceMetrics struct {
	histograms metrics.HistogramMetrics
	sums metrics.SumMetrics
	events metrics.SumMetrics
	attributes pcommon.Map
	// lastSeen captures when the last data points for this resource were recorded.
	lastSeen time.Time
	}

Below is an example of the SumMetrics struct. Please note the signature of the GetOrCreate function.

GetOrCreate(key Key, attributes pcommon.Map, startTimestamp pcommon.Timestamp) *Sum

opentelemetry-collector-contrib/connector/spanmetricsconnector/internal/metrics/metrics.go

Lines 243 to 293 in 9732ea5

	type Sum struct {
	attributes pcommon.Map
	count uint64
	exemplars pmetric.ExemplarSlice
	maxExemplarCount *int
	startTimestamp pcommon.Timestamp
	// isFirst is used to track if this datapoint is new to the Sum. This
	// is used to ensure that new Sum metrics being with 0, and then are incremented
	// to the desired value. This avoids Prometheus throwing away the first
	// value in the series, due to the transition from null -> x.
	isFirst bool
	}
	func (s *Sum) Add(value uint64) {
	s.count += value
	}
	func NewSumMetrics(maxExemplarCount *int, cardinalityLimit int) SumMetrics {
	return SumMetrics{
	metrics: make(map[Key]*Sum),
	maxExemplarCount: maxExemplarCount,
	cardinalityLimit: cardinalityLimit,
	}
	}
	type SumMetrics struct {
	metrics map[Key]*Sum
	maxExemplarCount *int
	cardinalityLimit int
	}
	func (m *SumMetrics) IsCardinalityLimitReached() bool {
	return m.cardinalityLimit > 0 && len(m.metrics) >= m.cardinalityLimit
	}
	func (m *SumMetrics) GetOrCreate(key Key, attributes pcommon.Map, startTimestamp pcommon.Timestamp) *Sum {
	s, ok := m.metrics[key]
	if !ok {
	s = &Sum{
	attributes: attributes,
	exemplars: pmetric.NewExemplarSlice(),
	maxExemplarCount: m.maxExemplarCount,
	startTimestamp: startTimestamp,
	isFirst: true,
	}
	m.metrics[key] = s
	}
	return s
	}

I found that https://pkg.go.dev/go.opentelemetry.io/collector/pdata/pcommon#Map is also a struct, so the attributes variable is passed by value when GetOrCreate is invoked.

It means that the attributes var in Sum struct have no relationship with the attributes value in metricKeyToDimensionsCache .

As per the analysis above, I believe that metricKeyToDimensionsCache can no longer help improve the collector's memory usage. We can use aggregation_cardinality_limit instead.

[github-actions]

Pinging code owners:

• connector/spanmetrics: @portertech @Frapschen @iblancasa

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