Make it possible to create constant summaries and histograms from static values.

prometheus/examples_test.go

@@ -455,6 +455,56 @@ func ExampleSummaryVec() {
 	// ]
 }
 
+func ExampleConstSummary() {
+	desc := prometheus.NewDesc(
+		"http_request_duration_seconds",
+		"A summary of the HTTP request durations.",
+		[]string{"code", "method"},
+		prometheus.Labels{"owner": "example"},
+	)
+
+	// Create a constant summary from values we got from a 3rd party telemetry system.
+	s := prometheus.MustNewConstSummary(
+		desc,
+		4711, 403.34,
+		map[float64]float64{0.5: 42.3, 0.9: 323.3},
+		"200", "get",
+	)
+
+	// Just for demonstration, let's check the state of the summary by
+	// (ab)using its Write method (which is usually only used by Prometheus
+	// internally).
+	metric := &dto.Metric{}
+	s.Write(metric)
+	fmt.Println(proto.MarshalTextString(metric))
+
+	// Output:
+	// label: <
+	//   name: "code"
+	//   value: "200"
+	// >
+	// label: <
+	//   name: "method"
+	//   value: "get"
+	// >
+	// label: <
+	//   name: "owner"
+	//   value: "example"
+	// >
+	// summary: <
+	//   sample_count: 4711
+	//   sample_sum: 403.34
+	//   quantile: <
+	//     quantile: 0.5
+	//     value: 42.3
+	//   >
+	//   quantile: <
+	//     quantile: 0.9
+	//     value: 323.3
+	//   >
+	// >
+}
+
 func ExampleHistogram() {
 	temps := prometheus.NewHistogram(prometheus.HistogramOpts{
 		Name:    "pond_temperature_celsius",
@@ -501,6 +551,64 @@ func ExampleHistogram() {
 	// >
 }
 
+func ExampleConstHistogram() {
+	desc := prometheus.NewDesc(
+		"http_request_duration_seconds",
+		"A histogram of the HTTP request durations.",
+		[]string{"code", "method"},
+		prometheus.Labels{"owner": "example"},
+	)
+
+	// Create a constant histogram from values we got from a 3rd party telemetry system.
+	h := prometheus.MustNewConstHistogram(
+		desc,
+		4711, 403.34,
+		map[float64]uint64{25: 121, 50: 2403, 100: 3221, 200: 4233},
+		"200", "get",
+	)
+
+	// Just for demonstration, let's check the state of the histogram by
+	// (ab)using its Write method (which is usually only used by Prometheus
+	// internally).
+	metric := &dto.Metric{}
+	h.Write(metric)
+	fmt.Println(proto.MarshalTextString(metric))
+
+	// Output:
+	// label: <
+	//   name: "code"
+	//   value: "200"
+	// >
+	// label: <
+	//   name: "method"
+	//   value: "get"
+	// >
+	// label: <
+	//   name: "owner"
+	//   value: "example"
+	// >
+	// histogram: <
+	//   sample_count: 4711
+	//   sample_sum: 403.34
+	//   bucket: <
+	//     cumulative_count: 121
+	//     upper_bound: 25
+	//   >
+	//   bucket: <
+	//     cumulative_count: 2403
+	//     upper_bound: 50
+	//   >
+	//   bucket: <
+	//     cumulative_count: 3221
+	//     upper_bound: 100
+	//   >
+	//   bucket: <
+	//     cumulative_count: 4233
+	//     upper_bound: 200
+	//   >
+	// >
+}
+
 func ExamplePushCollectors() {
 	hostname, _ := os.Hostname()
 	completionTime := prometheus.NewGauge(prometheus.GaugeOpts{

prometheus/histogram.go

@@ -342,3 +342,102 @@ func (m *HistogramVec) WithLabelValues(lvs ...string) Histogram {
 func (m *HistogramVec) With(labels Labels) Histogram {
 	return m.MetricVec.With(labels).(Histogram)
 }
+
+type constHistogram struct {
+	desc       *Desc
+	count      uint64
+	sum        float64
+	buckets    map[float64]uint64
+	labelPairs []*dto.LabelPair
+}
+
+func (h *constHistogram) Desc() *Desc {
+	return h.desc
+}
+
+func (h *constHistogram) Write(out *dto.Metric) error {
+	his := &dto.Histogram{}
+	buckets := make([]*dto.Bucket, 0, len(h.buckets))
+
+	his.SampleCount = proto.Uint64(h.count)
+	his.SampleSum = proto.Float64(h.sum)
+
+	for upperBound, count := range h.buckets {
+		buckets = append(buckets, &dto.Bucket{
+			CumulativeCount: proto.Uint64(count),
+			UpperBound:      proto.Float64(upperBound),
+		})
+	}
+
+	if len(buckets) > 0 {
+		sort.Sort(buckSort(buckets))
+	}
+	his.Bucket = buckets
+
+	out.Histogram = his
+	out.Label = h.labelPairs
+
+	return nil
+}
+
+// NewConstHistogram returns a metric representing a Prometheus histogram with
+// fixed values for the count, sum, and bucket counts. As those parameters
+// cannot be changed, the returned value does not implement the Histogram
+// interface (but only the Metric interface). Users of this package will not
+// have much use for it in regular operations. However, when implementing custom
+// Collectors, it is useful as a throw-away metric that is generated on the fly
+// to send it to Prometheus in the Collect method.
+//
+// buckets is a map of upper bounds to cumulative counts, excluding the +Inf
+// bucket.
+//
+// NewConstHistogram returns an error if the length of labelValues is not
+// consistent with the variable labels in Desc.
+func NewConstHistogram(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	buckets map[float64]uint64,
+	labelValues ...string,
+) (Metric, error) {
+	if len(desc.variableLabels) != len(labelValues) {
+		return nil, errInconsistentCardinality
+	}
+	return &constHistogram{
+		desc:       desc,
+		count:      count,
+		sum:        sum,
+		buckets:    buckets,
+		labelPairs: makeLabelPairs(desc, labelValues),
+	}, nil
+}
+
+// MustNewConstHistogram is a version of NewConstHistogram that panics where
+// NewConstMetric would have returned an error.
+func MustNewConstHistogram(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	buckets map[float64]uint64,
+	labelValues ...string,
+) Metric {
+	m, err := NewConstHistogram(desc, count, sum, buckets, labelValues...)
+	if err != nil {
+		panic(err)
+	}
+	return m
+}
+
+type buckSort []*dto.Bucket
+
+func (s buckSort) Len() int {
+	return len(s)
+}
+
+func (s buckSort) Swap(i, j int) {
+	s[i], s[j] = s[j], s[i]
+}
+
+func (s buckSort) Less(i, j int) bool {
+	return s[i].GetUpperBound() < s[j].GetUpperBound()
+}

prometheus/summary.go

@@ -448,3 +448,89 @@ func (m *SummaryVec) WithLabelValues(lvs ...string) Summary {
 func (m *SummaryVec) With(labels Labels) Summary {
 	return m.MetricVec.With(labels).(Summary)
 }
+
+type constSummary struct {
+	desc       *Desc
+	count      uint64
+	sum        float64
+	quantiles  map[float64]float64
+	labelPairs []*dto.LabelPair
+}
+
+func (s *constSummary) Desc() *Desc {
+	return s.desc
+}
+
+func (s *constSummary) Write(out *dto.Metric) error {
+	sum := &dto.Summary{}
+	qs := make([]*dto.Quantile, 0, len(s.quantiles))
+
+	sum.SampleCount = proto.Uint64(s.count)
+	sum.SampleSum = proto.Float64(s.sum)
+
+	for rank, q := range s.quantiles {
+		qs = append(qs, &dto.Quantile{
+			Quantile: proto.Float64(rank),
+			Value:    proto.Float64(q),
+		})
+	}
+
+	if len(qs) > 0 {
+		sort.Sort(quantSort(qs))
+	}
+	sum.Quantile = qs
+
+	out.Summary = sum
+	out.Label = s.labelPairs
+
+	return nil
+}
+
+// NewConstSummary returns a metric representing a Prometheus summary with fixed
+// values for the count, sum, and quantiles. As those parameters cannot be
+// changed, the returned value does not implement the Summary interface (but
+// only the Metric interface). Users of this package will not have much use for
+// it in regular operations. However, when implementing custom Collectors, it is
+// useful as a throw-away metric that is generated on the fly to send it to
+// Prometheus in the Collect method.
+//
+// quantiles maps ranks to quantile values. For example, a median latency of
+// 0.23s and a 99th percentile latency of 0.56s would be expressed as:
+//     map[float64]float64{0.5: 0.23, 0.99: 0.56}
+//
+// NewConstSummary returns an error if the length of labelValues is not
+// consistent with the variable labels in Desc.
+func NewConstSummary(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	quantiles map[float64]float64,
+	labelValues ...string,
+) (Metric, error) {
+	if len(desc.variableLabels) != len(labelValues) {
+		return nil, errInconsistentCardinality
+	}
+	return &constSummary{
+		desc:       desc,
+		count:      count,
+		sum:        sum,
+		quantiles:  quantiles,
+		labelPairs: makeLabelPairs(desc, labelValues),
+	}, nil
+}
+
+// MustNewConstSummary is a version of NewConstSummary that panics where
+// NewConstMetric would have returned an error.
+func MustNewConstSummary(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	quantiles map[float64]float64,
+	labelValues ...string,
+) Metric {
+	m, err := NewConstSummary(desc, count, sum, quantiles, labelValues...)
+	if err != nil {
+		panic(err)
+	}
+	return m
+}