metric: export quantiles from prometheus-based histogram

This change builds on the previous one and adds a function to export
quantiles from the Prometheus-based histogram. This functionality is
used to store histogram data in the internal timeseries database. The
hdr library came with a function to do this, while Prometheus does not
have a public API for exporting quantiles.

The function implemented here is very similar to the one found
internally in Prometheus, using linear interpolation to calculate values
at a given quantile.

This commit also includes some additional testing and general
refactoring of the metrics code.

Release note: None

pkg/server/status/BUILD.bazel

@@ -65,11 +65,8 @@ go_library(
         "//pkg/util/timeutil",
         "@com_github_cockroachdb_errors//:errors",
         "@com_github_cockroachdb_redact//:redact",
-        "@com_github_codahale_hdrhistogram//:hdrhistogram",
         "@com_github_dustin_go_humanize//:go-humanize",
         "@com_github_elastic_gosigar//:gosigar",
-        "@com_github_prometheus_client_golang//prometheus",
-        "@com_github_prometheus_client_model//go",
         "@com_github_shirou_gopsutil_v3//net",
     ] + select({
         "@io_bazel_rules_go//go/platform:aix": [

pkg/server/status/recorder.go

@@ -46,7 +46,6 @@ import (
 	"github.com/cockroachdb/redact"
 	"github.com/dustin/go-humanize"
 	"github.com/elastic/gosigar"
-	prometheusgo "github.com/prometheus/client_model/go"
 )
 
 const (
@@ -528,23 +527,7 @@ type registryRecorder struct {
 }
 
 func extractValue(name string, mtr interface{}, fn func(string, float64)) error {
-	// TODO(tschottdorf,ajwerner): consider moving this switch to a single
-	// interface implemented by the individual metric types.
-	type (
-		float64Valuer          interface{ Value() float64 }
-		int64Valuer            interface{ Value() int64 }
-		int64Counter           interface{ Count() int64 }
-		prometheusMetricValuer interface{ ToPrometheusMetric() *prometheusgo.Metric }
-	)
 	switch mtr := mtr.(type) {
-	case float64:
-		fn(name, mtr)
-	case float64Valuer:
-		fn(name, mtr.Value())
-	case int64Valuer:
-		fn(name, float64(mtr.Value()))
-	case int64Counter:
-		fn(name, float64(mtr.Count()))
 	case *metric.Histogram:
 		// TODO(mrtracy): Where should this comment go for better
 		// visibility?
@@ -566,19 +549,24 @@ func extractValue(name string, mtr interface{}, fn func(string, float64)) error
 		}
 		fn(name+"-count", float64(curr.TotalCount()))
 	case *metric.HistogramV2:
-		// NB: this branch is intentionally at the bottom since all metrics implement it.
-		cur := mtr.Windowed()
-		var m prometheusgo.Metric
-		_ = cur.Write(&m)
-		hist := m.Histogram
-		n := float64(*hist.SampleCount)
+		n := float64(mtr.TotalCountWindowed())
 		fn(name+"-count", n)
-		fn(name+"-avg", *hist.SampleSum/n)
-		// TODO(obs-inf): add quantiles like for the hdrhistogram.
-	case prometheusMetricValuer:
-		// TODO we should be able to handle all non-histogram branches using this, i.e.
-		// can delete the float, int, etc, cases above.
-		_ = mtr.ToPrometheusMetric()
+		avg := mtr.TotalSumWindowed() / n
+		if n == 0 {
+			avg = 0
+		}
+		fn(name+"-avg", avg)
+		for _, pt := range recordHistogramQuantiles {
+			fn(name+pt.suffix, mtr.ValueAtQuantileWindowed(pt.quantile))
+		}
+	case metric.PrometheusExportable:
+		// NB: this branch is intentionally at the bottom since all metrics implement it.
+		m := mtr.ToPrometheusMetric()
+		if m.Gauge != nil {
+			fn(name, *m.Gauge.Value)
+		} else if m.Counter != nil {
+			fn(name, *m.Counter.Value)
+		}
 
 	default:
 		return errors.Errorf("cannot extract value for type %T", mtr)

pkg/util/metric/BUILD.bazel

@@ -51,6 +51,7 @@ go_test(
     deps = [
         "//pkg/util/log",
         "@com_github_kr_pretty//:pretty",
+        "@com_github_prometheus_client_golang//prometheus",
         "@com_github_prometheus_client_model//go",
         "@com_github_stretchr_testify//require",
     ],

pkg/util/metric/metric.go

@@ -13,6 +13,7 @@ package metric
 import (
 	"encoding/json"
 	"math"
+	"sort"
 	"sync/atomic"
 	"time"
 
@@ -231,6 +232,7 @@ func NewLatency(metadata Metadata, histogramWindow time.Duration) *Histogram {
 func (h *Histogram) Windowed() (*hdrhistogram.Histogram, time.Duration) {
 	h.mu.Lock()
 	defer h.mu.Unlock()
+	maybeTick(h.mu.tickHelper)
 	// TODO(obs-inf): not sure we should multiply by histWrapNum here, but it
 	// has been the behavior for a long time.
 	return cloneHistogram(h.mu.sliding.Merge()), histWrapNum * h.mu.tickInterval
@@ -404,6 +406,10 @@ func NewHistogramV2(
 var _ periodic = (*HistogramV2)(nil)
 var _ PrometheusExportable = (*HistogramV2)(nil)
 
+// HistogramV2 is a prometheus-backed histogram. It collects observed values by
+// keeping bucketed counts. For convenience, internally two sets of buckets are
+// kept: A cumulative set (i.e. data is never evicted) and a windowed set (which
+// keeps only recently collected samples).
 type HistogramV2 struct {
 	Metadata
 	cum prometheus.Histogram
@@ -440,26 +446,29 @@ func (h *HistogramV2) tick() {
 	h.windowed.tick()
 }
 
+// Windowed returns a copy of the current windowed histogram.
 func (h *HistogramV2) Windowed() prometheus.Histogram {
 	h.windowed.RLock()
 	defer h.windowed.RUnlock()
 	return h.windowed.cur
 }
 
+// RecordValue adds the given value to the histogram.
 func (h *HistogramV2) RecordValue(n int64) {
 	v := float64(n)
 	h.cum.Observe(v)
 
 	h.windowed.RLock()
 	defer h.windowed.RUnlock()
 	h.windowed.cur.Observe(v)
-
 }
 
+// GetType returns the prometheus type enum for this metric.
 func (h *HistogramV2) GetType() *prometheusgo.MetricType {
 	return prometheusgo.MetricType_HISTOGRAM.Enum()
 }
 
+// ToPrometheusMetric returns a filled-in prometheus metric of the right type.
 func (h *HistogramV2) ToPrometheusMetric() *prometheusgo.Metric {
 	m := &prometheusgo.Metric{}
 	if err := h.cum.Write(m); err != nil {
@@ -468,17 +477,89 @@ func (h *HistogramV2) ToPrometheusMetric() *prometheusgo.Metric {
 	return m
 }
 
+// ToPrometheusMetricWindowed returns a filled-in prometheus metric of the right type.
+func (h *HistogramV2) ToPrometheusMetricWindowed() *prometheusgo.Metric {
+	h.windowed.Lock()
+	defer h.windowed.Unlock()
+	m := &prometheusgo.Metric{}
+	if err := h.windowed.cur.Write(m); err != nil {
+		panic(err) // TODD
+	}
+	return m
+}
+
+// GetMetadata returns the metric's metadata including the Prometheus
+// MetricType.
 func (h *HistogramV2) GetMetadata() Metadata {
 	return h.Metadata
 }
 
+// Inspect calls the closure.
 func (h *HistogramV2) Inspect(f func(interface{})) {
 	h.windowed.Lock()
 	maybeTick(&h.windowed)
 	h.windowed.Unlock()
 	f(h)
 }
 
+// TotalCount returns the (cumulative) number of samples.
+func (h *HistogramV2) TotalCount() int64 {
+	return int64(h.ToPrometheusMetric().Histogram.GetSampleCount())
+}
+
+// TotalCountWindowed returns the number of samples in the current window.
+func (h *HistogramV2) TotalCountWindowed() int64 {
+	return int64(h.ToPrometheusMetricWindowed().Histogram.GetSampleCount())
+}
+
+// TotalSum returns the (cumulative) number of samples.
+func (h *HistogramV2) TotalSum() float64 {
+	return h.ToPrometheusMetric().Histogram.GetSampleSum()
+}
+
+// TotalSumWindowed returns the number of samples in the current window.
+func (h *HistogramV2) TotalSumWindowed() float64 {
+	return h.ToPrometheusMetricWindowed().Histogram.GetSampleSum()
+}
+
+// ValueAtQuantileWindowed takes a quantile value [0,100] and returns the
+// interpolated value at that quantile for the windowed histogram.
+//
+// https://github.com/prometheus/prometheus/blob/d91621890a2ccb3191a6d74812cc1827dd4093bf/promql/quantile.go#L75
+// This function is mostly taken from a prometheus internal function that
+// does the same thing. There are a few differences for our use case:
+// 		1. As a user of the prometheus go client library, we don't have access
+//			 to the implicit +Inf bucket, so we don't need special cases to deal
+//			 with the quantiles that include the +Inf bucket.
+//		2. Since the prometheus client library ensures buckets are in a strictly
+//			 increasing order at creation, we do not sort them.
+func (h *HistogramV2) ValueAtQuantileWindowed(q float64) float64 {
+	m := h.ToPrometheusMetricWindowed()
+
+	buckets := m.Histogram.Bucket
+	n := float64(*m.Histogram.SampleCount)
+	if n == 0 {
+		return 0
+	}
+
+	rank := uint64((q / 100) * n)
+	b := sort.Search(len(buckets)-1, func(i int) bool { return *buckets[i].CumulativeCount >= rank })
+
+	var (
+		bucketStart float64
+		bucketEnd   = *buckets[b].UpperBound
+		count       = *buckets[b].CumulativeCount
+	)
+
+	// Calculate the linearly interpolated value within the bucket
+	if b > 0 {
+		bucketStart = *buckets[b-1].UpperBound
+		count -= *buckets[b-1].CumulativeCount
+		rank -= *buckets[b-1].CumulativeCount
+	}
+	return bucketStart + (bucketEnd-bucketStart)*(float64(rank)/float64(count))
+}
+
 // A Counter holds a single mutable atomic value.
 type Counter struct {
 	Metadata

pkg/util/metric/metric_test.go

@@ -139,6 +139,72 @@ func TestHistogramPrometheus(t *testing.T) {
 	}
 }
 
+func TestHistogramV2(t *testing.T) {
+	u := func(v int) *uint64 {
+		n := uint64(v)
+		return &n
+	}
+
+	f := func(v int) *float64 {
+		n := float64(v)
+		return &n
+	}
+
+	h := NewHistogramV2(
+		Metadata{},
+		time.Hour,
+		prometheus.HistogramOpts{
+			Namespace:   "",
+			Subsystem:   "",
+			Name:        "",
+			Help:        "",
+			ConstLabels: nil,
+			Buckets: []float64{
+				1.0,
+				5.0,
+				10.0,
+				25.0,
+				100.0,
+			},
+		},
+	)
+
+	// should return 0 if no observations are made
+	require.Equal(t, 0.0, h.ValueAtQuantileWindowed(0))
+
+	measurements := []int64{0, 4, 5, 10, 20, 25, 30, 40, 90, 200}
+	var expSum float64
+	for _, m := range measurements {
+		h.RecordValue(m)
+		expSum += float64(m)
+	}
+
+	act := *h.ToPrometheusMetric().Histogram
+	exp := prometheusgo.Histogram{
+		SampleCount: u(len(measurements)),
+		SampleSum:   &expSum,
+		Bucket: []*prometheusgo.Bucket{
+			{CumulativeCount: u(1), UpperBound: f(1)},
+			{CumulativeCount: u(3), UpperBound: f(5)},
+			{CumulativeCount: u(4), UpperBound: f(10)},
+			{CumulativeCount: u(6), UpperBound: f(25)},
+			{CumulativeCount: u(9), UpperBound: f(100)},
+			// NB: 200 is greater than the largest defined bucket so prometheus
+			// puts it in an implicit bucket with +Inf as the upper bound.
+		},
+	}
+
+	if !reflect.DeepEqual(act, exp) {
+		t.Fatalf("expected differs from actual: %s", pretty.Diff(exp, act))
+	}
+
+	require.Equal(t, 0.0, h.ValueAtQuantileWindowed(0))
+	require.Equal(t, 1.0, h.ValueAtQuantileWindowed(10))
+	require.Equal(t, 17.5, h.ValueAtQuantileWindowed(50))
+	require.Equal(t, 75.0, h.ValueAtQuantileWindowed(80))
+	require.Equal(t, 125.0, h.ValueAtQuantileWindowed(100))
+}
+
 func TestHistogramBuckets(t *testing.T) {
 	verifyAndPrint := func(t *testing.T, exp, act []float64) {
 		t.Helper()