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refactor(metrics): single source of truth for summary quantile ranks/labels #791

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leoparente merged 1 commit into from
Jun 26, 2026
Merged

refactor(metrics): single source of truth for summary quantile ranks/labels #791

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59 changes: 35 additions & 24 deletions src/Metrics.h
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@
#ifdef __GNUC__
#pragma GCC diagnostic pop
#endif
#include <array>
#include <chrono>
#include <math.h>
#include <regex>
Expand Down Expand Up @@ -60,10 +61,30 @@ static inline uint64_t timespec_to_uint64(timespec &stamp)
return stamp.tv_sec * 1000000000ULL + stamp.tv_nsec;
}

// Single source of truth for the SLA quantiles pktvisor reports, in the order
// get_quantiles() produces them. Each emit path (JSON, Prometheus, OpenTelemetry)
// derives its label/rank from this table, so values and labels can never desync.
struct QuantileSpec {
double rank; // normalized rank, used to query the sketch and for OTEL
const char *json_key; // to_json field name, e.g. "p50"
const char *prom_label; // Prometheus/summary "quantile" label, e.g. "0.5"
};
static constexpr std::array<QuantileSpec, 4> QUANTILES{{
{0.50, "p50", "0.5"},
{0.90, "p90", "0.9"},
{0.95, "p95", "0.95"},
{0.99, "p99", "0.99"},
}};

template <typename T>
static inline std::vector<T> get_quantiles(const datasketches::kll_sketch<T> &quatile)
static inline std::vector<T> get_quantiles(const datasketches::kll_sketch<T> &quantile)
{
return {quatile.get_quantile(0.50), quatile.get_quantile(0.90), quatile.get_quantile(0.95), quatile.get_quantile(0.99)};
std::vector<T> quantiles;
quantiles.reserve(QUANTILES.size());
for (const auto &q : QUANTILES) {
quantiles.push_back(quantile.get_quantile(q.rank));
}
return quantiles;
}

class Metric
Expand Down Expand Up @@ -362,7 +383,7 @@ class Quantile final : public Metric
if (_quantiles_sum.empty()) {
_quantiles_sum = get_quantiles(_quantile);
}
for (uint8_t i = 0; i < 4; i++) {
for (size_t i = 0; i < QUANTILES.size(); i++) {
_quantiles_sum[i] += other_quantiles[i];
}
} else {
Expand Down Expand Up @@ -405,10 +426,9 @@ class Quantile final : public Metric
}

if (quantiles.size()) {
name_json_assign(j, {"p50"}, quantiles[0]);
name_json_assign(j, {"p90"}, quantiles[1]);
name_json_assign(j, {"p95"}, quantiles[2]);
name_json_assign(j, {"p99"}, quantiles[3]);
for (size_t i = 0; i < QUANTILES.size(); i++) {
name_json_assign(j, {QUANTILES[i].json_key}, quantiles[i]);
}
}
}

Expand All @@ -425,21 +445,13 @@ class Quantile final : public Metric
quantiles = _quantiles_sum;
}

LabelMap l5(add_labels);
l5["quantile"] = "0.5";
LabelMap l9(add_labels);
l9["quantile"] = "0.9";
LabelMap l95(add_labels);
l95["quantile"] = "0.95";
LabelMap l99(add_labels);
l99["quantile"] = "0.99";

if (quantiles.size()) {
const auto base = base_name_snake();
ser.write(base, PrometheusSerializer::Type::Summary, _desc, {}, l5, quantiles[0]);
ser.write(base, PrometheusSerializer::Type::Summary, _desc, {}, l9, quantiles[1]);
ser.write(base, PrometheusSerializer::Type::Summary, _desc, {}, l95, quantiles[2]);
ser.write(base, PrometheusSerializer::Type::Summary, _desc, {}, l99, quantiles[3]);
for (size_t i = 0; i < QUANTILES.size(); i++) {
LabelMap l(add_labels);
l["quantile"] = QUANTILES[i].prom_label;
ser.write(base, PrometheusSerializer::Type::Summary, _desc, {}, l, quantiles[i]);
}
ser.write(base, PrometheusSerializer::Type::Summary, _desc, {"sum"}, add_labels, _quantile.get_max_item());
ser.write(base, PrometheusSerializer::Type::Summary, _desc, {"count"}, add_labels, _quantile.get_n());
}
Expand All @@ -464,11 +476,10 @@ class Quantile final : public Metric
auto summary_data_point = metric->mutable_summary()->add_data_points();
summary_data_point->set_start_time_unix_nano(timespec_to_uint64(start));
summary_data_point->set_time_unix_nano(timespec_to_uint64(end));
const double fractions[4]{0.50, 0.90, 0.95, 0.99};
for (auto it = quantiles.begin(); it != quantiles.end(); ++it) {
for (size_t i = 0; i < QUANTILES.size(); i++) {
auto quantile = summary_data_point->add_quantile_values();
quantile->set_quantile(fractions[it - quantiles.begin()]);
quantile->set_value(*it);
quantile->set_quantile(QUANTILES[i].rank);
quantile->set_value(quantiles[i]);
}
for (const auto &label : add_labels) {
auto attribute = summary_data_point->add_attributes();
Expand Down
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