From a182728101d040f9a4c55358c1cd14c112cd6b3a Mon Sep 17 00:00:00 2001
From: Martin Sucha <martin.sucha@kiwi.com>
Date: Wed, 12 Jun 2019 13:16:07 +0200
Subject: [PATCH] dogstatsd: Drop metrics according to sample rate

Sample rate is for allowing to lower traffic generated by
the statsd packets so the observations need to be dropped.
statsd server will multiple the metrics with the inverse
of sample rate to compenstate.

Fixes #778
---
 metrics/dogstatsd/dogstatsd.go | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/metrics/dogstatsd/dogstatsd.go b/metrics/dogstatsd/dogstatsd.go
index bb2f003b3..d9618fd25 100644
--- a/metrics/dogstatsd/dogstatsd.go
+++ b/metrics/dogstatsd/dogstatsd.go
@@ -14,6 +14,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"math/rand"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -73,7 +74,7 @@ func (d *Dogstatsd) NewCounter(name string, sampleRate float64) *Counter {
 	d.rates.Set(name, sampleRate)
 	return &Counter{
 		name: name,
-		obs:  d.counters.Observe,
+		obs:  sampleObservations(d.counters.Observe, sampleRate),
 	}
 }
 
@@ -95,7 +96,7 @@ func (d *Dogstatsd) NewTiming(name string, sampleRate float64) *Timing {
 	d.rates.Set(name, sampleRate)
 	return &Timing{
 		name: name,
-		obs:  d.timings.Observe,
+		obs:  sampleObservations(d.timings.Observe, sampleRate),
 	}
 }
 
@@ -105,7 +106,7 @@ func (d *Dogstatsd) NewHistogram(name string, sampleRate float64) *Histogram {
 	d.rates.Set(name, sampleRate)
 	return &Histogram{
 		name: name,
-		obs:  d.histograms.Observe,
+		obs:  sampleObservations(d.histograms.Observe, sampleRate),
 	}
 }
 
@@ -239,6 +240,19 @@ func (d *Dogstatsd) tagValues(labelValues []string) string {
 
 type observeFunc func(name string, lvs lv.LabelValues, value float64)
 
+// sampleObservations returns a modified observeFunc that samples observations.
+func sampleObservations(obs observeFunc, sampleRate float64) observeFunc {
+	if sampleRate >= 1 {
+		return obs
+	}
+	return func(name string, lvs lv.LabelValues, value float64) {
+		if rand.Float64() > sampleRate {
+			return
+		}
+		obs(name, lvs, value)
+	}
+}
+
 // Counter is a DogStatsD counter. Observations are forwarded to a Dogstatsd
 // object, and aggregated (summed) per timeseries.
 type Counter struct {