Update sample rate precision handling (#834)

* Update sample rate precision handling

We already had sampling rate rounding,
but it was using the older proposal of
3 decimal places, and did not clamp the
lower bound.

Updated to use the same logic in both the
ratio sampler, and the implementation-
independent interpretation of the sample
rate.

Also, use the "round" function instead
of math.Round directly, to support older
versions of Go.

* scripts: pin ugorji/go

sampler.go

@@ -62,7 +62,15 @@ type SampleResult struct {
 	Sampled bool
 
 	// SampleRate holds the sample rate in effect at the
-	// time of the sampling decision.
+	// time of the sampling decision. This is used for
+	// propagating the value downstream, and for inclusion
+	// in events sent to APM Server.
+	//
+	// The sample rate will be rounded to 4 decimal places
+	// half away from zero, except if it is in the interval
+	// (0, 0.0001], in which case it is set to 0.0001. The
+	// Sampler implementation should also internally apply
+	// this logic to ensure consistency.
 	SampleRate float64
 }
 
@@ -82,10 +90,7 @@ func NewRatioSampler(r float64) Sampler {
 	if r < 0 || r > 1.0 {
 		panic(errors.Errorf("ratio %v out of range [0,1.0]", r))
 	}
-	if r > 0 && r < 0.0001 {
-		r = 0.0001
-	}
-	r = math.Round(r*10000) / 10000
+	r = roundSampleRate(r)
 	var x big.Float
 	x.SetUint64(math.MaxUint64)
 	x.Mul(&x, big.NewFloat(r))
@@ -114,3 +119,12 @@ func (s ratioSampler) SampleExtended(args SampleParams) SampleResult {
 	}
 	return result
 }
+
+// roundSampleRate rounds r to 4 decimal places half away from zero,
+// with the exception of values > 0 and < 0.0001, which are set to 0.0001.
+func roundSampleRate(r float64) float64 {
+	if r > 0 && r < 0.0001 {
+		r = 0.0001
+	}
+	return round(r*10000) / 10000
+}

scripts/before_install.sh

@@ -31,6 +31,7 @@ if (! go run scripts/mingoversion.go 1.11 &>/dev/null); then
   pin github.com/elastic/go-sysinfo v1.3.0
   pin google.golang.org/grpc v1.30.0 https://github.com/grpc/grpc-go
   pin github.com/jinzhu/gorm v1.9.16
+  pin github.com/ugorji/go v1.1.10
 fi
 
 if (! go run scripts/mingoversion.go 1.10 &>/dev/null); then

span_test.go

@@ -150,7 +150,7 @@ func TestTracerStartSpanIDSpecified(t *testing.T) {
 func TestSpanSampleRate(t *testing.T) {
 	tracer := apmtest.NewRecordingTracer()
 	defer tracer.Close()
-	tracer.SetSampler(apm.NewRatioSampler(0.5555))
+	tracer.SetSampler(apm.NewRatioSampler(0.55555))
 
 	tx := tracer.StartTransactionOptions("name", "type", apm.TransactionOptions{
 		// Use a known transaction ID for deterministic sampling.
@@ -164,7 +164,7 @@ func TestSpanSampleRate(t *testing.T) {
 	tracer.Flush(nil)
 
 	payloads := tracer.Payloads()
-	assert.Equal(t, 0.556, *payloads.Transactions[0].SampleRate)
-	assert.Equal(t, 0.556, *payloads.Spans[0].SampleRate)
-	assert.Equal(t, 0.556, *payloads.Spans[1].SampleRate)
+	assert.Equal(t, 0.5556, *payloads.Transactions[0].SampleRate)
+	assert.Equal(t, 0.5556, *payloads.Spans[0].SampleRate)
+	assert.Equal(t, 0.5556, *payloads.Spans[1].SampleRate)
 }

tracecontext.go

@@ -330,8 +330,8 @@ func (e *TraceStateEntry) validateValue() error {
 }
 
 func formatElasticTracestateValue(sampleRate float64) string {
-	// 0      -> "s:0"
-	// 1      -> "s:1"
-	// 0.5555 -> "s:0.555" (any rounding should be applied prior)
-	return fmt.Sprintf("s:%.3g", sampleRate)
+	// 0       -> "s:0"
+	// 1       -> "s:1"
+	// 0.55555 -> "s:0.5555" (any rounding should be applied prior)
+	return fmt.Sprintf("s:%.4g", sampleRate)
 }

transaction.go

@@ -110,7 +110,7 @@ func (t *Tracer) StartTransactionOptions(name, transactionType string, opts Tran
 				// we will scale the sampled transactions.
 				result.SampleRate = 0
 			}
-			sampleRate := round(1000*result.SampleRate) / 1000
+			sampleRate := roundSampleRate(result.SampleRate)
 			tx.traceContext.State = NewTraceState(TraceStateEntry{
 				Key:   elasticTracestateVendorKey,
 				Value: formatElasticTracestateValue(sampleRate),

transaction_test.go

@@ -184,7 +184,10 @@ func TestTransactionSampleRate(t *testing.T) {
 	tests := []test{
 		{0, 0, "es=s:0"},
 		{1, 1, "es=s:1"},
-		{0.5555, 0.556, "es=s:0.556"},
+		{0.00001, 0.0001, "es=s:0.0001"},
+		{0.55554, 0.5555, "es=s:0.5555"},
+		{0.55555, 0.5556, "es=s:0.5556"},
+		{0.55556, 0.5556, "es=s:0.5556"},
 	}
 	for _, test := range tests {
 		test := test // copy for closure
@@ -195,7 +198,7 @@ func TestTransactionSampleRate(t *testing.T) {
 			tracer.SetSampler(apm.NewRatioSampler(test.actualSampleRate))
 			tx := tracer.StartTransactionOptions("name", "type", apm.TransactionOptions{
 				// Use a known transaction ID for deterministic sampling.
-				TransactionID: apm.SpanID{1, 2, 3, 4, 5, 6, 7, 8},
+				TransactionID: apm.SpanID{0, 1, 2, 3, 4, 5, 6, 7},
 			})
 			tx.End()
 			tracer.Flush(nil)