Backport server: Add sampling rate to distributed tracing.

ExperimentalDistributedTracingSamplingRatePerMillion is the
number of samples to collect per million spans.
Defaults to 0.

Signed-off-by: James Blair <mail@jamesblair.net>

server/embed/config.go

@@ -365,6 +365,9 @@ type Config struct {
 	// that exist at the same time.
 	// Can only be used if ExperimentalEnableDistributedTracing is true.
 	ExperimentalDistributedTracingServiceInstanceID string `json:"experimental-distributed-tracing-instance-id"`
+	// ExperimentalDistributedTracingSamplingRatePerMillion is the number of samples to collect per million spans.
+	// Defaults to 0.
+	ExperimentalDistributedTracingSamplingRatePerMillion int `json:"experimental-distributed-tracing-sampling-rate"`
 
 	// Logger is logger options: currently only supports "zap".
 	// "capnslog" is removed in v3.5.
@@ -758,6 +761,13 @@ func (cfg *Config) Validate() error {
 		return fmt.Errorf("cipher suites cannot be configured when only TLS1.3 is enabled")
 	}
 
+	// Validate distributed tracing configuration but only if enabled.
+	if cfg.ExperimentalEnableDistributedTracing {
+		if err := validateTracingConfig(cfg.ExperimentalDistributedTracingSamplingRatePerMillion); err != nil {
+			return fmt.Errorf("distributed tracing configurition is not valid: (%v)", err)
+		}
+	}
+
 	return nil
 }
 

server/embed/config_tracing.go

@@ -16,6 +16,7 @@ package embed
 
 import (
 	"context"
+	"fmt"
 
 	"go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc"
 	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
@@ -26,6 +27,19 @@ import (
 	"go.uber.org/zap"
 )
 
+const maxSamplingRatePerMillion = 1000000
+
+func validateTracingConfig(samplingRate int) error {
+	if samplingRate < 0 {
+		return fmt.Errorf("tracing sampling rate must be positive")
+	}
+	if samplingRate > maxSamplingRatePerMillion {
+		return fmt.Errorf("tracing sampling rate must be less than %d", maxSamplingRatePerMillion)
+	}
+
+	return nil
+}
+
 func setupTracingExporter(ctx context.Context, cfg *Config) (exporter tracesdk.SpanExporter, options []otelgrpc.Option, err error) {
 	exporter, err = otlptracegrpc.New(ctx,
 		otlptracegrpc.WithInsecure(),
@@ -79,6 +93,14 @@ func setupTracingExporter(ctx context.Context, cfg *Config) (exporter tracesdk.S
 	return exporter, options, err
 }
 
+func determineSampler(samplingRate int) tracesdk.Sampler {
+	sampler := tracesdk.NeverSample()
+	if samplingRate == 0 {
+		return sampler
+	}
+	return tracesdk.TraceIDRatioBased(float64(samplingRate) / float64(maxSamplingRatePerMillion))
+}
+
 // As Tracing service Instance ID must be unique, it should
 // never use the empty default string value, it's set if
 // if it's a non empty string.

server/embed/config_tracing_test.go

@@ -0,0 +1,83 @@
+// Copyright 2021 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package embed
+
+import (
+	"testing"
+)
+
+const neverSampleDescription = "AlwaysOffSampler"
+
+func TestDetermineSampler(t *testing.T) {
+	tests := []struct {
+		name                   string
+		sampleRate             int
+		wantSamplerDescription string
+	}{
+		{
+			name:                   "sample rate is disabled",
+			sampleRate:             0,
+			wantSamplerDescription: neverSampleDescription,
+		},
+		{
+			name:                   "sample rate is 100",
+			sampleRate:             100,
+			wantSamplerDescription: "TraceIDRatioBased{0.0001}",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			sampler := determineSampler(tc.sampleRate)
+			if tc.wantSamplerDescription != sampler.Description() {
+				t.Errorf("tracing sampler was not as expected; expected sampler: %#+v, got sampler: %#+v", tc.wantSamplerDescription, sampler.Description())
+			}
+		})
+	}
+}
+
+func TestTracingConfig(t *testing.T) {
+	tests := []struct {
+		name       string
+		sampleRate int
+		wantErr    bool
+	}{
+		{
+			name:       "invalid - sample rate is less than 0",
+			sampleRate: -1,
+			wantErr:    true,
+		},
+		{
+			name:       "invalid - sample rate is more than allowed value",
+			sampleRate: maxSamplingRatePerMillion + 1,
+			wantErr:    true,
+		},
+		{
+			name:       "valid - sample rate is 100",
+			sampleRate: 100,
+			wantErr:    false,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			err := validateTracingConfig(tc.sampleRate)
+			if err == nil && tc.wantErr {
+				t.Errorf("expected error got (%v) error", err)
+			}
+			if err != nil && !tc.wantErr {
+				t.Errorf("expected no errors, got error: (%v)", err)
+			}
+		})
+	}
+}

server/etcdmain/config.go

@@ -276,6 +276,7 @@ func newConfig() *config {
 	fs.StringVar(&cfg.ec.ExperimentalDistributedTracingAddress, "experimental-distributed-tracing-address", embed.ExperimentalDistributedTracingAddress, "Address for distributed tracing used for OpenTelemetry Tracing (if enabled with experimental-enable-distributed-tracing flag).")
 	fs.StringVar(&cfg.ec.ExperimentalDistributedTracingServiceName, "experimental-distributed-tracing-service-name", embed.ExperimentalDistributedTracingServiceName, "Configures service name for distributed tracing to be used to define service name for OpenTelemetry Tracing (if enabled with experimental-enable-distributed-tracing flag). 'etcd' is the default service name. Use the same service name for all instances of etcd.")
 	fs.StringVar(&cfg.ec.ExperimentalDistributedTracingServiceInstanceID, "experimental-distributed-tracing-instance-id", "", "Configures service instance ID for distributed tracing to be used to define service instance ID key for OpenTelemetry Tracing (if enabled with experimental-enable-distributed-tracing flag). There is no default value set. This ID must be unique per etcd instance.")
+	fs.IntVar(&cfg.ec.ExperimentalDistributedTracingSamplingRatePerMillion, "experimental-distributed-tracing-sampling-rate", 0, "Number of samples to collect per million spans for OpenTelemetry Tracing (if enabled with experimental-enable-distributed-tracing flag).")
 
 	// auth
 	fs.StringVar(&cfg.ec.AuthToken, "auth-token", cfg.ec.AuthToken, "Specify auth token specific options.")

server/etcdmain/help.go

@@ -219,6 +219,8 @@ Experimental distributed tracing:
     Distributed tracing service name, must be same across all etcd instances.
   --experimental-distributed-tracing-instance-id ''
     Distributed tracing instance ID, must be unique per each etcd instance.
+  --experimental-distributed-tracing-sampling-rate '0'
+    Number of samples to collect per million spans for distributed tracing. Disabled by default.
 
 v2 Proxy (to be deprecated in v3.6):
   --proxy 'off'