Tracing: make sampling frequency configurable

tracing/tracing.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
+	"strconv"
 	"sync"
 
 	openzipkin "github.com/openzipkin/zipkin-go"
@@ -31,6 +32,7 @@ const globalTracingConfigPath = "/etc/tracing/client.conf"
 
 type tracingConfig struct {
 	ReportURL string `json:"report_url"`
+	Sample    string `json:"sample"`
 }
 
 // Read the global tracing configuration file. Its location is
@@ -100,9 +102,23 @@ func initTracing(endpointAddr string) {
 
 		reporter := zipkinHTTP.NewReporter(config.ReportURL)
 		ze := zipkin.NewExporter(reporter, localEndpoint)
-
 		trace.RegisterExporter(ze)
-		trace.ApplyConfig(trace.Config{DefaultSampler: trace.AlwaysSample()})
+
+		var tc trace.Config
+		switch config.Sample {
+		case "", "always":
+			tc.DefaultSampler = trace.AlwaysSample()
+		case "never":
+			tc.DefaultSampler = trace.NeverSample()
+		default:
+			frac, err := strconv.ParseFloat(config.Sample, 64)
+			if err != nil {
+				log.Printf("warning: error in tracing configuration: sample: %v, tracing disabled", err)
+				return
+			}
+			tc.DefaultSampler = trace.ProbabilitySampler(frac)
+		}
+		trace.ApplyConfig(tc)
 
 		log.Printf("tracing enabled (report_url %s)", config.ReportURL)