trace: allow for concurrent uploads to Stackdriver (#246)

* trace: allow for concurrent uploads to Stackdriver

In situations where a large number of spans need to be exported from a
single instance of the exporter, bundle uploads are limited to using a
single goroutine. This limits the overall throughput of the exporter.

Make use of the NumberOfWorkers option to allow the exporter to use
multiple, concurrent goroutines to upload spans to Stackdriver.

Closes #245.

Signed-off-by: Nick Travers <n.e.travers@gmail.com>

* Update documentation

Signed-off-by: Nick Travers <n.e.travers@gmail.com>

* Count number of exported spans

Signed-off-by: Nick Travers <n.e.travers@gmail.com>

* Fix data race in test case

Signed-off-by: Nick Travers <n.e.travers@gmail.com>

stackdriver.go

@@ -259,8 +259,7 @@ type Options struct {
 	ReportingInterval time.Duration
 
 	// NumberOfWorkers sets the number of go rountines that send requests
-	// to Stackdriver Monitoring. This is only used for Proto metrics export
-	// for now. The minimum number of workers is 1.
+	// to Stackdriver Monitoring and Trace. The minimum number of workers is 1.
 	NumberOfWorkers int
 
 	// ResourceByDescriptor may be provided to supply monitored resource dynamically

trace.go

@@ -76,6 +76,9 @@ func newTraceExporterWithClient(o Options, c *tracingclient.Client) *traceExport
 	} else {
 		b.BundleCountThreshold = 50
 	}
+	if o.NumberOfWorkers > 0 {
+		b.HandlerLimit = o.NumberOfWorkers
+	}
 	// The measured "bytes" are not really bytes, see exportReceiver.
 	b.BundleByteThreshold = b.BundleCountThreshold * 200
 	b.BundleByteLimit = b.BundleCountThreshold * 1000

trace_test.go

@@ -17,6 +17,7 @@ package stackdriver
 import (
 	"context"
 	"fmt"
+	"sync"
 	"testing"
 	"time"
 
@@ -63,6 +64,72 @@ func TestBundling(t *testing.T) {
 	}
 }
 
+func TestBundling_ConcurrentExports(t *testing.T) {
+	workers := 2
+	spansPerWorker := 10
+	delay := 2 * time.Second
+	exporter := newTraceExporterWithClient(Options{
+		ProjectID:            "fakeProjectID",
+		BundleCountThreshold: spansPerWorker,
+		BundleDelayThreshold: delay,
+		NumberOfWorkers:      workers,
+	}, nil)
+
+	wg := sync.WaitGroup{}
+	waitCh := make(chan struct{})
+	wg.Add(workers)
+
+	var exportMap sync.Map // maintain a collection of the spans exported
+	exporter.uploadFn = func(spans []*tracepb.Span) {
+		for _, s := range spans {
+			exportMap.Store(s.SpanId, true)
+		}
+		wg.Done()
+
+		// Don't complete the function until the WaitGroup is done.
+		// This ensures the semaphore limiting the concurrent uploads is not
+		// released by one goroutine completing before the other.
+		wg.Wait()
+	}
+	trace.RegisterExporter(exporter)
+
+	totalSpans := workers * spansPerWorker
+	var expectedSpanIDs []string
+	go func() {
+		// Release enough spans to form two bundles
+		for i := 0; i < totalSpans; i++ {
+			_, span := trace.StartSpan(context.Background(), "span", trace.WithSampler(trace.AlwaysSample()))
+			expectedSpanIDs = append(expectedSpanIDs, span.SpanContext().SpanID.String())
+			span.End()
+		}
+
+		// Wait for the desired concurrency before completing
+		wg.Wait()
+		close(waitCh)
+	}()
+
+	select {
+	case <-waitCh:
+	case <-time.After(delay / 2): // fail before a time-based flush is triggered
+		t.Fatal("timed out waiting for concurrent uploads")
+	}
+
+	// all the spans are accounted for
+	var exportedSpans []string
+	exportMap.Range(func(key, value interface{}) bool {
+		exportedSpans = append(exportedSpans, key.(string))
+		return true
+	})
+	if len(exportedSpans) != totalSpans {
+		t.Errorf("got %d spans, want %d", len(exportedSpans), totalSpans)
+	}
+	for _, id := range expectedSpanIDs {
+		if _, ok := exportMap.Load(id); !ok {
+			t.Errorf("want %s; missing from exported spans", id)
+		}
+	}
+}
+
 func TestNewContext_Timeout(t *testing.T) {
 	e := newTraceExporterWithClient(Options{
 		Timeout: 10 * time.Millisecond,