Add ForceFlush() method to SpanProcessor interface (#1166)

* Add ForceFlush() method to SpanProcessor interface

* Add a stub implementation to SimpleSpanProcessor
* Add a working implementation to BatchSpanProcessor

* add CHANGELOG.md entry

* Eliminate sleep from BatchSpanProcessor.ForceFlush() test

* Generating test spans serially should reduce test flakiness

CHANGELOG.md

@@ -14,6 +14,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - In the `go.opentelemetry.io/otel/api/trace` package, `NewTracerConfig` was added to construct new `TracerConfig`s.
    This addition was made to conform with our project option conventions. (#1155)
 - Instrumentation library information was added to the Zipkin exporter. (#1119)
+- The `SpanProcessor` interface now has a `ForceFlush()` method. (#1166)
 - More semantic conventions for k8s as resource attributes. (#1167)
 
 ### Changed

sdk/trace/batch_span_processor.go

@@ -67,6 +67,7 @@ type BatchSpanProcessor struct {
 	dropped uint32
 
 	batch      []*export.SpanData
+	batchMutex sync.Mutex
 	timer      *time.Timer
 	stopWait   sync.WaitGroup
 	stopOnce   sync.Once
@@ -131,6 +132,11 @@ func (bsp *BatchSpanProcessor) Shutdown() {
 	})
 }
 
+// ForceFlush exports all ended spans that have not yet been exported.
+func (bsp *BatchSpanProcessor) ForceFlush() {
+	bsp.exportSpans()
+}
+
 func WithMaxQueueSize(size int) BatchSpanProcessorOption {
 	return func(o *BatchSpanProcessorOptions) {
 		o.MaxQueueSize = size
@@ -159,6 +165,9 @@ func WithBlocking() BatchSpanProcessorOption {
 func (bsp *BatchSpanProcessor) exportSpans() {
 	bsp.timer.Reset(bsp.o.BatchTimeout)
 
+	bsp.batchMutex.Lock()
+	defer bsp.batchMutex.Unlock()
+
 	if len(bsp.batch) > 0 {
 		if err := bsp.e.ExportSpans(context.Background(), bsp.batch); err != nil {
 			global.Handle(err)
@@ -180,8 +189,11 @@ func (bsp *BatchSpanProcessor) processQueue() {
 		case <-bsp.timer.C:
 			bsp.exportSpans()
 		case sd := <-bsp.queue:
+			bsp.batchMutex.Lock()
 			bsp.batch = append(bsp.batch, sd)
-			if len(bsp.batch) == bsp.o.MaxExportBatchSize {
+			shouldExport := len(bsp.batch) == bsp.o.MaxExportBatchSize
+			bsp.batchMutex.Unlock()
+			if shouldExport {
 				if !bsp.timer.Stop() {
 					<-bsp.timer.C
 				}
@@ -202,8 +214,12 @@ func (bsp *BatchSpanProcessor) drainQueue() {
 				return
 			}
 
+			bsp.batchMutex.Lock()
 			bsp.batch = append(bsp.batch, sd)
-			if len(bsp.batch) == bsp.o.MaxExportBatchSize {
+			shouldExport := len(bsp.batch) == bsp.o.MaxExportBatchSize
+			bsp.batchMutex.Unlock()
+
+			if shouldExport {
 				bsp.exportSpans()
 			}
 		default:

sdk/trace/batch_span_processor_test.go

@@ -64,6 +64,7 @@ func TestNewBatchSpanProcessorWithNilExporter(t *testing.T) {
 	// These should not panic.
 	bsp.OnStart(&export.SpanData{})
 	bsp.OnEnd(&export.SpanData{})
+	bsp.ForceFlush()
 	bsp.Shutdown()
 }
 
@@ -180,6 +181,53 @@ func TestNewBatchSpanProcessorWithOptions(t *testing.T) {
 	}
 }
 
+func TestBatchSpanProcessorForceFlush(t *testing.T) {
+	option := testOption{
+		name: "ForceFlush()",
+		o: []sdktrace.BatchSpanProcessorOption{
+			sdktrace.WithBatchTimeout(10 * time.Second),
+			sdktrace.WithMaxQueueSize(2000),
+			sdktrace.WithMaxExportBatchSize(2000),
+		},
+		wantNumSpans:   205,
+		wantBatchCount: 1,
+		genNumSpans:    205,
+	}
+
+	te := testBatchExporter{}
+	tp := basicProvider(t)
+	ssp := createAndRegisterBatchSP(option, &te)
+	if ssp == nil {
+		t.Fatalf("%s: Error creating new instance of BatchSpanProcessor\n", option.name)
+	}
+	tp.RegisterSpanProcessor(ssp)
+	tr := tp.Tracer("BatchSpanProcessorWithOptions")
+
+	generateSpan(t, false, tr, option)
+
+	ssp.ForceFlush()
+
+	gotNumOfSpans := te.len()
+	if 0 == gotNumOfSpans {
+		t.Errorf("number of flushed spans is zero")
+	}
+
+	tp.UnregisterSpanProcessor(ssp)
+
+	gotNumOfSpans = te.len()
+	if option.wantNumSpans != gotNumOfSpans {
+		t.Errorf("number of exported span: got %+v, want %+v\n",
+			gotNumOfSpans, option.wantNumSpans)
+	}
+
+	gotBatchCount := te.getBatchCount()
+	if gotBatchCount < option.wantBatchCount {
+		t.Errorf("number batches: got %+v, want >= %+v\n",
+			gotBatchCount, option.wantBatchCount)
+		t.Errorf("Batches %v\n", te.sizes)
+	}
+}
+
 func createAndRegisterBatchSP(option testOption, te *testBatchExporter) *sdktrace.BatchSpanProcessor {
 	// Always use blocking queue to avoid flaky tests.
 	options := append(option.o, sdktrace.WithBlocking())

sdk/trace/simple_span_processor.go

@@ -54,3 +54,7 @@ func (ssp *SimpleSpanProcessor) OnEnd(sd *export.SpanData) {
 // Shutdown method does nothing. There is no data to cleanup.
 func (ssp *SimpleSpanProcessor) Shutdown() {
 }
+
+// ForceFlush does nothing as there is no data to flush.
+func (ssp *SimpleSpanProcessor) ForceFlush() {
+}

sdk/trace/span_processor.go

@@ -35,6 +35,12 @@ type SpanProcessor interface {
 	// data. No calls to OnStart and OnEnd method is invoked after Shutdown call is
 	// made. It should not be blocked indefinitely.
 	Shutdown()
+
+	// ForceFlush exports all ended spans to the configured Exporter that have not yet
+	// been exported.  It should only be called when absolutely necessary, such as when
+	// using a FaaS provider that may suspend the process after an invocation, but before
+	// the Processor can export the completed spans.
+	ForceFlush()
 }
 
 type spanProcessorMap map[SpanProcessor]*sync.Once

sdk/trace/span_processor_test.go

@@ -39,6 +39,9 @@ func (t *testSpanProcesor) Shutdown() {
 	t.shutdownCount++
 }
 
+func (t *testSpanProcesor) ForceFlush() {
+}
+
 func TestRegisterSpanProcessort(t *testing.T) {
 	name := "Register span processor before span starts"
 	tp := basicProvider(t)