sdk/metric: Remove Reader.ForceFlush and ManualReader.ForceFlush (#4375)

CHANGELOG.md

@@ -45,6 +45,11 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - `PeriodicReader.Shutdown` and `PeriodicReader.ForceFlush` in `go.opentelemetry.io/otel/sdk/metric` now apply the periodic reader's timeout to the operation if the user provided context does not contain a deadline. (#4356, #4377)
 - Upgrade all use of `go.opentelemetry.io/otel/semconv` to use `v1.21.0`. (#4408)
 
+### Removed
+
+- Remove `Reader.ForceFlush` in `go.opentelemetry.io/otel/metric`.
+  Notice that `PeriodicReader.ForceFlush` is still available. (#4375)
+
 ### Fixed
 
 - Correctly format log messages from the `go.opentelemetry.io/otel/exporters/zipkin` exporter. (#4143)

sdk/metric/config.go

@@ -37,7 +37,9 @@ func (c config) readerSignals() (forceFlush, shutdown func(context.Context) erro
 	var fFuncs, sFuncs []func(context.Context) error
 	for _, r := range c.readers {
 		sFuncs = append(sFuncs, r.Shutdown)
-		fFuncs = append(fFuncs, r.ForceFlush)
+		if f, ok := r.(interface{ ForceFlush(context.Context) error }); ok {
+			fFuncs = append(fFuncs, f.ForceFlush)
+		}
 	}
 
 	return unify(fFuncs), unifyShutdown(sFuncs)

sdk/metric/manual_reader.go

@@ -91,13 +91,6 @@ func (mr *ManualReader) aggregation(kind InstrumentKind) aggregation.Aggregation
 	return mr.aggregationSelector(kind)
 }
 
-// ForceFlush is a no-op, it always returns nil.
-//
-// This method is safe to call concurrently.
-func (mr *ManualReader) ForceFlush(context.Context) error {
-	return nil
-}
-
 // Shutdown closes any connections and frees any resources used by the reader.
 //
 // This method is safe to call concurrently.

sdk/metric/periodic_reader_test.go

@@ -20,6 +20,7 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
 
 	"go.opentelemetry.io/otel"
@@ -198,7 +199,7 @@ func (e *fnExporter) Shutdown(ctx context.Context) error {
 type periodicReaderTestSuite struct {
 	*readerTestSuite
 
-	ErrReader Reader
+	ErrReader *PeriodicReader
 }
 
 func (ts *periodicReaderTestSuite) SetupTest() {
@@ -425,6 +426,15 @@ func TestPeriodicReaderFlushesPending(t *testing.T) {
 	})
 }
 
+func TestPeriodicReaderMultipleForceFlush(t *testing.T) {
+	ctx := context.Background()
+	r := NewPeriodicReader(new(fnExporter))
+	r.register(testSDKProducer{})
+	r.RegisterProducer(testExternalProducer{})
+	require.NoError(t, r.ForceFlush(ctx))
+	require.NoError(t, r.ForceFlush(ctx))
+}
+
 func BenchmarkPeriodicReader(b *testing.B) {
 	b.Run("Collect", benchReaderCollectFunc(
 		NewPeriodicReader(new(fnExporter)),

sdk/metric/provider.go

@@ -110,6 +110,9 @@ func (mp *MeterProvider) Meter(name string, options ...metric.MeterOption) metri
 // telemetry be flushed or all resources have been released in these
 // situations.
 //
+// ForceFlush calls ForceFlush(context.Context) error
+// on all Readers that implements this method.
+//
 // This method is safe to call concurrently.
 func (mp *MeterProvider) ForceFlush(ctx context.Context) error {
 	if mp.forceFlush != nil {

sdk/metric/reader.go

@@ -84,16 +84,6 @@ type Reader interface {
 	// passed context is expected to be honored.
 	Collect(ctx context.Context, rm *metricdata.ResourceMetrics) error
 
-	// ForceFlush flushes all metric measurements held in an export pipeline.
-	//
-	// This deadline or cancellation of the passed context are honored. An appropriate
-	// error will be returned in these situations. There is no guaranteed that all
-	// telemetry be flushed or all resources have been released in these
-	// situations.
-	//
-	// This method needs to be concurrent safe.
-	ForceFlush(context.Context) error
-
 	// Shutdown flushes all metric measurements held in an export pipeline and releases any
 	// held computational resources.
 	//

sdk/metric/reader_test.go

@@ -93,14 +93,6 @@ func (ts *readerTestSuite) TestShutdownTwice() {
 	ts.ErrorIs(ts.Reader.Shutdown(ctx), ErrReaderShutdown)
 }
 
-func (ts *readerTestSuite) TestMultipleForceFlush() {
-	ctx := context.Background()
-	ts.Reader.register(testSDKProducer{})
-	ts.Reader.RegisterProducer(testExternalProducer{})
-	ts.Require().NoError(ts.Reader.ForceFlush(ctx))
-	ts.NoError(ts.Reader.ForceFlush(ctx))
-}
-
 func (ts *readerTestSuite) TestMultipleRegister() {
 	p0 := testSDKProducer{
 		produceFunc: func(ctx context.Context, rm *metricdata.ResourceMetrics) error {
@@ -186,11 +178,13 @@ func (ts *readerTestSuite) TestMethodConcurrentSafe() {
 			_ = ts.Reader.Collect(ctx, nil)
 		}()
 
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
-			_ = ts.Reader.ForceFlush(ctx)
-		}()
+		if f, ok := ts.Reader.(interface{ ForceFlush(context.Context) error }); ok {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				_ = f.ForceFlush(ctx)
+			}()
+		}
 
 		wg.Add(1)
 		go func() {