Log records dropped by the BatchProcessor (#5276)

* Add dropped count to queue

* Log dropped records

* Add changelog entry

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

CHANGELOG.md

@@ -13,6 +13,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Add `RecordFactory` in `go.opentelemetry.io/otel/log/logtest` to facilitate testing the bridge implementations. (#5263)
 - Add `RecordFactory` in `go.opentelemetry.io/otel/sdk/log/logtest` to facilitate testing the exporter and processor implementations. (#5258)
 - Add example for `go.opentelemetry.io/otel/exporters/stdout/stdoutlog`. (#5242)
+- The count of dropped records from the `BatchProcessor` in `go.opentelemetry.io/otel/sdk/log` is logged. (#5276)
 
 ### Changed
 

sdk/log/batch.go

@@ -10,6 +10,8 @@ import (
 	"sync"
 	"sync/atomic"
 	"time"
+
+	"go.opentelemetry.io/otel/internal/global"
 )
 
 const (
@@ -148,6 +150,10 @@ func (b *BatchProcessor) poll(interval time.Duration) (done chan struct{}) {
 				return
 			}
 
+			if d := b.q.Dropped(); d > 0 {
+				global.Warn("dropped log records", "dropped", d)
+			}
+
 			qLen := b.q.TryDequeue(buf, func(r []Record) bool {
 				ok := b.exporter.EnqueueExport(r)
 				if ok {
@@ -253,6 +259,7 @@ func (b *BatchProcessor) ForceFlush(ctx context.Context) error {
 type queue struct {
 	sync.Mutex
 
+	dropped     atomic.Uint64
 	cap, len    int
 	read, write *ring
 }
@@ -266,6 +273,12 @@ func newQueue(size int) *queue {
 	}
 }
 
+// Dropped returns the number of Records dropped during enqueueing since the
+// last time Dropped was called.
+func (q *queue) Dropped() uint64 {
+	return q.dropped.Swap(0)
+}
+
 // Enqueue adds r to the queue. The queue size, including the addition of r, is
 // returned.
 //
@@ -283,6 +296,7 @@ func (q *queue) Enqueue(r Record) int {
 		// Overflow. Advance read to be the new "oldest".
 		q.len = q.cap
 		q.read = q.read.Next()
+		q.dropped.Add(1)
 	}
 	return q.len
 }

sdk/log/batch_test.go

@@ -4,7 +4,9 @@
 package log // import "go.opentelemetry.io/otel/sdk/log"
 
 import (
+	"bytes"
 	"context"
+	stdlog "log"
 	"slices"
 	"strconv"
 	"sync"
@@ -12,10 +14,12 @@ import (
 	"time"
 	"unsafe"
 
+	"github.com/go-logr/stdr"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/internal/global"
 	"go.opentelemetry.io/otel/log"
 )
 
@@ -413,6 +417,41 @@ func TestBatchProcessor(t *testing.T) {
 		})
 	})
 
+	t.Run("DroppedLogs", func(t *testing.T) {
+		orig := global.GetLogger()
+		t.Cleanup(func() { global.SetLogger(orig) })
+		buf := new(bytes.Buffer)
+		stdr.SetVerbosity(1)
+		global.SetLogger(stdr.New(stdlog.New(buf, "", 0)))
+
+		e := newTestExporter(nil)
+		e.ExportTrigger = make(chan struct{})
+
+		b := NewBatchProcessor(
+			e,
+			WithMaxQueueSize(1),
+			WithExportMaxBatchSize(1),
+			WithExportInterval(time.Hour),
+			WithExportTimeout(time.Hour),
+		)
+		var r Record
+		assert.NoError(t, b.OnEmit(ctx, r), "queued")
+		assert.NoError(t, b.OnEmit(ctx, r), "dropped")
+
+		var n int
+		require.Eventually(t, func() bool {
+			n = e.ExportN()
+			return n > 0
+		}, 2*time.Second, time.Microsecond, "blocked export not attempted")
+
+		got := buf.String()
+		want := `"level"=1 "msg"="dropped log records" "dropped"=1`
+		assert.Contains(t, got, want)
+
+		close(e.ExportTrigger)
+		_ = b.Shutdown(ctx)
+	})
+
 	t.Run("ConcurrentSafe", func(t *testing.T) {
 		const goRoutines = 10
 
@@ -488,6 +527,18 @@ func TestQueue(t *testing.T) {
 		assert.Equal(t, []Record{r, r}, q.Flush(), "flushed Records")
 	})
 
+	t.Run("Dropped", func(t *testing.T) {
+		q := newQueue(1)
+
+		_ = q.Enqueue(r)
+		_ = q.Enqueue(r)
+		assert.Equal(t, uint64(1), q.Dropped(), "fist")
+
+		_ = q.Enqueue(r)
+		_ = q.Enqueue(r)
+		assert.Equal(t, uint64(2), q.Dropped(), "second")
+	})
+
 	t.Run("Flush", func(t *testing.T) {
 		const size = 2
 		q := newQueue(size)

sdk/log/go.mod

@@ -4,6 +4,7 @@ go 1.21
 
 require (
 	github.com/go-logr/logr v1.4.1
+	github.com/go-logr/stdr v1.2.2
 	github.com/stretchr/testify v1.9.0
 	go.opentelemetry.io/otel v1.26.0
 	go.opentelemetry.io/otel/log v0.2.0-alpha
@@ -13,7 +14,6 @@ require (
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	go.opentelemetry.io/otel/metric v1.26.0 // indirect
 	golang.org/x/sys v0.20.0 // indirect