Do not export empty span lists (open-telemetry#896)

* chore: remove in-memory polling timer

Author: dyladan

packages/opentelemetry-tracing/src/export/BatchSpanProcessor.ts

@@ -31,9 +31,9 @@ const DEFAULT_BUFFER_TIMEOUT_MS = 20_000;
 export class BatchSpanProcessor implements SpanProcessor {
   private readonly _bufferSize: number;
   private readonly _bufferTimeout: number;
+
   private _finishedSpans: ReadableSpan[] = [];
-  private _lastSpanFlush = Date.now();
-  private _timer: NodeJS.Timeout;
+  private _timer: NodeJS.Timeout | undefined;
   private _isShutdown = false;
 
   constructor(private readonly _exporter: SpanExporter, config?: BufferConfig) {
@@ -43,13 +43,6 @@ export class BatchSpanProcessor implements SpanProcessor {
       config && typeof config.bufferTimeout === 'number'
         ? config.bufferTimeout
         : DEFAULT_BUFFER_TIMEOUT_MS;
-
-    this._timer = setInterval(() => {
-      if (this._shouldFlush()) {
-        this._flush();
-      }
-    }, this._bufferTimeout);
-    unrefTimer(this._timer);
   }
 
   forceFlush(): void {
@@ -73,7 +66,6 @@ export class BatchSpanProcessor implements SpanProcessor {
     if (this._isShutdown) {
       return;
     }
-    clearInterval(this._timer);
     this.forceFlush();
     this._isShutdown = true;
     this._exporter.shutdown();
@@ -82,22 +74,33 @@ export class BatchSpanProcessor implements SpanProcessor {
   /** Add a span in the buffer. */
   private _addToBuffer(span: ReadableSpan) {
     this._finishedSpans.push(span);
+    this._maybeStartTimer();
     if (this._finishedSpans.length > this._bufferSize) {
       this._flush();
     }
   }
 
-  private _shouldFlush(): boolean {
-    return (
-      this._finishedSpans.length >= 0 &&
-      Date.now() - this._lastSpanFlush >= this._bufferTimeout
-    );
-  }
-
   /** Send the span data list to exporter */
   private _flush() {
+    this._clearTimer();
+    if (this._finishedSpans.length === 0) return;
     this._exporter.export(this._finishedSpans, () => {});
     this._finishedSpans = [];
-    this._lastSpanFlush = Date.now();
+  }
+
+  private _maybeStartTimer() {
+    if (this._timer !== undefined) return;
+
+    this._timer = setTimeout(() => {
+      this._flush();
+    }, this._bufferTimeout);
+    unrefTimer(this._timer);
+  }
+
+  private _clearTimer() {
+    if (this._timer !== undefined) {
+      clearTimeout(this._timer);
+      this._timer = undefined;
+    }
   }
 }

packages/opentelemetry-tracing/test/export/BatchSpanProcessor.test.ts

@@ -45,6 +45,7 @@ describe('BatchSpanProcessor', () => {
   });
   afterEach(() => {
     exporter.reset();
+    sinon.restore();
   });
 
   describe('constructor', () => {
@@ -142,5 +143,35 @@ describe('BatchSpanProcessor', () => {
       processor.forceFlush();
       assert.strictEqual(exporter.getFinishedSpans().length, 5);
     });
+
+    it('should not export empty span lists', done => {
+      const spy = sinon.spy(exporter, 'export');
+      const clock = sinon.useFakeTimers();
+
+      const tracer = new BasicTracerProvider({
+        sampler: ALWAYS_SAMPLER,
+      }).getTracer('default');
+      const processor = new BatchSpanProcessor(exporter, defaultBufferConfig);
+
+      // start but do not end spans
+      for (let i = 0; i < defaultBufferConfig.bufferSize; i++) {
+        const span = tracer.startSpan('spanName');
+        processor.onStart(span as Span);
+      }
+
+      setTimeout(() => {
+        assert.strictEqual(exporter.getFinishedSpans().length, 0);
+        // after the timeout, export should not have been called
+        // because no spans are ended
+        sinon.assert.notCalled(spy);
+        done();
+      }, defaultBufferConfig.bufferTimeout + 1000);
+
+      // no spans have been finished
+      assert.strictEqual(exporter.getFinishedSpans().length, 0);
+      clock.tick(defaultBufferConfig.bufferTimeout + 1000);
+
+      clock.restore();
+    });
   });
 });