From fb6d8dc8d0d89df0fa399d132bffb975daa7bf1b Mon Sep 17 00:00:00 2001
From: Martin Kubliniak <kublim@gmail.com>
Date: Fri, 15 Dec 2023 08:36:29 +0100
Subject: [PATCH] feat(sdk-trace-base): improve log messages when dropping span
 events (#4223)

---
 CHANGELOG.md                                          |  2 ++
 packages/opentelemetry-sdk-trace-base/src/Span.ts     | 10 +++++++++-
 .../test/common/Span.test.ts                          | 11 +++++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1d81af36519..28759606768 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,8 @@ For experimental package changes, see the [experimental CHANGELOG](experimental/
 
 ### :rocket: (Enhancement)
 
+* feat(sdk-trace-base): improve log messages when dropping span events [#4223](https://github.com/open-telemetry/opentelemetry-js/pull/4223) @mkubliniak
+
 ### :bug: (Bug Fix)
 
 ### :books: (Refine Doc)
diff --git a/packages/opentelemetry-sdk-trace-base/src/Span.ts b/packages/opentelemetry-sdk-trace-base/src/Span.ts
index e3e37d82136..29af9fc6513 100644
--- a/packages/opentelemetry-sdk-trace-base/src/Span.ts
+++ b/packages/opentelemetry-sdk-trace-base/src/Span.ts
@@ -185,7 +185,9 @@ export class Span implements APISpan, ReadableSpan {
       return this;
     }
     if (this.events.length >= this._spanLimits.eventCountLimit!) {
-      diag.warn('Dropping extra events.');
+      if (this._droppedEventsCount === 0) {
+        diag.debug('Dropping extra events.');
+      }
       this.events.shift();
       this._droppedEventsCount++;
     }
@@ -242,6 +244,12 @@ export class Span implements APISpan, ReadableSpan {
       this._duration = [0, 0];
     }
 
+    if (this._droppedEventsCount > 0) {
+      diag.warn(
+        `Dropped ${this._droppedEventsCount} events because eventCountLimit reached`
+      );
+    }
+
     this._spanProcessor.onEnd(this);
   }
 
diff --git a/packages/opentelemetry-sdk-trace-base/test/common/Span.test.ts b/packages/opentelemetry-sdk-trace-base/test/common/Span.test.ts
index 11b0f3f318e..0067f85c897 100644
--- a/packages/opentelemetry-sdk-trace-base/test/common/Span.test.ts
+++ b/packages/opentelemetry-sdk-trace-base/test/common/Span.test.ts
@@ -15,6 +15,7 @@
  */
 
 import {
+  diag,
   SpanStatusCode,
   Exception,
   ROOT_CONTEXT,
@@ -786,6 +787,10 @@ describe('Span', () => {
       spanContext,
       SpanKind.CLIENT
     );
+
+    const debugStub = sinon.spy(diag, 'debug');
+    const warnStub = sinon.spy(diag, 'warn');
+
     for (let i = 0; i < 150; i++) {
       span.addEvent('sent' + i);
     }
@@ -793,6 +798,12 @@ describe('Span', () => {
 
     assert.strictEqual(span.events.length, 100);
     assert.strictEqual(span.events[span.events.length - 1].name, 'sent149');
+
+    sinon.assert.calledOnceWithExactly(debugStub, 'Dropping extra events.');
+    sinon.assert.calledOnceWithExactly(
+      warnStub,
+      'Dropped 50 events because eventCountLimit reached'
+    );
   });
 
   it('should store the count of dropped events in droppedEventsCount', () => {