From 60a17ee8e343f0234443faeb6ba2b39413474dae Mon Sep 17 00:00:00 2001
From: Ran Nozik <ran@gethelios.dev>
Date: Wed, 27 Apr 2022 18:45:10 +0300
Subject: [PATCH] feat: warn when hooked module is already loaded (#2926)

---
 CHANGELOG.md                                     |  1 +
 .../src/platform/node/instrumentation.ts         | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index accaf406d4b..c2c6e27cc1b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ All notable changes to this project will be documented in this file.
 ### :rocket: (Enhancement)
 
 * feat(ConsoleSpanExporter): export span links [#2917](https://github.com/open-telemetry/opentelemetry-js/pull/2917) @trentm
+* feat: warn when hooked module is already loaded [#2926](https://github.com/open-telemetry/opentelemetry-js/pull/2926) @nozik
 
 ### :bug: (Bug Fix)
 
diff --git a/experimental/packages/opentelemetry-instrumentation/src/platform/node/instrumentation.ts b/experimental/packages/opentelemetry-instrumentation/src/platform/node/instrumentation.ts
index f67476930d7..09d6a86b292 100644
--- a/experimental/packages/opentelemetry-instrumentation/src/platform/node/instrumentation.ts
+++ b/experimental/packages/opentelemetry-instrumentation/src/platform/node/instrumentation.ts
@@ -59,6 +59,21 @@ export abstract class InstrumentationBase<T = any>
     }
   }
 
+  private _warnOnPreloadedModules(): void {
+    this._modules.forEach((module: InstrumentationModuleDefinition<T>) => {
+      const { name } = module;
+      try {
+        const resolvedModule = require.resolve(name);
+        if (require.cache[resolvedModule]) {
+          // Module is already cached, which means the instrumentation hook might not work
+          this._diag.warn(`Module ${name} has been loaded before ${this.instrumentationName} so it might not work, please initialize it before requiring ${name}`);
+        }
+      } catch {
+        // Module isn't available, we can simply skip
+      }
+    });
+  }
+
   private _extractPackageVersion(baseDir: string): string | undefined {
     try {
       // eslint-disable-next-line @typescript-eslint/no-var-requires
@@ -134,6 +149,7 @@ export abstract class InstrumentationBase<T = any>
       return;
     }
 
+    this._warnOnPreloadedModules();
     for (const module of this._modules) {
       this._hooks.push(
         RequireInTheMiddle(