warn when hooked module is already loaded

CHANGELOG.md

@@ -8,6 +8,8 @@ All notable changes to this project will be documented in this file.
 
 ### :rocket: (Enhancement)
 
+* feat: warn when hooked module is already loaded [#2926](https://github.com/open-telemetry/opentelemetry-js/pull/2926) @nozik
+
 ### :bug: (Bug Fix)
 
 * fix: sanitize attributes inputs [#2881](https://github.com/open-telemetry/opentelemetry-js/pull/2881) @legendecas

experimental/packages/opentelemetry-instrumentation/src/platform/node/instrumentation.ts

@@ -52,13 +52,37 @@ export abstract class InstrumentationBase<T = any>
         'No modules instrumentation has been defined,' +
         ' nothing will be patched'
       );
+    } else {
+      this._warnOnPreloadedModules();
     }
 
     if (this._config.enabled) {
       this.enable();
     }
   }
 
+  private _warnOnPreloadedModules(): void {
+    const preloadedModules: string[] = [];
+    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
+          preloadedModules.push(name);
+        }
+      } catch {
+        // Module isn't available, we can simply skip
+      }
+    });
+
+    if (!preloadedModules.length) {
+      return;
+    }
+
+    this._diag.warn(`Some modules (${preloadedModules.join(', ')}) were already required when their respective plugin was loaded, some plugins might not work. Make sure the SDK is setup before you require in other modules.`);
+  }
+
   private _extractPackageVersion(baseDir: string): string | undefined {
     try {
       // eslint-disable-next-line @typescript-eslint/no-var-requires