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docs: Describe support for ESM (#4876)
Co-authored-by: Trent Mick <trentm@gmail.com>
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| # ECMAScript Modules vs. CommonJS | ||
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| Node.js uses a different module loader for ECMAScript Modules (ESM) vs. CommonJS (CJS). | ||
| To verify whether your application is ESM or CJS, refer to [Node.js docs for Determining Module System](https://nodejs.org/api/packages.html#determining-module-system). | ||
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| An `.mjs` extension or `type:module` in the built app's `package.json` indicates the app is ESM. | ||
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| **Much of OpenTelemetry JS documentation is written assuming the compiled application is run as CJS.** | ||
| ESM support is ongoing; a few adjustments are needed for configuration and startup commands. | ||
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| For more explanation about CJS and ESM, see the [Node.js docs](https://nodejs.org/api/modules.html#enabling). | ||
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| ## TypeScript | ||
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| Many TypeScript projects today are written using ESM syntax, regardless of how they are compiled. | ||
| In the `tsconfig.json`, there is an option to compile to ESM or CJS. | ||
| If the compiled code is ESM, those import statements will remain the same (e.g. `import { foo } from 'bar';`). | ||
| If the compiled code is CJS, those import statements will become `require()` statements (e.g. `const { foo } = require('bar');`) | ||
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| ## Initializing the SDK | ||
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| Instrumentation setup and configuration must be run before your application code. | ||
| If the SDK is initialized in a separate file (recommended), ensure it is imported first in application startup, or use the `--require` or `--import` flag during startup to preload the module. | ||
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| For CJS, the `NODE_OPTIONS` for the startup command should include `--require ./telemetry.js`. | ||
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| For ESM, minimum Node.js version of `18.19.0` is required. | ||
| The `NODE_OPTIONS` for the startup command should include `--import ./telemetry.js`. | ||
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| ## Instrumentation Hook Required for ESM | ||
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| If your application is written in JavaScript as ESM, or compiled to ESM from TypeScript, then a loader hook is required to properly patch instrumentation. | ||
| The custom hook for ESM instrumentation is `--experimental-loader=@opentelemetry/instrumentation/hook.mjs`. | ||
| This flag must be passed to the `node` binary, which is often done as a startup command and/or in the `NODE_OPTIONS` environment variable. | ||
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| ### Additional Notes on Experimental Loaders | ||
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| Though the OpenTelemetry loader currently relies on `import-in-the-middle`, direct usage of `import-in-the-middle/hook.mjs` may cease to work in the future. | ||
| The only currently supported loader hook is `@opentelemetry/instrumentation/hook.mjs`. | ||
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| **Note:** Eventually the recommendation for how to setup OpenTelemetry for usage with ESM will change to no longer require `--experimental-loader=@opentelemetry/instrumentation/hook.mjs`. | ||
| Instead the bootstrap code (in `./telemetry.js`) will use Node.js's newer `module.register(...)`. | ||
| Refer to this [issue](https://github.com/open-telemetry/opentelemetry-js/issues/4933) for details. | ||
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| Because of ongoing issues with loaders running TypeScript code as ESM in development environments, results may vary. | ||
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| To use `ts-node` to run the uncompiled TypeScript code, the module must be CJS. | ||
| To use `tsx` to run the uncompiled TypeScript code as ESM, the `--import` flag must be used. | ||
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| ## Using the Zero Code Option with `auto-instrumentations-node` | ||
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| The `auto-instrumentations-node` package contains a `register` entry-point that can be used with `--require` or `--import` to setup and start the SDK easily, without application code changes. | ||
| For ESM, the package also requires the usage of the loader hook. | ||
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| Startup command for CJS: | ||
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| ```sh | ||
| node --require @opentelemetry/auto-instrumentations-node/register app.js | ||
| ``` | ||
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| Startup command for ESM: | ||
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| ```sh | ||
| node --experimental-loader=@opentelemetry/instrumentation/hook.mjs --import @opentelemetry/auto-instrumentations-node/register app.js | ||
| ``` | ||
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| ## Examples | ||
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| ### Example Written in JavaScript as CJS | ||
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| ```javascript | ||
| /*telemetry.cjs*/ | ||
| const { NodeSDK } = require('@opentelemetry/sdk-node'); | ||
| const { ConsoleSpanExporter } = require('@opentelemetry/sdk-trace-node'); | ||
| const { | ||
| getNodeAutoInstrumentations, | ||
| } = require('@opentelemetry/auto-instrumentations-node'); | ||
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| const sdk = new NodeSDK({ | ||
| traceExporter: new ConsoleSpanExporter(), | ||
| instrumentations: [getNodeAutoInstrumentations()], | ||
| }); | ||
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| sdk.start(); | ||
| ``` | ||
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| Startup command: | ||
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| ```sh | ||
| node --require ./telemetry.cjs app.js | ||
| ``` | ||
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| ### Example Written in JavaScript as ESM or TypeScript | ||
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| ```typescript | ||
| /*telemetry.ts | telemetry.mjs*/ | ||
| import { NodeSDK } from '@opentelemetry/sdk-node'; | ||
| import { ConsoleSpanExporter } from '@opentelemetry/sdk-trace-node'; | ||
| import { getNodeAutoInstrumentations } from '@opentelemetry/auto-instrumentations-node'; | ||
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| const sdk = new NodeSDK({ | ||
| traceExporter: new ConsoleSpanExporter(), | ||
| instrumentations: [getNodeAutoInstrumentations()], | ||
| }); | ||
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| sdk.start(); | ||
| ``` | ||
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| Startup command for compiled CJS: | ||
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| ```sh | ||
| node --require ./telemetry.js app.js | ||
| ``` | ||
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| Startup command for compiled ESM: | ||
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| ```sh | ||
| node --experimental-loader=@opentelemetry/instrumentation/hook.mjs --import ./telemetry.js app.js | ||
| ``` | ||
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| ### ESM Options for Different Versions of Node.js | ||
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| The entire startup command should include the following `NODE_OPTIONS`: | ||
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| | Node.js Version | NODE_OPTIONS | | ||
| | ----------------- | ----------------------------------------------------------------------------------------- | | ||
| | 16.x | `--require ./telemetry.cjs --experimental-loader=@opentelemetry/instrumentation/hook.mjs` | | ||
| | >=18.1.0 <18.19.0 | `--require ./telemetry.cjs --experimental-loader=@opentelemetry/instrumentation/hook.mjs` | | ||
| | ^18.19.0 | `--import ./telemetry.mjs --experimental-loader=@opentelemetry/instrumentation/hook.mjs` | | ||
| | 20.x | `--import ./telemetry.mjs --experimental-loader=@opentelemetry/instrumentation/hook.mjs` | | ||
| | 22.x | `--import ./telemetry.mjs --experimental-loader=@opentelemetry/instrumentation/hook.mjs` | |
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