Asynchronous context tracking

These classes are used to associate state and propagate it throughout callbacks and promise chains. They allow storing data throughout the lifetime of a web request or any other asynchronous duration. It is similar to thread-local storage in other languages.

The AsyncLocalStorage and AsyncResource classes are part of the node:async_hooks module:

import { AsyncLocalStorage, AsyncResource } from 'node:async_hooks';

This class creates stores that stay coherent through asynchronous operations.

While you can create your own implementation on top of the node:async_hooks module, AsyncLocalStorage should be preferred as it is a performant and memory safe implementation that involves significant optimizations that are non-obvious to implement.

The following example uses AsyncLocalStorage to build a simple logger that assigns IDs to incoming HTTP requests and includes them in messages logged within each request.

import http from 'node:http';
import { AsyncLocalStorage } from 'node:async_hooks';

const asyncLocalStorage = new AsyncLocalStorage();

function logWithId(msg) {
  const id = asyncLocalStorage.getStore();
  console.log(`${id !== undefined ? id : '-'}:`, msg);
}

let idSeq = 0;
http.createServer((req, res) => {
  asyncLocalStorage.run(idSeq++, () => {
    logWithId('start');
    // Imagine any chain of async operations here
    setImmediate(() => {
      logWithId('finish');
      res.end();
    });
  });
}).listen(8080);

http.get('http://localhost:8080');
http.get('http://localhost:8080');
// Prints:
//   0: start
//   0: finish
//   1: start
//   1: finish

Each instance of AsyncLocalStorage maintains an independent storage context. Multiple instances can safely exist simultaneously without risk of interfering with each other's data.

new AsyncLocalStorage(options?): void

Creates a new instance of AsyncLocalStorage. Store is only provided within a run() call or after an enterWith() call.

AsyncLocalStorage.bind(fn): Function

Binds the given function to the current execution context.

AsyncLocalStorage.snapshot(): Function

Captures the current execution context and returns a function that accepts a function as an argument. Whenever the returned function is called, it calls the function passed to it within the captured context.

const asyncLocalStorage = new AsyncLocalStorage();
const runInAsyncScope = asyncLocalStorage.run(123, () => AsyncLocalStorage.snapshot());
const result = asyncLocalStorage.run(321, () => runInAsyncScope(() => asyncLocalStorage.getStore()));
console.log(result);  // returns 123

AsyncLocalStorage.snapshot() can replace the use of AsyncResource for simple async context tracking purposes, for example:

class Foo {
  #runInAsyncScope = AsyncLocalStorage.snapshot();

  get() { return this.#runInAsyncScope(() => asyncLocalStorage.getStore()); }
}

const foo = asyncLocalStorage.run(123, () => new Foo());
console.log(asyncLocalStorage.run(321, () => foo.get())); // returns 123

asyncLocalStorage.disable(): void

Disables the instance of AsyncLocalStorage. All subsequent calls to asyncLocalStorage.getStore() will return undefined until asyncLocalStorage.run() or asyncLocalStorage.enterWith() is called again.

When calling asyncLocalStorage.disable(), all current contexts linked to the instance will be exited.

Calling asyncLocalStorage.disable() is required before the asyncLocalStorage can be garbage collected. This does not apply to stores provided by the asyncLocalStorage, as those objects are garbage collected along with the corresponding async resources.

Use this method when the asyncLocalStorage is not in use anymore in the current process.

asyncLocalStorage.getStore(): any

Returns the current store. If called outside of an asynchronous context initialized by calling asyncLocalStorage.run() or asyncLocalStorage.enterWith(), it returns undefined.

asyncLocalStorage.enterWith(store): void

Transitions into the context for the remainder of the current synchronous execution and then persists the store through any following asynchronous calls.

Example:

const store = { id: 1 };
// Replaces previous store with the given store object
asyncLocalStorage.enterWith(store);
asyncLocalStorage.getStore(); // Returns the store object
someAsyncOperation(() => {
  asyncLocalStorage.getStore(); // Returns the same object
});

This transition will continue for the entire synchronous execution. This means that if, for example, the context is entered within an event handler subsequent event handlers will also run within that context unless specifically bound to another context with an AsyncResource. That is why run() should be preferred over enterWith() unless there are strong reasons to use the latter method.

const store = { id: 1 };

emitter.on('my-event', () => {
  asyncLocalStorage.enterWith(store);
});
emitter.on('my-event', () => {
  asyncLocalStorage.getStore(); // Returns the same object
});

asyncLocalStorage.getStore(); // Returns undefined
emitter.emit('my-event');
asyncLocalStorage.getStore(); // Returns the same object

The name of the AsyncLocalStorage instance if provided.

asyncLocalStorage.run(store, callback, ...args?): void

Runs a function synchronously within a context and returns its return value. The store is not accessible outside of the callback function. The store is accessible to any asynchronous operations created within the callback.

The optional args are passed to the callback function.

If the callback function throws an error, the error is thrown by run() too. The stacktrace is not impacted by this call and the context is exited.

Example:

const store = { id: 2 };
try {
  asyncLocalStorage.run(store, () => {
    asyncLocalStorage.getStore(); // Returns the store object
    setTimeout(() => {
      asyncLocalStorage.getStore(); // Returns the store object
    }, 200);
    throw new Error();
  });
} catch (e) {
  asyncLocalStorage.getStore(); // Returns undefined
  // The error will be caught here
}

asyncLocalStorage.exit(callback, ...args?): void

Runs a function synchronously outside of a context and returns its return value. The store is not accessible within the callback function or the asynchronous operations created within the callback. Any getStore() call done within the callback function will always return undefined.

The optional args are passed to the callback function.

If the callback function throws an error, the error is thrown by exit() too. The stacktrace is not impacted by this call and the context is re-entered.

Example:

// Within a call to run
try {
  asyncLocalStorage.getStore(); // Returns the store object or value
  asyncLocalStorage.exit(() => {
    asyncLocalStorage.getStore(); // Returns undefined
    throw new Error();
  });
} catch (e) {
  asyncLocalStorage.getStore(); // Returns the same object or value
  // The error will be caught here
}

asyncLocalStorage.withScope(store): RunScope

Creates a disposable scope that enters the given store and automatically restores the previous store value when the scope is disposed. This method is designed to work with JavaScript's explicit resource management (using syntax).

Example:

import { AsyncLocalStorage } from 'node:async_hooks';

const asyncLocalStorage = new AsyncLocalStorage();

{
  using _ = asyncLocalStorage.withScope('my-store');
  console.log(asyncLocalStorage.getStore()); // Prints: my-store
}

console.log(asyncLocalStorage.getStore()); // Prints: undefined

The withScope() method is particularly useful for managing context in synchronous code where you want to ensure the previous store value is restored when exiting a block, even if an error is thrown.

import { AsyncLocalStorage } from 'node:async_hooks';

const asyncLocalStorage = new AsyncLocalStorage();

try {
  using _ = asyncLocalStorage.withScope('my-store');
  console.log(asyncLocalStorage.getStore()); // Prints: my-store
  throw new Error('test');
} catch (e) {
  // Store is automatically restored even after error
  console.log(asyncLocalStorage.getStore()); // Prints: undefined
}

Important: When using withScope() in async functions before the first await, be aware that the scope change will affect the caller's context. The synchronous portion of an async function (before the first await) runs immediately when called, and when it reaches the first await, it returns the promise to the caller. At that point, the scope change becomes visible in the caller's context and will persist in subsequent synchronous code until something else changes the scope value. For async operations, prefer using run() which properly isolates context across async boundaries.

import { AsyncLocalStorage } from 'node:async_hooks';

const asyncLocalStorage = new AsyncLocalStorage();

async function example() {
  using _ = asyncLocalStorage.withScope('my-store');
  console.log(asyncLocalStorage.getStore()); // Prints: my-store
  await someAsyncOperation(); // Function pauses here and returns promise
  console.log(asyncLocalStorage.getStore()); // Prints: my-store
}

// Calling without await
example(); // Synchronous portion runs, then pauses at first await
// After the promise is returned, the scope 'my-store' is now active in caller!
console.log(asyncLocalStorage.getStore()); // Prints: my-store (unexpected!)

If, within an async function, only one await call is to run within a context, the following pattern should be used:

async function fn() {
  await asyncLocalStorage.run(new Map(), () => {
    asyncLocalStorage.getStore().set('key', value);
    return foo(); // The return value of foo will be awaited
  });
}

In this example, the store is only available in the callback function and the functions called by foo. Outside of run, calling getStore will return undefined.

In most cases, AsyncLocalStorage works without issues. In rare situations, the current store is lost in one of the asynchronous operations.

If your code is callback-based, it is enough to promisify it with util.promisify() so it starts working with native promises.

If you need to use a callback-based API or your code assumes a custom thenable implementation, use the AsyncResource class to associate the asynchronous operation with the correct execution context. Find the function call responsible for the context loss by logging the content of asyncLocalStorage.getStore() after the calls you suspect are responsible for the loss. When the code logs undefined, the last callback called is probably responsible for the context loss.

A disposable scope returned by asyncLocalStorage.withScope() that automatically restores the previous store value when disposed. This class implements the Explicit Resource Management protocol and is designed to work with JavaScript's using syntax.

The scope automatically restores the previous store value when the using block exits, whether through normal completion or by throwing an error.

scope.dispose(): void

Explicitly ends the scope and restores the previous store value. This method is idempotent: calling it multiple times has the same effect as calling it once.

The [Symbol.dispose]() method defers to dispose().

If withScope() is called without the using keyword, dispose() must be called manually to restore the previous store value. Forgetting to call dispose() will cause the store value to persist for the remainder of the current execution context:

import { AsyncLocalStorage } from 'node:async_hooks';

const storage = new AsyncLocalStorage();

// Without using, the scope must be disposed manually
const scope = storage.withScope('my-store');
// storage.getStore() === 'my-store' here

scope.dispose(); // Restore previous value
// storage.getStore() === undefined here

The class AsyncResource is designed to be extended by the embedder's async resources. Using this, users can easily trigger the lifetime events of their own resources.

The init hook will trigger when an AsyncResource is instantiated.

The following is an overview of the AsyncResource API.

import { AsyncResource, executionAsyncId } from 'node:async_hooks';

// AsyncResource() is meant to be extended. Instantiating a
// new AsyncResource() also triggers init. If triggerAsyncId is omitted then
// async_hook.executionAsyncId() is used.
const asyncResource = new AsyncResource(
  type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false },
);

// Run a function in the execution context of the resource. This will
// * establish the context of the resource
// * trigger the AsyncHooks before callbacks
// * call the provided function `fn` with the supplied arguments
// * trigger the AsyncHooks after callbacks
// * restore the original execution context
asyncResource.runInAsyncScope(fn, thisArg, ...args);

// Call AsyncHooks destroy callbacks.
asyncResource.emitDestroy();

// Return the unique ID assigned to the AsyncResource instance.
asyncResource.asyncId();

// Return the trigger ID for the AsyncResource instance.
asyncResource.triggerAsyncId();

new AsyncResource(type, options?): void

Example usage:

class DBQuery extends AsyncResource {
  constructor(db) {
    super('DBQuery');
    this.db = db;
  }

  getInfo(query, callback) {
    this.db.get(query, (err, data) => {
      this.runInAsyncScope(callback, null, err, data);
    });
  }

  close() {
    this.db = null;
    this.emitDestroy();
  }
}

AsyncResource.bind(fn, type?, thisArg?): void

Binds the given function to the current execution context.

asyncResource.bind(fn, thisArg?): void

Binds the given function to execute to this AsyncResource's scope.

asyncResource.runInAsyncScope(fn, thisArg?, ...args?): void

Call the provided function with the provided arguments in the execution context of the async resource. This will establish the context, trigger the AsyncHooks before callbacks, call the function, trigger the AsyncHooks after callbacks, and then restore the original execution context.

asyncResource.emitDestroy(): AsyncResource

Call all destroy hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the destroy hooks will never be called.

asyncResource.asyncId(): number

asyncResource.triggerAsyncId(): number

The following example shows how to use the AsyncResource class to properly provide async tracking for a Worker pool. Other resource pools, such as database connection pools, can follow a similar model.

Assuming that the task is adding two numbers, using a file named task_processor.js with the following content:

import { parentPort } from 'node:worker_threads';
parentPort.on('message', (task) => {
  parentPort.postMessage(task.a + task.b);
});

a Worker pool around it could use the following structure:

import { AsyncResource } from 'node:async_hooks';
import { EventEmitter } from 'node:events';
import { Worker } from 'node:worker_threads';

const kTaskInfo = Symbol('kTaskInfo');
const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');

class WorkerPoolTaskInfo extends AsyncResource {
  constructor(callback) {
    super('WorkerPoolTaskInfo');
    this.callback = callback;
  }

  done(err, result) {
    this.runInAsyncScope(this.callback, null, err, result);
    this.emitDestroy();  // `TaskInfo`s are used only once.
  }
}

export default class WorkerPool extends EventEmitter {
  constructor(numThreads) {
    super();
    this.numThreads = numThreads;
    this.workers = [];
    this.freeWorkers = [];
    this.tasks = [];

    for (let i = 0; i < numThreads; i++)
      this.addNewWorker();

    // Any time the kWorkerFreedEvent is emitted, dispatch
    // the next task pending in the queue, if any.
    this.on(kWorkerFreedEvent, () => {
      if (this.tasks.length > 0) {
        const { task, callback } = this.tasks.shift();
        this.runTask(task, callback);
      }
    });
  }

  addNewWorker() {
    const worker = new Worker(new URL('task_processor.js', import.meta.url));
    worker.on('message', (result) => {
      // In case of success: Call the callback that was passed to `runTask`,
      // remove the `TaskInfo` associated with the Worker, and mark it as free
      // again.
      worker[kTaskInfo].done(null, result);
      worker[kTaskInfo] = null;
      this.freeWorkers.push(worker);
      this.emit(kWorkerFreedEvent);
    });
    worker.on('error', (err) => {
      // In case of an uncaught exception: Call the callback that was passed to
      // `runTask` with the error.
      if (worker[kTaskInfo])
        worker[kTaskInfo].done(err, null);
      else
        this.emit('error', err);
      // Remove the worker from the list and start a new Worker to replace the
      // current one.
      this.workers.splice(this.workers.indexOf(worker), 1);
      this.addNewWorker();
    });
    this.workers.push(worker);
    this.freeWorkers.push(worker);
    this.emit(kWorkerFreedEvent);
  }

  runTask(task, callback) {
    if (this.freeWorkers.length === 0) {
      // No free threads, wait until a worker thread becomes free.
      this.tasks.push({ task, callback });
      return;
    }

    const worker = this.freeWorkers.pop();
    worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
    worker.postMessage(task);
  }

  close() {
    for (const worker of this.workers) worker.terminate();
  }
}

Without the explicit tracking added by the WorkerPoolTaskInfo objects, it would appear that the callbacks are associated with the individual Worker objects. However, the creation of the Workers is not associated with the creation of the tasks and does not provide information about when tasks were scheduled.

This pool could be used as follows:

import WorkerPool from './worker_pool.js';
import os from 'node:os';

const pool = new WorkerPool(os.availableParallelism());

let finished = 0;
for (let i = 0; i < 10; i++) {
  pool.runTask({ a: 42, b: 100 }, (err, result) => {
    console.log(i, err, result);
    if (++finished === 10)
      pool.close();
  });
}

Event listeners triggered by an EventEmitter may be run in a different execution context than the one that was active when eventEmitter.on() was called.

The following example shows how to use the AsyncResource class to properly associate an event listener with the correct execution context. The same approach can be applied to a Stream or a similar event-driven class.

import { createServer } from 'node:http';
import { AsyncResource, executionAsyncId } from 'node:async_hooks';

const server = createServer((req, res) => {
  req.on('close', AsyncResource.bind(() => {
    // Execution context is bound to the current outer scope.
  }));
  req.on('close', () => {
    // Execution context is bound to the scope that caused 'close' to emit.
  });
  res.end();
}).listen(3000);