AsyncLocalStorage - async_hooks - Node

class AsyncLocalStorage

Usage in Deno

import { AsyncLocalStorage } 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
//   1: start
//   0: finish
//   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.

Type Parameters

Methods



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.



enterWith(store: T): 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



exit<R,
TArgs extends any[],
>(callback: (...args: TArgs) => R,
...args: TArgs,
): R

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
}



getStore(): T | undefined

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



run<R>(store: T,
callback: () => R,
): R

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
}



run<R,
TArgs extends any[],
>(store: T,
callback: (...args: TArgs) => R,
...args: TArgs,
): R

Static Methods



bind<Func extends (...args: any[]) => any>(fn: Func): Func

Binds the given function to the current execution context.



snapshot(): <R,
TArgs extends any[],
>(fn: (...args: TArgs) => R,
...args: TArgs,
) => R

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