ALS.md

AsyncLocalStorage API

The AsyncLocalStorage class has been introduced in the Async Hooks module.

This API allows keeping a context across asynchronous operations. For instance, if an sequence id is stored within an instance of AsyncLocalStorage for each entering HTTP requests in a server, it will be possible to retrieve this id without knowing the current HTTP request.

const http = require('http');
const { AsyncLocalStorage } = require('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);

In this example, the logWithId function, will always know what the current request id is.

What can this API be used for

Use cases of this API include:

• logging
• User identification
• Performance tracking
• Error trancking and handling
• much more!

Note: This API is still experimental and some methods might change in the next releases of Node.js