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.