Async Local Storage in Node.js

• Provides us a way to store and manage context-specific data across asynchronous operations without needing to pass it explicitly through function arguments.
• Built on the async_hooks module, which tracks asynchronous resource lifecycle events.
• We need to use asyncLocalStorage.run(store, callback) to create a new context.
• Asynchronous operations initiated within this callback inherit that context.
• Each context created with asyncLocalStorage.run() is unique and does not interfere with other contexts.
• Common use cases can be maintaining custom context in our web app (e.g., request data, user ID...) across multiple layers (controllers, services, etc.), can help us with tracing how a request propagates through different async functions.
• Automatically cleans up the context after the asynchronous operations are complete.
• run(store, callback): Creates a new context and runs the callback with the provided store (like a Map or a primitive value).
• Set a value: asyncLocalStorage.getStore().set('requestId', requestId);
• Get a value: const requestId = asyncLocalStorage.getStore().get('requestId');

const http = require('node:http');
const { AsyncLocalStorage } = require('node:async_hooks');
const { v4: uuid } = require('uuid');

const asyncLocalStorage = new AsyncLocalStorage();

function logWithId(msg) {
  const requestId = asyncLocalStorage.getStore(); 
  console.log(`${requestId} - ${msg}`);
}

function serviceA() {
  logWithId('Service A log');
}

function serviceB() {
  logWithId('Service B log');
}

http.createServer((req, res) => {
  const requestId = uuid();
  asyncLocalStorage.run(requestId, () => { 
    logWithId('Request received');
    serviceA();
    serviceB();
    logWithId('All services called');
    res.end('Response sent');
  });
}).listen(4040);

http.get('');

// Output:
<generated-request-id> - Request received
<generated-request-id> - Service A log
<generated-request-id> - Service B log
<generated-request-id> - All services called