Correlation IDs: tracing requests across services and logs

A request touches 5 services and fails. Finding all related log entries across services requires searching by timestamp range and guessing which logs belong to the same request.

Propagate a correlation ID through all service calls and log it with every entry:

import { AsyncLocalStorage } from 'node:async_hooks';
import { v4 as uuidv4 } from 'uuid';

// Store correlation ID in async context (no thread-local in Node.js)
const correlationStorage = new AsyncLocalStorage();

// Middleware: create or propagate correlation ID
app.use((req, res, next) => {
  const correlationId = req.headers['x-correlation-id'] || uuidv4();
  res.setHeader('x-correlation-id', correlationId);
  correlationStorage.run({ correlationId }, next);
});

// Helper to get current correlation ID
const getCorrelationId = () => correlationStorage.getStore()?.correlationId;

// Logger automatically includes it
const logger = pino({
  mixin: () => ({ correlationId: getCorrelationId() }),
});

// Forward to downstream services
async function callUserService(userId) {
  return fetch(`http://user-service/users/${userId}`, {
    headers: {
      'x-correlation-id': getCorrelationId(),
    },
  });
}

AsyncLocalStorage provides Node.js equivalent of thread-local storage, propagating context through async call chains without passing it as a parameter to every function.