Code Quiz

Stateless Service Behind a Load Balancer

Spot why this Express auth service breaks when scaled horizontally behind a load balancer.

Codejavascript

const express = require('express');
const app = express();
app.use(express.json());

// Auth service deployed as multiple replicas behind a load balancer
const sessions = {};

app.post('/login', (req, res) => {
  const { userId } = req.body;
  const token = Math.random().toString(36).slice(2);
  sessions[token] = { userId, createdAt: Date.now() };
  res.json({ token });
});

app.get('/profile', (req, res) => {
  const token = req.headers.authorization;
  const session = sessions[token];
  if (!session) return res.status(401).json({ error: 'unauthorized' });
  res.json({ userId: session.userId });
});

app.listen(3000);

This service runs as several replicas behind a round-robin load balancer, and users randomly get 401s on /profile even with a valid token. What is the bug?

Next →Scaling a Stateless Service