What is horizontal scaling for a Node.js application, and what specific architectural requirement does it impose?

Horizontal scaling adds more instances behind a load balancer rather than making one instance more powerful -- this requires the application to be stateless: any instance must handle any request without depending on data that only exists on one instance, which is why session state needs a shared store like Redis and uploads need shared storage like S3.

// Stateful (breaks horizontal scaling)
app.use(session({ secret: 'x' }));

// Stateless: session in shared Redis
app.use(session({ store: new RedisStore({ client: redisClient }), secret: process.env.SESSION_SECRET }));