How does the cluster module work? What’s the difference between it and a load balancer?

The cluster module performs fork from your server (at that moment it is already an OS process), thus creating several slave processes. The cluster module supports two methods of distributing incoming connections.

• The first one (and the default one on all platforms except Windows), is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.
• The second approach is where the master process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.

The difference between a cluster module and a load balancer is that instead of distributing load between processes, the balancer distributes requests.