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What is the os.cpus() function used for when setting up clustering in a Node.js application? Why does Node.js provide a cluster module, given that a single Node.js process is single-threaded? How does PM2's cluster mode compare to manually using Node's built-in cluster module directly in application code? How do you pass data between the main thread and a worker thread using postMessage(), and what are the limits on what can be sent? What is the difference between the number of CPU cores and the optimal number of cluster workers for a given Node.js application? What is the difference between spawning a Worker Thread and spawning a Child Process for running a CPU-intensive JavaScript function? How do you gracefully terminate a worker thread, and what's the difference between worker.terminate() and letting it exit naturally? What happens to in-memory state (like an in-memory cache) when using the Node.js cluster module, and how do you handle this correctly? How do you use SharedArrayBuffer to share memory directly between Node.js worker threads, and what are the safety considerations? What is a worker thread pool, and why would you build one rather than creating a new Worker for every task?
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What is the difference between the 'sticky' and default load-balancing strategies when using Node's cluster module, and when does it matter?

Intermediate

Answer

By default the primary distributes connections across workers round-robin-like. 'Sticky' load balancing ensures requests from the same client always route to the same worker -- important for WebSocket connections or scenarios relying on in-memory, per-connection state on a specific worker.
const sticky = require('sticky-session');
if (!sticky.listen(server, 3000)) {
  server.once('listening', () => console.log('Server started'));
}
Real-world example A chat app clustering across four cores discovers WebSocket reconnections sometimes landed on a different worker than the one holding session state; sticky load balancing by IP resolved dropped connections.

Common follow-ups: Why is sticky load balancing generally unnecessary if session state is already externalized to Redis?;How does sticky behavior interact with a cloud load balancer in front of the whole cluster?

WebSockets & Real-Time Communication;HTTP & HTTPS Modules
All 15 Clustering & Worker Threads questions