TL;DR

Event loop must never block. Sync file reads, heavy CPU work, and long loops freeze all requests. Use async APIs ( fs.promises ), offload CPU to worker threads, and chunk large arrays with setImmediate .

Clustering utilizes all CPU cores. Node.js single-thread leaves cores idle. Use cluster module or PM2 ( pm2 start app.js -i max ). Requires stateless design – move sessions and caches to Redis.

Redis caching > in-memory. node-cache works per worker but not shared. ioredis provides shared cache across processes and servers, plus persistence and pub/sub.

Monitor event loop lag, heap usage, and active handles. Use Clinic.js for profiling ( clinic doctor -- node app.js ).…