All 15 Clustering & Worker Threads questions
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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? How do you handle an uncaught exception inside a specific cluster worker without taking down the entire application? What is the WorkerData option when creating a Worker, and how does it differ from communicating via postMessage() after creation? What is the difference in behavior between the 'primary' and 'worker' role in Node's cluster module regarding which code paths execute?
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Node.js › Clustering & Worker Threads

How does zero-downtime restart work with Node's cluster module or PM2, and why is it important for production deployments?

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Answer

Zero-downtime restart replaces workers one at a time -- each old worker finishes in-flight requests and stops accepting new ones while a new worker starts, so at every point some workers remain available, meaning no dropped requests during a deployment.
pm2 reload my-app
// PM2 starts a new worker, waits for it to be ready, then gracefully
// shuts down one old worker at a time until all run the new code
Real-world example An e-commerce API deploys several times daily using 'pm2 reload' rather than 'pm2 restart', so customers mid-checkout never experience a dropped connection during routine deployments.

Common follow-ups: What's the difference between 'pm2 restart' and 'pm2 reload' regarding downtime?;How long should a worker be given to drain requests before forced termination during a reload?

Deployment & Process Managers (PM2);Docker & Containerization for Node.js
All 15 Clustering & Worker Threads questions