Node.js Event Loop Phases
Design a small subsystem that relies on Node.js Event Loop Phases. Outline components, data flow, failure modes, and metrics.
Answers use simple, clear English.
Quick interview answer
Use Node.js Event Loop Phases as the core idea. Example shape: API server: heavy sync JSON.parse in request handler blocks the poll phase and delays all concurrent connections..
Detailed answer
Use Node.js Event Loop Phases as the core idea. Example shape: API server: heavy sync JSON.parse in request handler blocks the poll phase and delays all concurrent connections.. Watch for: CPU-bound work blocks the loop; microtask flooding delays timers.. Measure success via latency/error/saturation. Core: Node runs libuv phases: timers → pending → idle/prepare → poll → check (setImmediate) → close. process.nextTick and Promise microtasks run between phases and can starve I/O if abused. Real-time example: API server: heavy sync JSON.parse in request handler blocks the poll phase and delays all concurrent connections. Pros: Single-threaded model simplifies reasoning; non-blocking I/O scales connection count. Cons: CPU-bound work blocks the loop; microtask flooding delays timers. Common mistakes: Assuming setTimeout(0) runs before setImmediate always (order varies at startup). Best practices: Offload CPU work to worker_threads; never block the event loop in hot paths. Audience level: Fresher.
Full explanation
Node runs libuv phases: timers → pending → idle/prepare → poll → check (setImmediate) → close. process.nextTick and Promise microtasks run between phases and can starve I/O if abused.
Real example & use case
API server: heavy sync JSON.parse in request handler blocks the poll phase and delays all concurrent connections.
Pros & cons
Pros: Single-threaded model simplifies reasoning; non-blocking I/O scales connection count. Cons: CPU-bound work blocks the loop; microtask flooding delays timers.
Common mistakes
Assuming setTimeout(0) runs before setImmediate always (order varies at startup).
Best practices
Offload CPU work to worker_threads; never block the event loop in hot paths.
Follow-up questions
- How would you test Node.js Event Loop Phases?
- What metrics prove Node.js Event Loop Phases is healthy in prod?
- How does Node.js Event Loop Phases change at 10× traffic?