Questions
1 of 2
What are worker threads?
Difficulty: 5/10
worker_threads API, message passing, CPU-bound offloading
The node:worker_threads module enables the use of threads that execute JavaScript in parallel. Workers (threads) are useful for performing CPU-intensive JavaScript operations. They don't help much with I/O-intensive work. The Node.js built-in asynchronous I/O operations are more efficient than Workers can be.
List important properties available on worker_threads module:
- 1
Worker
- 2
isMainThread
- 3
parentPort
- 4
workerData
- 5
THERE ARE MORE
Unlike child_process or cluster, worker_threads can share memory. They do so by transferring ArrayBuffer instances or sharing SharedArrayBuffer instances.
Scenario Questions
0-2 years experience
- 1You need to parse a large CSV file without blocking the event loop. How would you use a Worker thread to accomplish this, and what steps are required to get the parsed data back to the main thread?
- 2If you create a Worker with `new Worker('./worker.js')` and forget to call `worker.unref()`, what effect does that have on process shutdown?
- 3What happens if the Worker throws an uncaught exception? How does the main thread observe it?
2-5 years experience
- 1Your new feature streams video frames to a client and you offload frame encoding to a Worker. Midway through testing, the encoding sometimes stalls and the main thread logs 'worker exited with code 1'. Walk me through how you'd debug this.
- 2You need to share a large read‑only buffer between the main thread and multiple Workers. Which API would you use and what trade‑offs does it introduce?
- 3Explain why using a Worker for a simple async I/O operation might be a bad idea, and how you would refactor it.
5-8 years experience
- 1Our service processes thousands of concurrent image transformations. We currently spin up a new Worker per request, leading to high memory usage. How would you redesign the worker usage to improve throughput and resource efficiency?
- 2Describe how you would implement a pool of reusable Workers that can handle variable‑size jobs while ensuring back‑pressure and graceful shutdown.
- 3When deploying to a container orchestration platform, what considerations around Worker thread limits, CPU quotas, and process signals must you account for?
8+ years experience
- 1We are migrating a legacy monolith that uses child_process for CPU‑bound work to a microservice architecture. How would you evaluate moving to Worker threads across services, considering maintainability, observability, and cross‑team ownership?
- 2Design a cross‑team strategy for standardizing Worker thread usage, including guidelines for error handling, logging, and shared memory, while allowing teams to opt‑in to custom implementations.
- 3At scale, how would you monitor and alert on Worker thread health across hundreds of nodes, and what metrics would you prioritize?
Follow-up Questions
- How would you decide between a Worker and a child process for a given task?
- What are the pitfalls of sharing state between the main thread and a Worker?
- Can you describe how you would handle errors emitted by a Worker?