Node.js is single-threaded, but it provides two primary mechanisms for parallel processing: the `cluster` module and `worker_threads`. They solve different problems and it's crucial to know when to use each.
The `cluster` Module
The `cluster` module allows you to create multiple Node.js processes that can all share the same server port. It's essentially a load balancer for your application.
- Mechanism: Creates separate processes with their own memory and event loop.
- Best for: I/O-bound workloads. Ideal for scaling a web server to handle a large number of concurrent network connections across multiple CPU cores.
- Use Case: An Express API server. You can fork the server process for each CPU core, and the master process will distribute incoming HTTP requests among the workers.
The `worker_threads` Module
Worker threads allow you to run multiple JavaScript threads within a single Node.js process. They can share memory efficiently using `SharedArrayBuffer`.
- Mechanism: Creates separate threads within the same process.
- Best for: CPU-bound workloads. Ideal for offloading a single, heavy computation from the main event loop to prevent it from blocking.
- Use Case: A web server receives a request to process a large image or a complex video file. It can spawn a worker thread to handle the processing, while the main thread remains free to handle other incoming requests.
Key Takeaway
Use `cluster` to scale your entire application to handle more concurrent I/O (like network requests). Use `worker_threads` to offload a specific, long-running CPU-intensive task from your main thread to keep your application responsive.
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