How does the thread pool fit into Node.js concurrency?
For operations that cannot be done asynchronously, like certain file system operations, libuv uses a thread pool (4 threads by default). These run off the main thread so they do not block the event loop, but callbacks still run on the main thread.
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More FAQs in How the Node.js Event Loop Handles Concurrency on a Single Thread
Through the event loop and non-blocking I/O. Async operations are delegated to libuv, so the thread processes other requests while waiting. When operations complete, their callbacks are queued and processed one at a time by the event loop.
No. Node.js handles many concurrent connections. The JavaScript code runs on one thread, but I/O operations are delegated to libuv, so the thread is free to process other requests while waiting for I/O to complete.
Because it uses far less memory than thread-per-request. One thread serves thousands of connections, since the thread is never blocked waiting for I/O. The trade-off is that CPU-heavy work on the main thread blocks everything.
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