How does the event loop process async I/O callbacks?
The event loop processes callbacks in phases: timers, pending callbacks, poll, check, and close. When libuv queues a completed I/O callback, the event loop picks it up on its next poll phase and runs it on the main thread.
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More FAQs in How Async I/O Works in Node.js Through libuv
When JavaScript calls an async I/O operation, it is delegated to libuv, which handles it off the main thread using the OS's async capabilities or the thread pool. When complete, libuv queues the callback, and the event loop runs it on the main thread.
libuv handles the actual I/O operation off the main thread, using the OS's async capabilities (epoll, kqueue, IOCP) or the thread pool. When the operation completes, libuv queues the callback for the event loop to process.
Because the main thread is never blocked waiting for I/O. While one request waits for a database query, the event loop processes other requests' callbacks. This lets Node.js handle thousands of concurrent connections on one thread.
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