Non-blocking I/O in Node.js: Underlying Principles for High-Concurrency Scenarios
This article focuses on explaining Node.js non-blocking I/O and its advantages. Traditional synchronous blocking I/O causes programs to wait for I/O completion, leaving the CPU idle and resulting in extremely low efficiency under high concurrency. Non-blocking I/O, by contrast, initiates a request without waiting, immediately executing other tasks, and notifies completion through callback functions, which are uniformly scheduled by the event loop. Node.js implements non-blocking I/O through the event loop and the libuv library: asynchronous I/O requests are handed over to the kernel (e.g., Linux epoll) by libuv. The kernel monitors I/O completion status, and upon completion, the corresponding callback is added to the task queue. The main thread is not blocked and can continue processing other tasks. Its high concurrency capability arises from: a single-threaded JS engine that does not block, with a large number of I/O requests waiting concurrently. The total time consumed is only the average time per single request, not the sum. libuv abstracts cross-platform I/O models and maintains an event loop (handling microtasks, macrotasks, and I/O callbacks) to uniformly schedule callbacks. Non-blocking I/O enables Node.js to excel in scenarios such as web servers, real-time communication, and I/O-intensive data processing. It is the core of Node.js's high concurrency handling, efficiently supporting tasks like front-end engineering and API services.
Read MoreNode.js Event Loop: Why Is It So Fast?
This article uses the analogy of a coffee shop waiter to explain the core mechanism of Node.js for efficiently handling concurrent requests—the event loop. Despite being single-threaded, Node.js can process a large number of concurrent requests efficiently, with the key lying in the collaboration between non-blocking I/O and the event loop: when executing asynchronous operations (such as file reading and network requests), Node.js delegates the task to the underlying libuv library and immediately responds to other requests. Once the operation is completed, the callback function is placed into the task queue. The event loop is the core scheduler, processing tasks in fixed phases: starting with timer callbacks (Timers), system callbacks (Pending Callbacks), followed by the crucial Poll phase to wait for I/O events, and then handling immediate callbacks (Check) and close callbacks (Close Callbacks). It ensures the ordered execution of asynchronous tasks through the call stack, task queues, and phase-based processing. The efficient design stems from three points: non-blocking I/O avoids CPU waiting, callback scheduling is executed in an ordered manner across phases, and the combination of single-threaded execution with asynchronous concurrency achieves high throughput. Understanding the scheduling logic of the event loop helps developers write more efficient Node.js code.
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