Is node.js single-threaded?

Node.js is single-threaded by default. Because of the JavaScript engine, node.js adopts a single-threaded asynchronous non-blocking mode. One nodejs cannot utilize multi-core resources; however, there are third-party libraries that can provide multiple Thread support is provided, but the main thread is single-threaded, and operations in the main thread are synchronously blocked.

The operating environment of this tutorial: windows10 system, nodejs version 12.19.0, Dell G3 computer.

Is node.js single-threaded?

Node.js is single-threaded by default, and node.js adopts single-threaded asynchronous non-blocking mode.

node.js adopts single-threaded asynchronous non-blocking mode. Its single thread refers to the single thread of its own Javascript running environment. Node.js does not have the ability to create new threads when Javascript is executed. Asynchronous is achieved through Libuv and its event loop.

Node is just that the main thread is single-threaded, and the operations in the main thread are synchronously blocked. But it throws all asynchronous operations to the IO thread pool, and the main thread interacts with the IO thread pool through a certain event polling mechanism to obtain asynchronous data.

The performance of node.js is not the highest. Because of the JavaScript engine, node.js is single-threaded by default, and a node.js application cannot utilize multi-core resources.

However, there are third-party libraries that provide multi-threading support, but it is not seamless. Node.js solves I/O bottlenecks (compared to traditional technology, synchronous blocking calls waste threads). It does not improve I/O speed, but resource scheduling is more efficient. If the I/O speed is not solved, node.js can only say that it can handle many requests at the same time, but the response time of each request is still that long, or even longer.

Node is single-threaded, so how does it handle high concurrent requests?

Rely on event-driven.

Each Node.js process has only one main thread executing program code, forming an execution context stack.

In addition to the main thread, an "Event queue" is also maintained. When the user's network request or other asynchronous operation arrives, node will put it into the Event Queue. At this time, it will not be executed immediately, and the code will not be blocked. It will continue until the main thread code is executed. complete.

After the execution of the main thread code is completed, and then through the Event Loop, that is, the event loop mechanism, start to retrieve the first event from the beginning of the Event Queue, allocate a thread from the thread pool to execute the event, and then Come down and continue to take out the second event, and then allocate a thread from the thread pool to execute, then the third, and the fourth. The main thread continuously checks whether there are unexecuted events in the event queue until all events in the event queue have been executed. After that, whenever a new event is added to the event queue, the main thread will be notified to take them out in order and hand them over to EventLoop for processing. When an event is executed, the main thread will be notified, the main thread will execute the callback, and the thread will be returned to the thread pool.

The main thread continues to repeat the third step above.

NodeJs single thread is just a main thread. Essentially asynchronous operations are completed by the thread pool. Node hands over all blocking operations to the internal thread pool for implementation, and is only responsible for continuous round-trip scheduling. , no real I/O operations are performed, thereby achieving asynchronous non-blocking I/O. This is the essence of node's single-threaded and event-driven nature.

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