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Explain the working of Node.js

  • Difficulty Level : Medium
  • Last Updated : 19 Sep, 2021

Node.js is an open-source backend javascript runtime environment. It is a used as backend service where javascript works on the server-side of the application. This way javascript is used on both frontend and backend. Node.js runs on chrome v8 engine which converts javascript code into machine code, it is highly scalable, lightweight, fast, and data-intensive.

Working of Node.js: Node.js accepts the request from the clients and sends the response, while working with the request node.js handles them with a single thread. To operate I/O operations or requests node.js use the concept of threads. Thread is a sequence of instructions that the server needs to perform. It runs parallel on the server to provide the information to multiple clients. Node.js is an event loop single-threaded language. It can handle concurrent requests with a single thread without blocking it for one request.

Node.js basically works on two concept

  •  Asynchronous
  •  Non-blocking I/O

Non-blocking I/o: Non-blocking i/o  means working with multiple requests without blocking the thread for a single request. I/O basically interacts with external systems such as files, databases. Node.js is not used for CPU-intensive work means for calculations, video processing because a single thread cannot handle the CPU works.

Asynchronous: Asynchronous is executing a callback function. The moment we get the response from the other server or database it will execute a callback function. Callback functions are called as soon as some work is finished and this is because the node.js uses an event-driven architecture. The single thread doesn’t work with the request instead it sends the request to another system which resolves the request and it is accessible for another request.



To implement the concept of the system to handle the request  node.js uses the concept of Libuv.

Libuv is an open-source library built-in c++. It has a strong focus on asynchronous and  I/O, this gives node access to the underlying computer operating system, file system, and networking.

Libuv implements two extremely important features of node.js  

  •  Event loop
  •  Thread pool

Event loop: The event loop contains a single thread and is responsible for handling easy tasks like executing callbacks and network I/O. When the program is to initialize all the top-level code is executed, the code is not in the callback function. All the applications code that is inside callback functions will run in the event loop. EventLoop is the heart of node.js. When we start our node application the event loop starts running right away. Most of the work is done in the event loop.

Nodejs use event-driven-architecture.

  • Events are emitted.
  • Event loop picks them up.
  • Callbacks are called.

Event queue: As soon as the request is sent the thread places the request into a queue. It is known as an event queue. The process like app receiving HTTP request or server or a timer will emit event as soon as they are done with the work and event loop will pick up these events and call the callback functions that are associated with each event and response is sent to the client. 

The event loop is an indefinite loop that continuously receives the request and processes them. It checks the queue and waits for the incoming request indefinitely.

Thread pool: Though node.js is single-threaded it internally maintains a thread pool. When non-blocking requests are accepted there are processed in an event loop, but while accepting blocking requests it checks for available threads in a thread pool, assigns a thread to the client’s request which is then processed and send back to the event loop, and response is sent to the respective client.

The thread pool size can be change:

process.env.UV_THREADPOOL_SIZE = 1;

Working of Node.js

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