How to handle concurrency in Node.js ?

Node.js is an open-source, cross-platform runtime environment built on Chrome's V8 Engine. It is used to develop highly scalable backend as well as server-side applications.

Node.js uses a single-threaded event loop architecture. It is also asynchronous in nature. These are the two main reasons why node.js can handle multiple concurrent requests easily and why it becomes an obvious choice for the development of such applications.

Working of Node.js

Node.js works asynchronously. In other words, it does not block incoming requests from clients when the operating system has one I/O intensive request. Instead, it passes this I/O request to the internal C++ threads and takes up the next job from the event queue.

Note: The internal C++ threads may or may not be one, but that is not of concern here.

The event queue is nothing but a queue that stores the incoming requests in the order that it received them. It then passes on these requests to the Event Loop when it is available. For every I/O request, the event loop receives, passes it on to the internal C++ threads for processing and makes itself available for the other requests, and starts processing those. Then it uses the concept of callback functions from JavaScript to receive the responses of the tasks that were sent to the internal C++ threads earlier for processing and delivers them to the client.

Let's understand this with an example:

Example 1: Traditional Web Architecture

function func() {
    console.log("Hey I am Line number 1");
    setTimeout(function () {
        console.log("Hey I am Line number 2");
    }, 2000);
    console.log("Hey I am Line number 3");
}
console.log("Hey I am Line number 4");

func();

Here, we have 4 console.log() statements in the order of 1 to 4. Assuming this as a traditional architecture-based application, the output would look something like this.

Output:

Hey I am Line number 4
Hey I am Line number 1
Hey I am Line number 3
Hey I am Line number 2

Explanation: Here, the "Hey I am Line number 4" will get executed first as it is above the function call. After that, the function will be called. Then "Hey I am Line number 1" will be printed and the application will add callback passed to setTimeout into browser macrotask queue, print "Hey I am Line number 3" respectively and then when time specified as 2-nd argument of setTimeout exceedes, JS event loop will get callback function from queue and execute it, thus printing "Hey I am Line number 2".

Example 2: Node.js Application

function func() {
    console.log("Hey I am Line number 1");
    setTimeout(function () {
        console.log("Hey I am Line number 2");
    }, 2000);
    console.log("Hey I am Line number 3");
}
console.log("Hey I am Line number 4");
func();

Here, we have 4 console.log() statements in the order 1 to 4. Assuming this as a single-threaded non-blocking application, the output would look something like this.

Output:

Hey I am Line number 4
Hey I am Line number 1
Hey I am Line number 3
Hey I am Line number 2

Explanation: Here, the "Hey I am Line number 4" will get executed first as it is above the function call. After that, the function will be called. Then "Hey I am Line number 1" will be printed and the application will go into a timeout due to the function call, for 2 seconds (2000 ms).  Now, when the application is waiting for 2 seconds, it will not block further requests, it will instead process the next request and thus print "Hey I am Line number 3". Once the timeout is finished, the lines will now be executed and "Hey I am Line number 2" will be printed to the console.

In this way, the event loop never stays occupied or blocked by a particular request. This is what gives Node.js a competitive advantage over traditional web servers in terms of handling multiple user requests at the same time, i.e. concurrency.