Why Node.js is a single threaded language ?

Applications built on the top of node.js use Single Threaded Event Loop Model architecture to handle multiple concurrent clients like JSP, Spring MVC, ASP.NET, HTML, Ajax, jQuery, etc. are other web technologies that can be used rather than node.js but these listed technologies follow “Multi-Threaded Request-Response” architecture to handle multiple concurrent clients.

Single thread: Node JS Platform doesn’t follow the Multi-Threaded Request/Response Stateless Model. It follows the Single-Threaded with Event Loop Model. Node JS Processing model mainly inspired by JavaScript Event-based model with JavaScript callback mechanism. Because of which Node.js can handle more concurrent client requests with ease. The event loop is the heart of the Node.js processing model as shown below diagram.

Node.js app/server single thread event loop model

Description of the diagram:

n = Number of requests by clients to the Node.js web server.
Lets assume they are accessing our Web Application built on top of Node.js concurrently Clients are Client-1, Client-2 . . . Client-n.
m = number of threads in thread pool.

Web Server receives Client-1, Client-2 . . . till Client-n requests and places them in the Event Queue.

Advantages of Single-Threaded Event Loop over Multi-Threaded Request/Response Stateless Model:

1. Can handle more & more concurrent client’s requests with ease.
2. Eliminates the need of creating more and more threads, because of the Event loop.
3. Applications built on top of node.js use the least threads possible to reduce memory or resource usage.

Reason behind node.js uses Single Threaded Event Loop Model architecture:

• Initially, node.js was created as an experiment in asynchronous processing and in theory was that doing asynchronous processing on a single thread could provide more performance and scalability under typical web loads than the typical thread-based implementation when the application isn’t doing CPU intensive stuff and can run thousands more concurrent connections than Apache or IIS or other thread-based servers.
• The single-threaded, asynchronous nature of node.js does also make things complicated but threading is worse than this in terms of time taken to design an application, cost of development, deadlocks, priority inversions, and all the other things that come in the life cycle of an application.
• There is also a very well known and criticized issue with the one thread per request model for a server which is that they don’t scale very well for several scenarios compared to the event loop thread model, in short, they lack scalability as the application grows to meet the future demands and with the addition of new features.
• As Node.js follows Single-Threaded with Event Loop Model inspired by JavaScript Event-based model with JavaScript callback mechanism. So, node.js is single-threaded similar to JavaScript but not purely JavaScript code which implies things that are done asynchronously like network calls, file system tasks, DNS lookup, etc. are actually not handled by the main thread and are in C++ as Ryan Dahl is not a fan of JavaScript and also C++ has access to multiple threads that makes it a better language for asynchronous tasks.

Reference: https://nodejs.org/en/docs/