How the single threaded non blocking IO model works in Node.js

Question

I\'m not a Node programmer, but I\'m interested in how the single threaded non blocking IO model works. After I read the article understanding-the-node-js-e

Answer 1

Node.js is based on the event loop programming model. The event loop runs in single thread and repeatedly waits for events and then runs any event handlers subscribed to those events. Events can be for example

• timer wait is complete
• next chunk of data is ready to be written to this file
• theres a fresh new HTTP request coming our way

All of this runs in single thread and no JavaScript code is ever executed in parallel. As long as these event handlers are small and wait for yet more events themselves everything works out nicely. This allows multiple request to be handled concurrently by a single Node.js process.

(There's a little bit magic under the hood as where the events originate. Some of it involve low level worker threads running in parallel.)

In this SQL case, there's a lot of things (events) happening between making the database query and getting its results in the callback. During that time the event loop keeps pumping life into the application and advancing other requests one tiny event at a time. Therefore multiple requests are being served concurrently.

According to: "Event loop from 10,000ft - core concept behind Node.js".