Nodejs callback mechanism - which thread handles the callback?

Question

I'm new to nodeJS and was wondering about the single-instance model of Node. In a simple nodeJs application, when some blocking operation is asynchronously handled with callbacks, does the main thread running the nodeJs handle the callback as well?. If the request is to get some data off the database, and there are 100s of concurrent users, and each db operation takes couple of seconds, when the callback is finally fired (for each of the connections), is the main thread accepting these requests used for executing the callback as well? If so, how does nodeJs scale and how does it respond so fast?.

Answer 1

Each instance of nodejs runs in a single thread. Period. When you make an async call to, say, a network request, it doesn't wait around for it, not in your code or anywhere else. It has an event loop that runs through. When the response is ready, it invokes your callback.

This can be incredibly performant, because it doesn't need lots of threads and all the memory overhead, but it means you need to be careful not to do synchronous blocking stuff.

There is a pretty decent explanation of the event loop at http://blog.mixu.net/2011/02/01/understanding-the-node-js-event-loop/ and the original jsconf presentation by Ryan Dahl http://www.youtube.com/watch?v=ztspvPYybIY is worth watching. Ever seen an engineer get a standing ovation for a technical presentation?