NodeJS event loop vs event queue priority

Question

It is to my understanding that the node event loop will continue to handle requests until the event loop is empty, at which point it will look the the event queue to complete the blocking I/O requests.

My question is.. What happens if the event loop never becomes empty? Not due to bad code (ie never ending loop) but due to consistent client requests (thinking something like google that gets never ending requests)?

I realize there is a possibility I am misunderstanding a fundamental aspect of how client requests are handled by a server.

Answer 1

There are actually several different phases of the event loop (timers, I/O, check events, pending callbacks, etc...) and they are checked in a circular order. In addition some things (like promises and other microtasks) go to the front of the line no matter what phase of the event loop is currently processing.

It is possible that a never ending set of one type of event can block the event queue from serving other types of events. That would be a design/implementation problem that needs to be prevented.

You can read a bit more about the different types of things in the event loop here: https://developer.ibm.com/tutorials/learn-nodejs-the-event-loop/ and https://www.geeksforgeeks.org/node-js-event-loop/ and https://snyk.io/blog/nodejs-how-even-quick-async-functions-can-block-the-event-loop-starve-io .

While it is possible to overload the event loop such that is doesn't get out of one phase of the event loop, it's not very common because of the way most processing in nodejs consists of multiple events which gives other things a chance to interleave. For example, processing an incoming http request consists of connecting, reading, writing, closing, etc... and the processing of that event may involve other types of events. So, it can happen that you overload one type of event (I've done it only once in my coding and that was because of poorly written communication between the main thread and a bunch of WorkerThreads that was easily fixed to not have the problem once I realized what the problem was.