Nodejs event emitter implementation in Nextjs 13 or 14

[torver213]

How do i make nodejs event emitter object to listen to emitted events globally within my nextjs13 application just like in normal nodejs environment?

I have a file post.js where a user creates a new post. I want to promptly return the response back to the user, emit an event where a handler function will listen to the emitted event in another file & perform some other tasks in the background like updating post tags count and then finally emitting an sse(server sent event) to client app to notify other connected clients.

First of all, i created a file known as eventEmitter.js where I created a single instance to be used within my application below;

    //eventEmitter.js

    import { EventEmitter } from 'node:events';

    const eventEmitter = new EventEmitter({captureRejections: true})

    export default eventEmitter;

    //post.js
  
    import eventEmitter from './eventEmitter'
  
    const insertPost = async() => {
       // perform db post insert
       
       //emit an event to be listened to perform a background task
       eventEmitter.emit("PING", "hello world");
       
       //return 
       return {id: 1, title: 'hello' }
   }
   

   insertPost()
   
   
    // this only works when the listener is in the same file with the emitted event
   eventEmitter.on("PING", (ev) => {
        console.log(ev)
   });
   

    //listener.js
  
    import eventEmitter from './eventEmitter'
    // this doesn't work i.e the event is not captured in the file
   eventEmitter.on("PING", (ev) => {
        console.log(ev)
   });

 //route.js
  export function GET(request: Request) {
       const responseStream = new TransformStream();
       const writer = responseStream.writable.getWriter();
       const encoder = new TextEncoder();
       //... other code
      
       // this doesn't work i.e the event is not captured in the file
      eventEmitter.on("PING", (ev) => {
        console.log(ev)
     });
     
     // my streaming logic
}

My question is, how do i listen to the emitted event in other files like listener.js and route.js files other than just post.js file?

I want to listen to the emitted events globally within my nextjs app

The above works seamlessly in a nodejs environment and i expected it to work in nextjs environment too.

PS: I'm using nextjs App Router

Please help.

[jonshipman]

I mean, couldn't you have a src/lib/application.ts and listen from there?

import { EventEmitter } from 'node:events';

class ApplicationEmitter extends EventEmitter {}

export const app = new ApplicationEmitter();

app.on('something', function() {
	// do something...
});

The issue you're running into is that on that GET request, it only fires during the polling. Not knowing where the app is hosted is another issue as "serverless" hosts spin up instances on request. If you are on a traditional server then you can just import '@/lib/application' into 'src/middleware.ts' and it should work with emits.

Also, it's never a good idea to wrap listeners in functions. That's a quick way to a memory leak my friend. Instead emit a callback. e.g. eventEmitter.emit('PING', function callback() { /* awesome */ }); then in the application.ts, eventEmitter.on('PING', function(cb) { cb(); });

[latifs]

Hi @techfortified,
have you been able to solve that?
thanks

[arvilmena]

I tried putting it on instrumentation.ts but I am getting errors for when webpack is trying to bundle server packages. Even though I have these packages ignore in the nextjs webpack config. Is bundling intrumentation.ts skips the webpack config?

Curious as how others are able to implement eventEmitters in NextJs server

[jonshipman]

You need to inline the imports. const cron = await import('node-cron'); within the register function.

export async function register() {
  if (process.env.NEXT_RUNTIME === 'nodejs') {
    await import('@/server');

[arvilmena]

@jonshipman Thanks, that led me to fixing the build error in my actual project, but at the end the events are still not getting listened.

So, I created a minimal repro repo: https://github.com/arvilmena/test--nextjs--eventemitter

There's a server action that will emit an event. And some event listener inside my app should be invoked everytime that event was emitted. I tried 2 approach:

1. Is to instantiate the event listeners in the instrumentation.ts = did not work
2. Is to instantiate the event listeners at the top of the server action file = it worked! but looks ugly, it looks like that the event listeners will be instantiated everytime any server action in that server action file is called? will that cause memory leak?

If you can take a look here's the reproducible repo: https://github.com/arvilmena/test--nextjs--eventemitter
Branch for attempt 1: https://github.com/arvilmena/test--nextjs--eventemitter/tree/attempt/1-via-instrumentation-js
Branch for attempt 2: https://github.com/arvilmena/test--nextjs--eventemitter/tree/attempt/2-via-on-top-of-server-actions-file

Can you guide me on what's the best approach?

Thank you!