The objects initialised in instrumentation.ts are not available in the rest of the code (server side).

[MansourM61]

Summary

I am new to Next.JS and my previous experience is with Vite.

Let's say I have a module called global-vars.ts with the following content:

// global-vars.ts
const globalData = {}

export { globalData };

Then in my instrumentation.ts, I put some data in the globalData variable as below:

// instrumentation.ts
import { globalData } from "./lib/global-vars";

export function register() {
    Object.assign(globalData, { id: "test" });

    console.log(globalData);
}

Now the problem is that the content inside the variable globalData is not preserved when I access it in other places on the server. For example, in middleware.ts the following code:

import { NextResponse, NextRequest } from 'next/server'
import { globalData } from './global-vars';

// This function can be marked `async` if using `await` inside
export function middleware(request: NextRequest) {
    console.log(globalData);

    return Response.json({ message: 'Hello World' })
}

writes [] in the console.

In a normal Javascript/Typescript project, the state of a module is preserved when it is called at different places. So I wonder if I am doing something wrong here or this is the way Nest.JS works.

Additional information

The online documentation says that register() function in instrumentation.ts can be used to run the code once when the server is setting up. So I don't understand why this is happening.

Example

No response

[icyJoseph]

Just speaking to what's happening. If everything was running on the same thread, then it'd work, but there's different layers going on here. Some work is done inside workers, edge middleware is packed into a different environment, etc...

[MansourM61]

Thanks for replying.

In that case, I can think of two solutions:

1. to put the variables related to a certain API or page in the corresponding API or page itself.
2. create a mini server running in parallel that holds all global data and all APIs and pages interact with it.

I'll see what I can do.
Thanks.

[MansourM61]

Just did a quick test.
Option one is not working since for every call to the API or the page, a different thread is being used. I am assuming the design philosophy behind this architecture is to treat the server as a pure-function server. I think that makes sense considering the fact that Next.js is introuced as a stateless framework.

[icyJoseph]

Btw, you can append stuff to globalThis, the problem is typically more with singletons, but if you did something like:

// my module
// global-vars.ts
const globalData = { data: 1000}

export default globalData

And then in instrumentation:

// import globalData
// inside the register function
globalThis.data = globalData

And then in your server component, you can do console.log(globalThis.data) and there'll be data, but doing this:

// import globalData
console.log(globalThis.data === globalData) // false - not the same singleton
console.log(globalThis.data.data, globalData.data) // 1000 1000

I wouldn't rely this too much though. Middleware will also see the value. You can segment out by process.env.NEXT_RUNTIME too, etc.