A module can be imported multiple times on server

[duydang2311]

Link to the code that reproduces this issue

https://github.com/duydang2311/module-imported-multiple-times-issue-reproduction

To Reproduce

1. Start the application in development (next dev).
2. Connect to http://localhost:3000.

Current vs. Expected behavior

There are instrumentation.ts (NodeJS runtime) and page.tsx (runs on server) importing the same service.ts file.

Current behavior

The server prints "service.ts was imported" twice.

This behaviour, in my original code, causes my service configuration in instrumentation hook to be lost when service.ts is imported and used in my pages.

Expected behavior

The server prints "service.ts was imported" just once.

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP Fri Jan 27 02:56:13 UTC 2023
Binaries:
  Node: 18.18.0
  npm: 10.1.0
  Yarn: N/A
  pnpm: 8.7.5
Relevant Packages:
  next: 13.5.3-canary.3
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.1.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

App Router

Additional context

No response

[justLuiz]

for me this is not a bug, in your sample its a expected behavior

[duydang2311]

I think the expected behaviour is to load/initialize every module just once in the same environment, which is on the server in this sample. I'm using instrumentation as an entry point to configure my services. Once the pages import the service, it somehow treats the service as a new module as if it was never imported before, hence all the pre-configuration are lost at this point.

[MangoMarcus]

Is there any update on this issue?

Modules should only be evaluated once, so this isn't the expected behavior. OP's example includes a dynamic import which may throw a spanner in the works, but it should still only be evaluated once.

I've noticed the same behavior with static imports both during dev and during the build step, using the app router.

[leduyquang753]

Having the same issue. Setting up a websocket instance in a module causes multiple instances to be created on the server. So I have to resort to setting things into globalThis.

[VictorGlindasPaf]

This seems to happen both in dev and production mode.
The cause looks to be due to multiple chunks containing the same module code.

[PrinOrange]

Same here. And now I'm trapped in such testing problem for long time.