Next App Router (14.2.3) - Inconsistent Singleton

[vaneenige]

Link to the code that reproduces this issue

https://github.com/vaneenige/next-app-router-singleton

To Reproduce

1. Install dependencies
2. Run the build - npm run build
3. See the log of Create random multiple times

Current vs. Expected behavior

Current behavior:

• We are rendering 100 pages that all use a singleton module.
• During the build it logs the initialization multiple times (~5)

Expected behavior:

• It should run the initialization once
• It's also odd that it doesn't run it 100 times, but rather just a few times.

In the reproduction repository you can find a few different test cases:

• With a class that is initialized
• With the globalThis (check and override)
• With the React.cache (async) function

---

After quite a bit of research I found multiple sources claiming that this pattern does work with the pages directory, but also multiple sources that can't get it to work with the app directory.

• Using the globalThis method in pages: Global variables #15054 (comment)
• Multiple users having issues with it in app: Caching in Dev Mode #48481 (comment)
• Shiki (syntax highlighter) causing memory issues (the solution is in the reproduction, but doesn't behave as expected): RuntimeError: memory access out of bounds shikijs/shiki#567
• Prisma suggesting the globalThis solution to prevent too many connections: https://www.prisma.io/docs/orm/more/help-and-troubleshooting/help-articles/nextjs-prisma-client-dev-practices

---

Having a proper singleton could benefit a lot of use cases such as database connections or separate worker scripts.

I've seen the issue being discussed on Reddit and GitHub with regards to MongoDB (or other persistent API connections) too.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.0.0: Fri Sep 15 14:41:34 PDT 2023; root:xnu-10002.1.13~1/RELEASE_ARM64_T8103
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 18.17.1
  npm: 9.6.7
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 14.2.3 // Latest available version is detected (14.2.3).
  eslint-config-next: 14.2.3
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.4.5
Next.js Config:
  output: N/A

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

Module Resolution

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

next build (local)

Additional context

No response

[bitttttten]

Are you sure it's not becaues of the Webpack Build Worker? This can split your build over a few different workers. This means that webpack is run X times over your app and therefore you may see X amount of logs called.

You can opt out of it: https://nextjs.org/docs/messages/webpack-build-worker-opt-out#webpack-build-worker-opt-out although you will most likely see a degrade in build times.

[vaneenige]

I thought this could be it (as I was already thinking it's running separate builds in parallel), but it doesn't seem to change anything. When I add the following it still runs the initialization multiple times (1 time for every ~20 pages):

const nextConfig = {
  experimental: {
    webpackBuildWorker: false
  }
}