Help me understand how pnpm works and why deps sometimes cannot be found

[mobob]

I've had a few number of issues with deps lately in my pnpm monorepo, so i'm trying to dive a bit deeper to understand whats going on, and taking the issue below to do so.

For background; i'm on the latest pnpm (9.12.1 and have tried older), using a monorepo. My workspace file is as follows (and the project i'm working on rn is backend:

packages:
  - 'pipeline'
  - 'frontend'
  - 'backend'
  - 'packages/*'

Today, i got this error while running a test:

 FAIL  <snip>  ● Test suite failed to run

    Cannot find module '@aws-sdk/core' from '../node_modules/.pnpm/@[email protected]_@[email protected]/node_modules/@aws-sdk/lib-dynamodb/dist-cjs/index.js'

Ok - so where is @aws-sdk/core? My cwd is /backend, so i check backend/node_modules:

(base) ➜  backend git:(oct7-pdfs) ✗ ls node_modules/@aws-sdk     
client-dynamodb        client-s3              client-ses             client-sts             util-dynamodb
client-eventbridge     client-secrets-manager client-sso-oidc        lib-dynamodb

Hmm - this seems odd. From how i understand packages that are dependencies should always show up at the root, and be (soft) symbolically linked in.

Probably doesn't matter much - the above search within lib-dynamodb isn't in the backend dir anyways, its in the root node_modules. So lets take a look in there:

(base) ➜  backend git:(oct7-pdfs) ✗ ls ../node_modules/.pnpm/@[email protected]_@[email protected]/node_modules/@aws-sdk
client-dynamodb lib-dynamodb    util-dynamodb

Ok, sure enough, its not there either. Which perhaps isn't a huge surprise, because @aws-sdk/core is not listed as a dependency of lib-dynamodb - it is of client-dynamodb.

https://github.com/aws/aws-sdk-js-v3/blob/main/lib/lib-dynamodb/package.json
https://github.com/aws/aws-sdk-js-v3/blob/534e028f7f8cb6f20fe9be0b61aba19e05bbd461/clients/client-dynamodb/package.json#L25C4-L25C26

So how then do all these nested upon nested deps get resolved? This is where my understanding breaks down.

For the record there are seemingly a zillion references to different versions of @aws-sdk/core in my lock file, so i suspect this and me recently adding a gargantuan lib (cough cough llamaindex) isn't helping me here.

And the kicker; the only workaround i currently have for this is to install @aws-sdk/core in the workspace root 😭. It feels like something is broken in my setup as all this should just work, so any tips to fill in my understanding or where to look next would be appreciated!