How to Migrate an Express API to AWS Lambda Without Using Wrappers?

It sounds like you’re using TypeScript, right? If you are, you really don’t need to rely heavily on layers for things like models. They’re usually meant for sharing large dependencies or binaries. Just use normal imports and ensure your build process handles tree-shaking. This way, you can bundle your dependencies and shared code effectively. Tools like esbuild work great for this, and both CDK and SAM support it out of the box.

You really need to consider why you're migrating in the first place. A genuine serverless approach might require a complete rewrite—potentially moving all dependencies to managed services, like MongoDB alternatives, and using API Gateway instead of Express. If all you need to do is make it serverless, maybe a wrapper could suffice. But if they’re set on pure Lambdas, understand it’s going to mean more work on your end. Also, layers are primarily for sharing code or dependencies, not for managing warm connections.

Honestly, I’d recommend avoiding layers in this case. They can complicate things more than necessary. It’s better to write a package for any shared libraries, treating them like a regular dependency. Store secrets in Secrets Manager and use SSM for configuration. And about your express confusion—just remember, wrapping your API might mean you’re still using Express in some capacity. If you want to eliminate it, make sure to refactor the routing and everything else accordingly!

Uploading everything into a single lambda could be an option if you stay within the size limits. Just be cautious with layers since versioning can be tricky. It might work for you, but managing them can be a pain.