Spin python wasm not working as expected ... what am I missing ?

[michaelalang]

Hey guys,

I am trying to spin a python WASM with

spin build 

and than use it with envoy which reports (no matter how I push the build)

{"level":"error","time":"2025-05-18T08:51:40.432327Z","scope":"wasm","msg":"convert the wasm config: cannot fetch Wasm module oci://quay.chester.at/infrastructure/wasm:test: could not fetch Wasm binary: the given image is in invalid format as an OCI image: 2 errors occurred:\n\t* could not parse as compat variant: invalid media type application/vnd.fermyon.spin.application.v1+config (expect application/vnd.oci.image.layer.v1.tar+gzip)\n\t* could not parse as oci variant: could not find the layer of type application/vnd.module.wasm.content.layer.v1+wasm\n\n"}

I was trying

spin registry push 

as well as in OCI format after reading the PR #1014

spin oci push

still I cannot get it running except with

spin up -f registry/image:tag

What am I missing to make the spin-python-sdk run as WASM plugin with envoy ?

[itowlson]

I'm not sure what Envoy expects as an OCI format, but the Spin application OCI format contains a full Spin application, manifest, Wasm binaries, data files and all. It looks from the error as if Envoy is looking to pull a single Wasm binary.

Envoy's instructions for creating a Wasm OCI appear to recommend doing a docker build/push or buildah command (https://gateway.envoyproxy.io/latest/tasks/extensibility/build-wasm-image/) that copies the single Wasm file into a scratch image. What Spin does in a registry push is very different from this, because spin registry push expects to be dealing with assets, configuration, and multiple binaries. So for the OCI part of the process you are probably better off following their guidance. You can still use spin build to create the Wasm binary, and then point your Dockerfile or buildah command at the resulting app.wasm.

Note that this will only copy the raw binary, not any file assets or Spin configuration, because those are part of the Spin application not part of the Wasm binary (spin registry push does include them, but Envoy then doesn't know what to do with this more complex image).

Hope this helps and please forgive me if it doesn't, I'm somewhat guessing based off the Envoy docs!

[michaelalang]

@itowlson thanks and no worries ... I will try following the instructions from https://www.fermyon.com/blog/python-wagi

Therefore, the first step to running Python scripts in WebAssembly is to get a WebAssembly Python module. It is possible to build your own, as the CPython project supports this. We prefer SingleStore’s WASI-enabled Python WebAssembly build, which provides a minimal toolkit to bridge Python and WebAssembly. Building either of these from source requires a lot of configuration and can take quite a while. So for this example, we have prebuilt and pre-configured a Python environment that you can use for your own projects. (This repo also has a sample Wagi setup).

and finallizing this thread, is the assumption okay that spin registry push is not expected to work (or maybe) if something else picks up the WASM that does not understand the multilayer approach of spin ?

[itowlson]

That final assumption is correct. spin registry push always creates the multi-layer format. (We've discussed a way to push bare Wasm binaries but are not yet sure what that looks like - definitely don't hold your breath!)

You'll need to check with Envoy what module interface it expects. The Python WAGI blog post is quite old and reflects a CGI-style interface (the Wasm equivalent of a main() entry point that returns a response over stdout). The current SDK instead produces a "WASI HTTP" component which is more like a .so or .dll with a specific strong-typed entry point. I don't know which, if either Envoy is looking for, sorry.

[michaelalang]

thanks and ack on the format of envoy for WASM OCI ...
it expects a single layer wasm binary like defined in https://github.com/solo-io/wasm/blob/master/spec/spec.md#layers

the rest is something I have to figure out with envoy :)

thanks for your update <3