Support trigger command

[radu-matei]

This commit allows Spin apps to be init containers, based on https://github.com/fermyon/spin-trigger-command:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: spin-test-init
spec:
  replicas: 1
  selector:
    matchLabels:
      app: spin-test
  template:
    metadata:
      labels:
        app: spin-test
    spec:
      runtimeClassName: wasmtime-spin-v2
      initContainers:
      - name: spin-init
        image: ttl.sh/spinkube-init:24h
        command: ["/"]
      containers:
      - name: spin-test
        image: ghcr.io/deislabs/containerd-wasm-shims/examples/spin-rust-hello:v0.10.0
        command: ["/"]
        ports:
        - containerPort: 80

[devigned]

@radu-matei I think I get mechanically how this works, but I'm not quite clear on the use case for init container having a command trigger. Can you explain what it would be used for?

[radu-matei]

I should have added a bit more context to this — we've had a lot of conversations during KubeCon with people who are interested in using Wasm as init containers — where their init containers take dozens of seconds to start, even if all they do is a tiny bit of work.

So they are interested in seeing how to use Wasm because of its tiny footprint and fast startup.

At the same time, we've also long been interested in a Spin trigger that ran a WASI (CLI) component to completion (regardless of what it does exactly).

So this brings the two together and a) lets you execute a "main" function to completion through Spin, and b) enables using that as an init container.

Turning this into a draft for now, as we want to discuss this in the next SpinKube meeting.

[devigned]

Would you want to pass in the args from the entrypoint of the container spec?

[jsturtevant]

So this brings the two together and a) lets you execute a "main" function to completion through Spin, and b) enables using that as an init container.

There isn't anything in here that makes it specifically for init containers, this could be used to run the main pod to completion as well? Thinking something like a kubernetes Job.

[jsturtevant]

LGTM, We might add a workload test for the init container scenario: https://github.com/spinkube/containerd-shim-spin/tree/main/tests/workloads

[radu-matei]

@jsturtevant correct, this can definitely be used for other kinds of workloads, not just init containers.

[Mossaka]

Is the spin command trigger implementing the wasip2 wasi-cli world? That is, does it have the sockets APIs?

[radu-matei]

@Mossaka yeah, the trigger now supports both WASI Preview 1 and WASI Preview 2 components (using the CLI world). Tested with a Rust component that uses sockets, so that definitely works (will push an example on this over the next few days).

On passing arguments, we have a proposal for that here:

fermyon/spin-trigger-command#4 (comment)

[Mossaka]

LGTM

[radu-matei]

This app manifest works in the shim using the command trigger:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: spin-test-init
spec:
  replicas: 1
  selector:
    matchLabels:
      app: spin-test
  template:
    metadata:
      labels:
        app: spin-test
    spec:
      runtimeClassName: wasmtime-spin-v2
      initContainers:
      - name: spin-init
        image: ttl.sh/spinkube-init-args:24h
        args: ["these", "are", "some", "args"]
      containers:
      - name: spin-test
        image: ghcr.io/deislabs/containerd-wasm-shims/examples/spin-rust-hello:v0.10.0
        command: ["/"]
        ports:
        - containerPort: 80

Getting the logs from the init container:

$ kubectl logs pods/spin-test-init-5f4d994596-q8wfh -c spin-init
hiwasi
these
are
some
args

[radu-matei]

I'll merge this so we can iterate. Thank you all for the feedback!