07-extism-go-plug-in.md

Create an Extism plug-in with TinyGo

• run the plugins with the Extism CLI
• use the host functions provided by the PDK

Extism Go PDK: https://github.com/extism/go-pdk

Create a new project with source code

mkdir 07-extism-go-plug-in
cd 07-extism-go-plug-in
go mod init hello-extism-go

Create a main.go file

touch main.go

package main

//export hello()
func hello() int32 {
  // read the function argument from the memory

  // display the argument

  // read the config object
  
  // display the value of url

  // use the url to make an http request

  // display the response

  // return a message: output := "🎉 Extism is 💜"


  return 0
}

func main() {}

• //export hello(), then the function is "visible" for the host
• the empty main function is mandatory
• int32: the wasm functions return a number (if you make a void function it should work)

Install the Go plug-in development kit

go get github.com/extism/go-pdk

Specifications

The wasm plugin will be launched with the Extism CLI, like this:

extism call go-plugin.wasm hello \
  --input "😀 Hello World 🌍! (from TinyGo)" \
  --log-level "info" \
  --allow-host "*" \
  --set-config '{"url":"https://jsonplaceholder.typicode.com/todos/1"}' \
  --wasi

# info, debug, warn, error
extism call go-plugin.wasm hello \
  --input "😀 Hello World 🌍! (from TinyGo)" \
  --log-level "debug" \
  --allow-host "*" \
  --set-config '{"url":"https://jsonplaceholder.typicode.com/todos/1"}' \
  --wasi

We call the hello function in the wasm plugin

• --input will pass the string to the plugin
• --log-level will set the log level
• --allow-host "*" will allow the plugin to make http requests
• --set-config will set the configuration for the plugin (another way to pass data to the plugin)
• --wasi will enable WASI

The function will:

• read the function argument from the memory
• display the result
• get the value of url from the config object
• display the value of url
• make a GET HTTP request to the url
• display the response
• return a value to the host (the CLI)

Update the source code / how to use the Extism PDK

How to read the function argument fro the shared memory

// read the function argument from the memory
input := pdk.Input()

How to display the result

// display input
pdk.Log(pdk.LogInfo, string(input))

How to read a key from the config object

// read a key from the config object
url, _ := pdk.GetConfig("url")

How to make an HTTP request

// make an HTTP request
req := pdk.NewHTTPRequest("GET", url)
res := req.Send()
// you can get the result with res.Body()

• If you want to test the response status , use res.Status()
• If you want to display the response body, use res.Body()
• If you want to display the response status, use strconv.FormatUint(uint64(res.Status()), 10)

How to "return" a value to the Extism CLI

// return a message
// copy string to host memory
mem := pdk.AllocateString("🎉 Extism is 💜")
pdk.OutputMemory(mem)

Build the plugin

tinygo build -scheduler=none --no-debug \
  -o go-plugin.wasm \
  -target wasi main.go

ls -lh *.wasm

... And, then, run the plugin with the Extism CLI 🚀