Extism Go PDK

This library can be used to write Extism Plug-ins in Go.

Install

Include the library with Go get:

go get github.com/extism/go-pdk

Reference Documentation

You can find the reference documentation for this library on pkg.go.dev.

Getting Started

The goal of writing an Extism plug-in is to compile your Go code to a Wasm module with exported functions that the host application can invoke. The first thing you should understand is creating an export. Let's write a simple program that exports a greet function which will take a name as a string and return a greeting string. Paste this into your main.go:

package main

import (
	"github.com/extism/go-pdk"
)

//export greet
func greet() int32 {
	input := pdk.Input()
	greeting := `Hello, ` + string(input) + `!`
	pdk.OutputString(greeting)
	return 0
}

func main() {}

Some things to note about this code:

1. The //export greet comment is required. This marks the greet function as an export with the name greet that can be called by the host.
2. We need a main but it is unused.
3. Exports in the Go PDK are coded to the raw ABI. You get parameters from the host by calling pdk.Input* functions and you send returns back with the pdk.Output* functions.
4. An Extism export expects an i32 return code. 0 is success and 1 is a failure.

Install the tinygo compiler:

See https://tinygo.org/getting-started/install/ for instructions for your platform.

Note: while the core Go toolchain has support to target WebAssembly, we find tinygo to work well for plug-in code. Please open issues on this repository if you try building with go build instead & have problems!

Compile this with the command:

tinygo build -o plugin.wasm -target wasi main.go

We can now test plugin.wasm using the Extism CLI's run command:

extism call plugin.wasm greet --input "Benjamin" --wasi
# => Hello, Benjamin!

Note: Currently wasi must be provided for all Go plug-ins even if they don't need system access, however this will eventually be optional.

Note: We also have a web-based, plug-in tester called the Extism Playground

More Exports: Error Handling

Suppose we want to re-write our greeting module to never greet Benjamins. We can use pdk.SetError or pdk.SetErrorString:

//export greet
func greet() int32 {
	name := string(pdk.Input())
	if name == "Benjamin" {
		pdk.SetError(errors.New("Sorry, we don't greet Benjamins!"))
		return 1
	}
	greeting := `Hello, ` + name + `!`
	pdk.OutputString(greeting)
	return 0
}

Now when we try again:

extism call plugin.wasm greet --input="Benjamin" --wasi
# => Error: Sorry, we don't greet Benjamins!
echo $? # print last status code
# => 1
extism call plugin.wasm greet --input="Zach" --wasi
# => Hello, Zach!
echo $?
# => 0

Json

Extism export functions simply take bytes in and bytes out. Those can be whatever you want them to be. A common and simple way to get more complex types to and from the host is with json:

type Add struct {
	A int `json:"a"`
	B int `json:"b"`
}

type Sum struct {
	Sum int `json:"sum"`
}

//export add
func add() int32 {
	params := Add{}
	// use json input helper, which automatically unmarshals the plugin input into your struct
	err := pdk.InputJSON(&params)
	if err != nil {
		pdk.SetError(err)
		return 1
	}
	sum := Sum{Sum: params.A + params.B}
	// use json output helper, which automatically marshals your struct to the plugin output
	output, err := pdk.OutputJSON(sum)
	if err != nil {
		pdk.SetError(err)
		return 1
	}
	return 0
}

extism call plugin.wasm add --input='{"a": 20, "b": 21}' --wasi
# => {"sum":41}

Configs

Configs are key-value pairs that can be passed in by the host when creating a plug-in. These can be useful to statically configure the plug-in with some data that exists across every function call. Here is a trivial example using pdk.GetConfig:

//export greet
func greet() int32 {
	user, ok := pdk.GetConfig("user")
	if !ok {
		pdk.SetErrorString("This plug-in requires a 'user' key in the config")
		return 1
	}
	greeting := `Hello, ` + user + `!`
	pdk.OutputString(greeting)
	return 0
}

To test it, the Extism CLI has a --config option that lets you pass in key=value pairs:

extism call plugin.wasm greet --config user=Benjamin
# => Hello, Benjamin!

Variables

Variables are another key-value mechanism but it's a mutable data store that will persist across function calls. These variables will persist as long as the host has loaded and not freed the plug-in.

//export count
func count() int32 {
	count := pdk.GetVarInt("count")
	count = count + 1
	pdk.SetVarInt("count", count)
	pdk.OutputString(strconv.Itoa(count))
	return 0
}

Note: Use the untyped variants pdk.SetVar(string, []byte) and pdk.GetVar(string) []byte to handle your own types.

Logging

Because Wasm modules by default do not have access to the system, printing to stdout won't work (unless you use WASI). Extism provides a simple logging function that allows you to use the host application to log without having to give the plug-in permission to make syscalls.

//export log_stuff
func logStuff() int32 {
	pdk.Log(pdk.LogInfo, "An info log!")
	pdk.Log(pdk.LogDebug, "A debug log!")
	pdk.Log(pdk.LogWarn, "A warn log!")
	pdk.Log(pdk.LogError, "An error log!")
	return 0
}

From Extism CLI:

extism call plugin.wasm log_stuff --wasi --log-level=debug
2023/10/12 12:11:23 Calling function : log_stuff
2023/10/12 12:11:23 An info log!
2023/10/12 12:11:23 A debug log!
2023/10/12 12:11:23 A warn log!
2023/10/12 12:11:23 An error log!

Note: From the CLI you need to pass a level with --log-level. If you are running the plug-in in your own host using one of our SDKs, you need to make sure that you call set_log_file to "stdout" or some file location.

HTTP

Sometimes it is useful to let a plug-in make HTTP calls. See this example

//export http_get
func httpGet() int32 {
	// create an HTTP Request (withuot relying on WASI), set headers as needed
	req := pdk.NewHTTPRequest(pdk.MethodGet, "https://jsonplaceholder.typicode.com/todos/1")
	req.SetHeader("some-name", "some-value")
	req.SetHeader("another", "again")
	// send the request, get response back (can check status on response via res.Status())
	res := req.Send()

	pdk.OutputMemory(res.Memory())

	return 0
}

By default, Extism modules cannot make HTTP requests unless you specify which hosts it can connect to. You can use --alow-host in the Extism CLI to set this:

extism call plugin.wasm http_get --wasi --allow-host='*.typicode.com'
# => { "userId": 1, "id": 1, "title": "delectus aut autem", "completed": false }

Imports (Host Functions)

Like any other code module, Wasm not only let's you export functions to the outside world, you can import them too. Host Functions allow a plug-in to import functions defined in the host. For example, if you host application is written in Python, it can pass a Python function down to your Go plug-in where you can invoke it.

This topic can get fairly complicated and we have not yet fully abstracted the Wasm knowledge you need to do this correctly. So we recommend reading our concept doc on Host Functions before you get started.

A Simple Example

Host functions have a similar interface as exports. You just need to declare them as extern on the top of your main.go. You only declare the interface as it is the host's responsibility to provide the implementation:

//go:wasmimport extism:host/user a_python_func
func aPythonFunc(uint64) uint64

We should be able to call this function as a normal Go function. Note that we need to manually handle the pointer casting:

//export hello_from_python
func helloFromPython() int32 {
    msg := "An argument to send to Python"
    mem := pdk.AllocateString(msg)
    defer mem.Free()
    ptr := aPythonFunc(mem.Offset())
    rmem := pdk.FindMemory(ptr)
    response := string(rmem.ReadBytes())
    pdk.OutputString(response)
    return 0
}

Testing it out

We can't really test this from the Extism CLI as something must provide the implementation. So let's write out the Python side here. Check out the docs for Host SDKs to implement a host function in a language of your choice.

from extism import host_fn, Plugin

@host_fn()
def a_python_func(input: str) -> str:
    # just printing this out to prove we're in Python land
    print("Hello from Python!")

    # let's just add "!" to the input string
    # but you could imagine here we could add some
    # applicaiton code like query or manipulate the database
    # or our application APIs
    return input + "!"

Now when we load the plug-in we pass the host function:

manifest = {"wasm": [{"path": "/path/to/plugin.wasm"}]}
plugin = Plugin(manifest, functions=[a_python_func], wasi=True)
result = plugin.call('hello_from_python', b'').decode('utf-8')
print(result)

python3 app.py
# => Hello from Python!
# => An argument to send to Python!

Reactor modules

Since TinyGo doesn't support Reactor modules yet, If you want to use WASI inside your Reactor module functions (exported functions other than main), you'll need to import wasi-reactor module which makes sure libc and go runtime are properly initialized:

package main

import (
	"os"

	"github.com/extism/go-pdk"
	_ "github.com/extism/go-pdk/wasi-reactor"
)

//export read_file
func read_file() {
	name := pdk.InputString()

	content, err := os.ReadFile(name)
	if err != nil {
		pdk.Log(pdk.LogError, err.Error())
		return
	}

	pdk.Output(content)
}

func main() {}

tinygo build -target wasi -o reactor.wasm .\tiny_main.go
extism call ./reactor.wasm read_file --input "./test.txt" --allow-path . --wasi --log-level info
# => Hello World!

Note: this is not required if you only have the main function.

Reach Out!

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