Write a host function with the Extism Host SDK

Give power to the Wasm plugins

In this article, we will:

A little reminder about host functions

Excerpt from "Extism, WebAssembly Plugins & Host functions" :

The host application can give the guest (the Wasm module) extra powers. We call these "host functions". It is a function developed "in the source code of the host". The host exposes (export) it to the Wasm module, which will be able to execute it. For example, you can develop a host function to display messages and thus allow the Wasm module to display messages in a terminal during its execution...

... Extism's Plugin Development Kit (PDK) provides some ready-to-use host functions, especially for logging, HTTP requests or reading a configuration in memory.

But with Extism's Host SDK, you can develop your host functions. This can be useful, for example, for database access and interaction with MQTT or NATS brokers...

In this article, we will keep it simple and develop a host function that allows to retrieve messages stored in the memory of the host application from a key. We will use a JavaScript Map for this.

Let's start by modifying our Node.js application.

Development of the host functions

Modify the server.js file as follows:

import Fastify from 'fastify'
import process from "node:process"

// 1️⃣
import { Context, HostFunction, ValType } from '@extism/extism'
import { readFileSync } from 'fs'

// 2️⃣
let memoryMap = new Map()

memoryMap.set("hello", "👋 Hello World 🌍")
memoryMap.set("message", "I 💜 Extism 😍")

// 3️⃣ Host function (callable by the WASM plugin)
function memoryGet(plugin, inputs, outputs, userData) { 

  // 4️⃣ Read the value of inputs from the memory
  let memKey = plugin.memory(inputs[0].v.i64)
  // memKey is a buffer, 
  // use toString() to get the string value

  // 5️⃣ This is the return value
  const returnValue = memoryMap.get(memKey.toString())

  // 6️⃣ Allocate memory
  let offs = plugin.memoryAlloc(Buffer.byteLength(returnValue))
  // 7️⃣ Copy the value into memory

  // 8️⃣ return the position and the length for the wasm plugin
  outputs[0].v.i64 = offs 

// 9️⃣ Host functions list
let hostFunctions = [
  new HostFunction(

// location of the new plugin
let wasmFile = "../12-simple-go-mem-plugin/simple.wasm"
let functionName = "say_hello"
let httpPort = 7070

let wasm = readFileSync(wasmFile)

const fastify = Fastify({
  logger: true

const opts = {}

// Create the WASM plugin
let ctx = new Context()

// 1️⃣0️⃣
let plugin = ctx.plugin(wasm, true, hostFunctions)

// Create and start the HTTP server
const start = async () => {

  fastify.post('/', opts, async (request, reply) => {

    // Call the WASM function, 
    // the request body is the argument of the function
    let buf = await plugin.call(functionName, request.body); 
    let result = buf.toString()

    return result

  try {
    await fastify.listen({ port: httpPort, host: ''})
  } catch (err) {
start().then(r => console.log("😄 started"))
  • 1: import HostFunction (which allows the host to define functions callable by the Wasm plugin) and ValType (an enumeration of the possible types used by the host function).

  • 2: create and populate a JavaScript Map

  • 3: define the host function memoryGet

  • 4: when the Wasm plugin calls the host function, the parameter passing is done using the shared memory between the plugin and the host. plugin.memory(inputs[0].v.i64) is used to fetch this information from the shared memory. memKey is a buffer that contains the key to find a value in the JavaScript Map (and we use memKey.toString() to transform the buffer into a string).

  • 5: we get the value associated with the key.

  • 6: we allocate memory to copy the value associated with the key. offs corresponds to the position and length of the value in memory (thanks to the bit-shifting method, we can "fit two values into one").

  • 7: we copy the value returnValue into this memory at the indicated location offs.

  • 8: we copy into the return variable outputs (passed to the function by reference) the value of offs which will allow the Wasm plugin to read the result of the function in memory.

  • 9: we define an array of host functions. In our case, we create only one, where "hostMemoryGet" will be the alias of the function "seen" by the Wasm plugin, [ValType.I64] represents the type of the input parameter and the type of the output parameter (we remember that Wasm functions only accept numbers - and in our case, these numbers contain the positions and sizes of values in shared memory), and finally memory which is the definition of our host function.

  • 10: When instantiating the Wasm plugin, we pass the array of host functions as an argument.

Before we can rerun our HTTP server, we will have to modify our Wasm plugin.

Modify the Wasm plugin

package main

import (

//export hostMemoryGet // 1️⃣
func hostMemoryGet(x uint64) uint64

//export say_hello
func say_hello() int32 {

    // read function argument from the memory
    // this is the name passed to the function
    input := pdk.Input()

    // Call the host function
    // 2️⃣
    key1 := pdk.AllocateString("hello")
    // 3️⃣
    offs1 := hostMemoryGet(key1.Offset())

    // 4️⃣
    mem1 := pdk.FindMemory(offs1)
        mem1 is a struct instance
        type Memory struct {
            offset uint64
            length uint64

    // 5️⃣
    buffMem1 := make([]byte, mem1.Length())

    // 6️⃣ get the second message
    key2 := pdk.AllocateString("message")
    offs2 := hostMemoryGet(key2.Offset())
    mem2 := pdk.FindMemory(offs2)
    buffMem2 := make([]byte, mem2.Length())

  // 7️⃣
    data := []string{
        "👋 Hello " + string(input),
        "key: hello, value: " + string(buffMem1),
        "key: message, value: " + string(buffMem2),

    // Allocate space into the memory
    mem := pdk.AllocateString(strings.Join(data, "\n"))
    // copy output to host memory

    return 0

func main() {}
  • 1: the function hostMemoryGet must be exported to be usable.

  • 2: we want to call the host function to get the value corresponding to the key hello, so we have to copy this key into memory.

  • 3: we call the host function hostMemoryGet (key1.Offset() represents the position and length in memory of the key key1 into only one value).

  • 4: pdk.FindMemory(offs1) allows to retrieve a structure mem1 containing the position and length.

  • 5: we can now create a buffer buffMem1 with the size of the value to retrieve and load it with the content of the memory location (mem1). Then we have to read the string with string(buffMem1).

  • 6: we repeat the process of reading the second key.

  • 7: we build a slice of strings that we will then transform into a single string to return to the host function.

If you want to learn more about the shared memory between the host and the Wasm plugin, you can read this blog post: WASI communication between Node.js and Wasm modules with the Wasm Buffer Memory

Compile the new plugin

To compile the program, use TinyGo and the command below, which will produce a file simple.wasm :

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

It's time to test our modifications.

Start the server and call the MicroService

To start the server, use this command:

node server.js

Then, to call the MicroService, use this simple curl command :

curl -X POST http://localhost:7070 \
-H 'Content-Type: text/plain; charset=utf-8' \
-d 'Jane Doe'

And you will get the message value from each of the keys of the JavaScript Map :

👋 Hello Jane Doe
key: hello, value: 👋 Hello World 🌍
key: message, value: I 💜 Extism 😍

Remember that when the Wasm plugin calls the host function, it is not it that executes the processing but rather the host application. In the case of Node.js, this will eventually slow down the execution of the plugin because Node.js is generally slower than compiled Go. Nevertheless, the potential of host functions is exciting.

😥 This article was a bit more complicated than the previous ones, but this concept of host functions is essential. These last two articles also show how to evolve your Node.js applications with other languages. Feel free to contact me for more explanations. My next article will also explain how to make host functions, but this time in Go.