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Extism Ruby Host SDK

This repo contains the ruby gem for integrating with the Extism runtime. Install this library into your host ruby application to run Extism plug-ins.


Install the Extism Runtime Dependency

For this library, you first need to install the Extism Runtime. You can download the shared object directly from a release or use the Extism CLI to install it:

sudo extism lib install latest

#=> Fetching
#=> Copying libextism.dylib to /usr/local/lib/libextism.dylib
#=> Copying extism.h to /usr/local/include/extism.h

Install the Gem

Add this library to your Gemfile:

gem 'extism', '~> 1.0.0'

Or if installing on the system level:

gem install extism

Getting Started

This guide should walk you through some of the concepts in Extism and this ruby library.

Note: You should be able to follow this guide by copy pasting the code into irb.

Creating A Plug-in

The primary concept in Extism is the plug-in. You can think of a plug-in as a code module stored in a .wasm file.

Since you may not have an Extism plug-in on hand to test, let's load a demo plug-in from the web:

# First require the library
require "extism"

url = ""
manifest = Extism::Manifest.from_url url
plugin =

Note: See the Manifest docs as it has a rich schema and a lot of options.

Calling A Plug-in's Exports

This plug-in was written in Rust and it does one thing, it counts vowels in a string. As such, it exposes one "export" function: count_vowels. We can call exports using Extism::Plugin#call:"count_vowels", "Hello, World!")
# => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}

All exports have a simple interface of bytes-in and bytes-out. This plug-in happens to take a string and return a JSON encoded string with a report of results.

Plug-in State

Plug-ins may be stateful or stateless. Plug-ins can maintain state b/w calls by the use of variables. Our count vowels plug-in remembers the total number of vowels it's ever counted in the "total" key in the result. You can see this by making subsequent calls to the export:"count_vowels", "Hello, World!")
# => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}"count_vowels", "Hello, World!")
# => {"count": 3, "total": 9, "vowels": "aeiouAEIOU"}

These variables will persist until this plug-in is freed or you initialize a new one.


Plug-ins may optionally take a configuration object. This is a static way to configure the plug-in. Our count-vowels plugin takes an optional configuration to change out which characters are considered vowels. Example:

plugin ="count_vowels", "Yellow, World!")
# => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}

plugin =, config: { vowels: "aeiouyAEIOUY" })"count_vowels", "Yellow, World!")
# => {"count": 4, "total": 4, "vowels": "aeiouAEIOUY"}

Host Functions

Let's extend our count-vowels example a little bit: Instead of storing the total in an ephemeral plug-in var, let's store it in a persistent key-value store!

Wasm can't use our KV store on it's own. This is where Host Functions come in.

Host functions allow us to grant new capabilities to our plug-ins from our application. They are simply some ruby methods you write which can be passed down and invoked from any language inside the plug-in.

Let's load the manifest like usual but load up this count_vowels_kvstore plug-in:

url = ""
manifest = Extism::Manifest.from_url(url)

Note: The source code for this is here and is written in rust, but it could be written in any of our PDK languages.

Unlike our previous plug-in, this plug-in expects you to provide host functions that satisfy our its import interface for a KV store.

In the ruby sdk, we have a concept for this called a Host Environment. An environment is an instance of a class that implements any host functions your plug-in needs.

We want to expose two functions to our plugin, kv_write(key: String, value: Bytes) which writes a bytes value to a key and kv_read(key: String) -> Bytes which reads the bytes at the given key.

# pretend this is Redis or something :)

class KvEnvironment
  include Extism::HostEnvironment

  # We need to describe the wasm function signature of each host function
  # to register them to this environment
  register_import :kv_read, [Extism::ValType::PTR], [Extism::ValType::PTR]
  register_import :kv_write, [Extism::ValType::PTR, Extism::ValType::PTR], []

  def kv_read(plugin, inputs, outputs, _user_data)
    key = plugin.input_as_string(inputs.first)
    val = KV_STORE[key] || [0].pack('V') # get 4 LE bytes for 0 default
    puts "Read from key=#{key}"
    plugin.output_string(outputs.first, val)

  def kv_write(plugin, inputs, _outputs, _user_data)
    key = plugin.input_as_string(inputs.first)
    val = plugin.input_as_string(inputs[1])
    puts "Writing value=#{val.unpack1('V')} from key=#{key}"
    KV_STORE[key] = val

Note: In order to write host functions you should get familiar with the methods on the Extism::CurrentPlugin class. The plugin parameter is an instance of this class.

Now we just need to create a new host environment and pass it in when loading the plug-in. Here our environment initializer takes no arguments, but you could imagine putting some customer specific instance variables in there:

env =
plugin =, environment: env)

Now we can invoke the event:"count_vowels", "Hello, World!")
# => Read from key=count-vowels"
# => Writing value=3 from key=count-vowels"
# => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}"count_vowels", "Hello, World!")
# => Read from key=count-vowels"
# => Writing value=6 from key=count-vowels"
# => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}