Extism runtime and rust-sdk

This repo contains the code for the Extism runtime and rust-sdk. It can be embedded in any Rust application to call Extism plug-ins.

Note: If you're unsure what Extism is or what an SDK is see our homepage: https://extism.org.

Installation

Cargo

To use the extism crate, you can add it to your Cargo file:

[dependencies]
extism = "1.4.1"

Environment variables

There are a few environment variables that can be used for debugging purposes:

• EXTISM_ENABLE_WASI_OUTPUT=1: show WASI stdout/stderr
• EXTISM_MEMDUMP=extism.mem: dump Extism linear memory to a file
• EXTISM_COREDUMP=extism.core: write coredump to a file when a WebAssembly function traps
• EXTISM_DEBUG=1: generate debug information
• EXTISM_PROFILE=perf|jitdump|vtune: enable Wasmtime profiling
• EXTISM_CACHE_CONFIG=path/to/config.toml: enable Wasmtime cache, see the docs for details about configuration. Setting this to an empty string will disable caching.

Note: The debug and coredump info will only be written if the plug-in has an error.

Getting Started

This guide should walk you through some of the concepts in Extism and the extism crate.

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:

use extism::*;

fn main() {
  let url = Wasm::url(
    "https://github.com/extism/plugins/releases/latest/download/count_vowels.wasm"
  );
  let manifest = Manifest::new([url]);
  let mut plugin = Plugin::new(&manifest, [], true).unwrap();
  let res = plugin.call::<&str, &str>("count_vowels", "Hello, world!").unwrap();
  println!("{}", res);
}

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:

let res = plugin.call::<&str, &str>("count_vowels", "Hello, world!").unwrap();
println!("{}", res);
# => {"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.

The call function uses extism-convert to determine which input/output types can be used. If we wanted to use a concrete type for the count_vowels result, we could defined a struct:

#[derive(Debug, serde::Deserialize)]
struct VowelCount {
  count: usize,
  total: usize,
  vowels: String,
}

Then we can use Json to get the JSON results decoded into VowelCount:

let Json(res) = plugin.call::<&str, Json<VowelCount>>("count_vowels", "Hello, world!").unwrap();
println!("{:?}", res);
# => VowelCount {count: 3, total: 3, vowels: "aeiouAEIOU"}

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:

let res = plugin.call::<&str, &str>("count_vowels", "Hello, world!").unwrap();
println!("{}", res);
# => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}

let res = plugin.call::<&str, &str>("count_vowels", "Hello, world!").unwrap();
println!("{}", res);
# => {"count": 3, "total": 9, "vowels": "aeiouAEIOU"}

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

Configuration

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:

let manifest = Manifest::new([url]);
let mut plugin = Plugin::new(&manifest, [], true);
let res = plugin.call::<&str, &str>("count_vowels", "Yellow, world!").unwrap();
println!("{}", res);
# => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}
let mut plugin = Plugin::new(&manifest, [], true).with_config_key("vowels", "aeiouyAEIOUY");
let res = plugin.call::<&str, &str>("count_vowels", "Yellow, world!").unwrap();
println!("{}", res);
# => {"count": 4, "total": 4, "vowels": "aeiouyAEIOUY"}

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 Rust functions 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:

let url = Wasm::url(
  "https://github.com/extism/plugins/releases/latest/download/count_vowels_kvstore.wasm"
);
let manifest = Manifest::new([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.

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.

use extism::*;

// pretend this is redis or something :)
type KVStore = std::collections::BTreeMap<String, Vec<u8>>;

// When a first argument separated with a semicolon is provided to `host_fn` it is used as the
// variable name and type for the `UserData` parameter
host_fn!(kv_read(user_data: KVStore; key: String) -> u32 {
    let kv = user_data.get()?;
    let kv = kv.lock().unwrap();
    let value = kv
        .get(&key)
        .map(|x| u32::from_le_bytes(x.clone().try_into().unwrap()))
        .unwrap_or_else(|| 0u32);
    Ok(value)
});

host_fn!(kv_write(user_data: KVStore; key: String, value: u32) {
    let kv = user_data.get()?;
    let mut kv = kv.lock().unwrap();
    kv.insert(key, value.to_le_bytes().to_vec());
    Ok(())
});

fn main() {
    let kv_store = UserData::new(KVStore::default());

    let url = Wasm::url(
        "https://github.com/extism/plugins/releases/latest/download/count_vowels_kvstore.wasm",
    );
    let manifest = Manifest::new([url]);
    let mut plugin = PluginBuilder::new(manifest)
        .with_wasi(true)
        .with_function(
            "kv_read",
            [PTR],
            [PTR],
            kv_store.clone(),
            kv_read,
        )
        .with_function(
            "kv_write",
            [PTR, PTR],
            [],
            kv_store.clone(),
            kv_write,
        )
        .build()
        .unwrap();

    for _ in 0..5 {
        let res = plugin
            .call::<&str, &str>("count_vowels", "Hello, world!")
            .unwrap();
        println!("{}", res);
    }
}

Note: In order to write host functions you should get familiar with the methods on the CurrentPlugin and UserData types.

Now we can invoke the event:

let res = plugin.call::<&str, &str>("count_vowels", "Hello, world!").unwrap();
println!("{}", res);
# => Read from key=count-vowels"
# => Writing value=3 from key=count-vowels"
# => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}

let res = plugin.call::<&str, &str>("count_vowels", "Hello, world!").unwrap();
println!("{}", res);
# => Read from key=count-vowels"
# => Writing value=6 from key=count-vowels"
# => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}