Building WebAssembly platforms with waPC

Jarrod Overson
5 min readOct 19, 2021

WebAssembly is exciting at first glance but quickly turns into an adventure in software archeology. You spend most of your time piecing together clues from abandoned sites (github projects) and ancient texts (websites) searching for the holy grail.

While waPC is not the holy grail, it’s a satisfying solution to the headache of getting productive with WebAssembly. When WebAssembly’s “Hello World” can leave you with more questions than answers, waPC scales with you from starter projects to complete platforms.

What is waPC?

The WebAssembly Procedure Calls (waPC) project is like a standard module interface for WebAssembly on top of an extensible RPC framework. It irons out the wrinkles between native code and WebAssembly guests to make passing and receiving complex data types trivial. Under the hood, waPC defines an opaque protocol that allows you to broker arbitrary, dynamic calls across native logic, WebAssembly, from WebAssembly to other WebAssembly, or across the internet without knowing anything about the call’s data structure or serialization algorithms.

WaPC is great for platform builders but covers common use cases just as well. Even though the goal is broader than some other projects, the waPC experience is so intuitive and powerful that you should keep it in your toolbox no matter what you end up using.

waPC vs wasm-bindgen (et al)?

Similar looking tools, different audiences

If you write Rust and want to target the web browser alone, the wasm-bindgen project is mature and tailored specifically to this use case. WaPC is more generic. WaPC is better suited for building cross-platform applications than fast browser code.

WaPC has host implementations in Rust, Go, and JavaScript (nodejs + browser) and guest SDKs for Rust, TinyGo, AssemblyScript, and Zig. If you’re looking for the “portable” part of WebAssembly, waPC is for you. If you’re looking to make lightning fast web application, wasm-bindgen is forging the path there.

Both wasm-bindgen and waPC are made of layers that could complement each other, but they aren’t made to coexist at the moment.

The waPC suite

Do you learn better by copy/pasting code and running things yourself? Try heading to Getting started with waPC and use this post as a reference when you need it.

The waPC protocol

The waPC protocol is a handful of complementary host and guest methods that act as the communication contract across the WebAssembly boundary. They provide an interface for calling operations, reading data, communicating errors, and exposing a logger. Check out this TypeScript representation of the waPC protocol from the host’s side for a glimpse into the technicals.

waPC Hosts

A waPC host is the native implementation that loads and initializes a WebAssembly guest and makes requests over the waPC protocol. WaPC has host implementations for Rust, Go, and JavaScript. If you don’t see your platform, the protocol is small and you could create a host implementation in a day or two. This is another reason we committed to waPC at Vino, each layer of the technology revolves around a dense but understandable core.

waPC Guests

WaPC guests are WebAssembly modules that implement the guest portion of the waPC protocol. “Operations” are an important part of waPC guests and hosts. Operations are like the exported functions you could expose when compiling to wasm normally, but waPC adds a layer of abstraction that keeps the interface in and out of wasm consistent. This keeps reliable bindings for hosts while maintaining a dynamic interface to internal guest functionality.

Guests can also make host calls, i.e. native function calls. These are operations in the reverse direction that include the input payload and the requested operation name as well as strings representing a namespace and binding. The additional data gives hosts flexibility to define the interface it exposes to its guests. A host could provide native functionality like a custom stdlib, or it could dynamically load and forward those calls to another WebAssembly module. WaPC has guest implementations for Rust, TinyGo, AssemblyScript, and Zig.

The actual process of implementing the guest bindings is abstracted away from you via the wapc CLI and WIDL (see below).

WebAssembly IDL (WIDL, pronounced whittle)

WIDL is an interface definition language for describing waPC modules. It’s easier to understand with an example:

interface {
# This defines an add operation that takes
# two numbers and returns another
add(left: number, right: number): number
# This is a more complex example that shows an example
# HTTP Request operation
request(url: string): HttpResponse
type HttpResponse {
headers: [Header]
body: string
type Header {
name: string
value: string

The wapc CLI uses WIDL like this to generate type definitions and integration code for waPC guests and hosts. For most usage, you define your interface with WIDL and wapc generates everything else. You’re left writing only your business logic.

The WIDL spec defines the types and syntax of additional features. The widl parser and widl-codegen are written in JavaScript and available as npm modules or browser bundles.

The wapc CLI

wapc is a command line interface to helper methods:

  • wapc new automatically creates new waPC guests with default templates in the language of your choice.
  • wapc generate automatically generates integration code from your WIDL schemas.
  • wapc update keeps the wapc internals up-to-date.

The officially supported CLI is a Go program and the logic is written as JavaScript and available on npm with bundles for browser environments. Rust & nodejs versions of the CLI exist to show how you can embed wapc functionality like I did with the widl-validator. The validator does all the parsing and code generation on the client side. No servers were harmed in this demo.

Passing and receiving complex data types

The waPC protocol doesn’t prescribe a data serialization algorithm, but wapc generates code that uses MessagePack as a default. MessagePack is a sensible option while waPC’s format-agnostic stance means you can substitute something more efficient for your data or build brokers that pass the data along without deserialization.

How it looks

Linked here are some sample projects for you to get a feel for what Rust guests look like: jsoverson/wapc-guest-examples. The contained projects were all generated via the wapc new command.

The echo example sends returns the input string value as output and has the following schema WIDL:

interface {
echo(msg: string): string

You can see the compiled wasm running live in the browser at the end of the page here: and load your own modules here:

Writing your own waPC hosts and guests

That’s it! The next step is to start working with actual code yourself. Check out the guest and host tutorial over at Getting started with waPC.



Jarrod Overson

I write about JavaScript, Rust, WebAssembly, Security. Also a speaker, O'Reilly Author, creator of Plato, CTO @Candle