wasmCloud can help build simple components and complex components. While wash and wasmCloud utility libraries (in Rust, crates like wasmcloud-component) provide what you'll need to build components along with useful helpers, adding complexity can sometimes make it difficult to use ecosystem tooling.

This page documents some advanced use-cases for more complex components, along with implementation and configuration patterns.

Reusing code from helper libraries with custom interfaces (e.g. wasmcloud-component)

When building a component that contains custom interfaces in addition to using a standardized interfaces like wasi:http, your component's WIT interface might look something like this:

wit

package example:component;

interface transform {
   transform-bytes: func(data: list<u8>) -> result<list<u8>, string>;
}

world transformer {
   export transform;
   export wasi:http/incoming-handler@0.2.2;
}

While it's not important how the transform-bytes function works, what's important is that from the same component we expose both a wasi:http/incoming-handler and the original transform interface.

It's likely that this component makes it possible to run transformation logic both when called directly from a Wasm host (via example:component/transform.transform-bytes), and when called via a HTTP request (i.e. wasi:http/incoming-handler.handle, triggered via a web request to the wasmCloud HTTP provider).

wasmcloud-component makes it incredibly easy to deal with web requests, providing utilities familiar to Rust developers based on standard ecosystem types rather than the robust (but often verbose) fully generated wasi:http bindings.

Using wasmcloud-component means you can write code like this:

rust

// ...

/// Implementation of our WASI interfaces will hang from this struct
struct Component;

// ... implementation of the transform trait for the component ...

// Implementation of our standardized `wasi:http` interface
//
// By using the Server trait, we can fill out the wasi:http/incoming-handler impl via
// a much more native interface rather than only generated bindings
impl wasmcloud_component::http::Server for Component {
    fn handle(
        request: http::IncomingRequest,
    ) -> http::Result<http::Response<impl http::OutgoingBody>> {
        // ...
    }
}

Problem: Helper crates with pre-defined WASI exports

While wasmcloud_component is helpful, in the example component it causes an issue -- the component must implement two exports:

• wasi:http/incoming-handler (wasmcloud-component helps with this)
• transform

The wasmcloud_component crate can't know anything about transform, but this causes a problem because wasmcloud_component performs the primary interface export for all of the bindings of the component

This causes an issue when building our component that often looks like the following:

linking with `wasm-component-ld` failed: exit status: 1
error: failed to encode component

          Caused by:
              0: failed to decode world from module
              1: module was not valid
              2: failed to find export of interface `example:component/transform` function `transform-bytes`

Solution: Merging pre-defined exports with local custom interfaces, using wit-bindgen manually

To make this work properly, we'll need to write code that uses wit-bindgen, rather than just wasmcloud-component:

rust

mod bindings {
    use super::Component;

    wit_bindgen::generate!({
        with: {
            // Here we re-use the existing bindings that were generated in `wasmcloud_component`.
            //
            // Due to this re-use, the `wasmcloud::http::export!` will work properly and satisfy
            // the types that are expected by *our* component, because we instruct wit-bindgen
            // that the appropriate types for the import *are* the ones from `wasmcloud_component`.
            //
            // If we were to try to `generate` this binding ourselves, we'd have similarly *named*
            // interfaces, but the code in `wasmcloud_component` could not reference the modules &
            // types generated in this crate -- they'd be named similarly, but be different (so you'd
            // get an error that [your version of] the interface wasn't satisfied, despite
            // the wasmcloud_component export! below).
            //
            // For info on options to generate!, see:
            //   https://docs.rs/wit-bindgen/0.38.0/wit_bindgen/macro.generate.html
            //
            "wasi:http/incoming-handler@0.2.2": wasmcloud_component::wasi::exports::http::incoming_handler,
        },
        generate_all
    });

    export!(Component);
    wasmcloud_component::http::export!(Component);
}

/// Implementation of our WASI interfaces will hang from this struct
struct Component;

use bindings::exports::example::component::transform;

// Implementation of our custom `transform` interface
impl transform::Guest for Component {
    fn transform_bytes(_data: Vec<u8>) -> Result<Vec<u8>, String> {
        todo!()
    }
}

// Implementation of our standardized `wasi:http` interface
//
// By using the Server trait, we can fill out the wasi:http/incoming-handler impl via
// a much more native interface rather than only generated bindings
impl http::Server for Component {
    fn handle(
        request: http::IncomingRequest,
    ) -> http::Result<http::Response<impl http::OutgoingBody>> {
        todo!()
    }
}

By dropping down to "manual" wit-bindgen, we can use the with option to specify that we want to reuse bindings made available by another crate in our current crate, keeping the use of the better ergonomics of http::Server while still being able to export our transform::Guest implementation via the Component struct.