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4. Creating your own modules

Creating the scaffold for a new actor modules in Rust is easy. We will create an actor that accepts an HTTP request and responds with "Hello World". To create your new actor project, change to the directory where you want the project to be created, and enter the command below. The last term on the command hello is the project name. If you choose a different project name, the name of the subdirectory and some symbols in the generated code will be different from the example code in this guide.

cosmo new actor hello --template-name hello

Let's change into the newly-created folder hello and take a look at the generated project. The file src/ includes an imports section, a struct HelloActor, and an impl block that implements the HttpServer trait for the actor struct. Let's walk through these sections in detail.

Note the two lines near the top of the source code file:

use wasmbus_rpc::actor::prelude::*;
use wasmcloud_interface_httpserver::{HttpRequest, HttpResponse, HttpServer, HttpServerReceiver};

This shows us that we're using a core wasmCloud crate called wasmbus_rpc and we've also declared a dependency on the wasmcloud_interface_httpserver crate. By convention, all first-party wasmCloud interface crates begin with wasmcloud_interface.

Just below that, you'll see:

#[derive(Debug, Default, Actor, HealthResponder)]
#[services(Actor, HttpServer)]
struct HelloActor {}

HelloActor is the name of your actor - if you chose a different project name, the actor name will include your project name.

The two lines above the actor name invoke Rust macros that generate - at compile time - nearly all of the scaffolding needed to build an actor. The HealthResponder term generates a function that automatically responds to health check queries from the wasmCloud host. The #[services(...)] line declares the services ('traits', in Rust) that your actor implements, and generates message handling code for those interfaces. All actors implement the Actor interface. The HttpServer entry declares that the actor will also implement that interface, and requires an implementation of that trait's method: handle_request.

impl HttpServer for HelloActor {
    /// Returns a greeting, "Hello World", in the response body.
    /// If the request contains a query parameter 'name=NAME', the
    /// response is changed to "Hello NAME"
    async fn handle_request(
        _ctx: &Context,
        req: &HttpRequest,
    ) -> std::result::Result<HttpResponse, RpcError> {
        let text=form_urlencoded::parse(req.query_string.as_bytes())
            .find(|(n, _)| n == "name")
            .map(|(_, v)| v.to_string())
            .unwrap_or_else(|| "World".to_string());
        Ok(HttpResponse {
            body: format!("Hello {}", text).as_bytes().to_vec(),

Within the handle_request method, the actor receives the HTTP request, and returns an HttpResponse, which is sent back to the HTTP client (such as a curl command or a web browser). We'll look into the details of that method, and customize it, shortly.

If you use an IDE that comes with code completion and hover-tooltips, you'll be able to see documentation and get strongly-typed guidance as you develop code to interact with the wasmCloud interfaces.

Something's missing

Before we get into modifying the scaffolding to create the rest of this actor, take a look at what's not included in this code. This code returns an abstraction of an HTTP response. It is not tightly coupled to any particular HTTP server. Furthermore, you don't see the port number or server configuration options anywhere in the code. Finally, you can scale and compose this actor any way you see fit without ever having to recompile or redeploy it.

This is the way development was meant to be.

Pure business logic, with all of your non-functional requirements handled through loosely coupled abstractions by runtime-configurable hosts. No boilerplate, no fuss.