Set up Rust for FastEdge CDN apps

Set up Rust for FastEdge CDN apps by adding the wasm32-wasip1 target, configuring a Cargo project, and verifying the toolchain.

FastEdge CDN applications run inside Gcore’s CDN pipeline and can intercept HTTP traffic at four stages: on request headers, on request body, on response headers, and on response body. They are built on the Proxy-Wasm specification — an open standard for WebAssembly-based proxy extensions used across CDN and service mesh environments.

In Rust, Proxy-Wasm applications use the proxy-wasm crate. Instead of a single request handler, a CDN application implements callbacks such as on_http_request_headers and on_http_response_headers, which the CDN runtime invokes at different stages of request processing.

Rust and Cargo are required. On Windows, also install Visual Studio Build Tools with the Desktop development with C++ workload.

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Add the WebAssembly target

CDN apps compile to wasm32-wasip1. Add the target once — it applies to all future builds:

rustup target add wasm32-wasip1

If the Legacy Rust SDK is already set up, this target is already present.

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Configure a project

FastEdge applications compile to WebAssembly libraries. Two changes from the Cargo defaults are needed: the output type must be cdylib, and proxy-wasm must be listed as a dependency.

1. Create the library crate:

   cargo new --lib my-cdn-app
   cd my-cdn-app
   

2. Replace the contents of Cargo.toml:

   [package]
   name = "my_cdn_app"
   version = "0.1.0"
   edition = "2021"
   
   [lib]
   crate-type = ["cdylib"]
   
   [dependencies]
   proxy-wasm = "0.2"
   log = "0.4"
   

   proxy-wasm = "0.2" provides the filter traits and the proxy_wasm::main! entry point. log is used for structured logging inside the CDN pipeline.

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Verify the toolchain

A minimal filter that adds a custom request header is enough to confirm the toolchain produces a valid CDN binary. Replace src/lib.rs:

use proxy_wasm::traits::*;
use proxy_wasm::types::*;

proxy_wasm::main! {{
    proxy_wasm::set_log_level(LogLevel::Trace);
    proxy_wasm::set_root_context(|_| -> Box<dyn RootContext> {
        Box::new(MyRoot)
    });
}}

struct MyRoot;
impl Context for MyRoot {}
impl RootContext for MyRoot {
    fn create_http_context(&self, _: u32) -> Option<Box<dyn HttpContext>> {
        Some(Box::new(MyFilter))
    }
    fn get_type(&self) -> Option<ContextType> {
        Some(ContextType::HttpContext)
    }
}

struct MyFilter;
impl Context for MyFilter {}
impl HttpContext for MyFilter {
    fn on_http_request_headers(&mut self, _: usize, _: bool) -> Action {
        self.add_http_request_header("x-fastedge", "cdn");
        Action::Continue
    }
}

Do not set request headers with the x-cdn- or x-gcdn- prefixes. The CDN reserves these names for internal use and strips them before the request reaches the origin. The Proxy-Wasm header APIs do not return an error when the application sets a reserved name, so the header appears to be set in the app but never arrives at the origin.

Build it:

cargo build --release --target wasm32-wasip1

The first build downloads dependencies and takes one to two minutes. When it completes without errors, the toolchain is ready — the compiled binary is at ./target/wasm32-wasip1/release/my_cdn_app.wasm. That file can be uploaded to FastEdge as a CDN application.