Interceptor Example

This example demonstrates an interceptor that wraps a component with cross-cutting logging advice.

Structure

• wit/package.wit: WIT interfaces for the greeter and logger worlds
• greeter/: Wasm component that exports a greet function
• logger/: Wasm component that exports modulewise:interceptor/advice and implements generic logging before and after each invocation
• config.toml: Component and capability configuration
• wasi-logging-to-stdout.wasm: WASI logging adapter (fetched from an OCI registry on first build)

Building and Running

Change into the examples/interceptor directory.

1. Build the components:

2. Run the example:

Output:

... INFO  [interceptor]: Before greet(name: "World")
... INFO  [interceptor]: After greet -> "Hello, World!"
"Hello, World!"

Configuration

[component.greeter]
uri = "./target/wasm32-unknown-unknown/release/greeter.wasm"
interceptors = ["logging-advice"]

[component.logging-advice]
uri = "./target/wasm32-unknown-unknown/release/logger.wasm"
imports = ["stdout-logger"]

[component.stdout-logger]
uri = "./wasi-logging-to-stdout.wasm"
imports = ["wasip2"]

[capability.wasip2]
type = "wasi:p2"

How It Works

The interceptors property on the greeter refers to generic advice, so the runtime generates an interceptor component at startup. The generated interceptor composes the logging-advice component with the greeter component, so that the advice is applied to each exported function of the greeter.