service

Service Example

This example demonstrates a Service that owns a custom config category, provides a host capability, and has lifecycle hooks.

Structure

• wit/greeter.wit: WIT interface implemented by the host capability
• host/src/main.rs: Service, ConfigHandler, and HostCapability implementation
• greeter/: Wasm component that imports the host interface
• config.toml: Component, capability, and service configuration

Building and Running

Change into the examples/service directory.

1. Build the greeter component and host:

./build.sh

2. Run the example:

./run.sh

Output:

[GreetingService] started
Result: "HOWDY, World!"
[GreetingService] shutdown

How It Works

GreetingService implements Service with three responsibilities:

1. Config handling: A ConfigHandler claims the [greeting] category, parsing the message property. Config is shared with the service via Arc<Mutex<...>>.
2. Capability provisioning: After config is parsed, capabilities() uses .take() to pull the message out of the mutex and creates a GreetingCapability factory. The factory also reads uppercase from the capability's properties.
3. Lifecycle: start() and shutdown() are called around invocation.

Flow

1. Configuration: [greeting.default] sets message = "Howdy". [capability.greeting] sets uppercase = true.
2. Config phase: GreetingConfigHandler parses the greeting category and stores the message.
3. Capability phase: GreetingService.capabilities() takes the stored message, creates a factory that applies the uppercase transform.
4. Invocation: Guest calls get_greeting(), host returns "HOWDY", guest formats "HOWDY, World!".