Host Functions Example

This example shows how to embed functions defined in a host into plugins to expand the plugin functionalities.

Generate Go code

A proto file is under ./greeting.

// The host functions embedded into the plugin
// go:plugin type=host
service HostFunctions {
  // Sends a HTTP GET request
  rpc HttpGet(HttpGetRequest) returns (HttpGetResponse) {}
  // Shows a log message
  rpc Log(LogRequest) returns (google.protobuf.Empty) {}
}

NOTE: You must specify type=host in the comment. It represents the service is for host functions.

Then, generate source code.

$ protoc --go-plugin_out=. --go-plugin_opt=paths=source_relative greeting/greet.proto 

Implement host functions

The following interface is generated.

type HostFunctions interface {
	HttpGet(context.Context, HttpGetRequest) (HttpGetResponse, error)
	Log(context.Context, LogRequest) (emptypb.Empty, error)
}

Implement that interface.

// myHostFunctions implements greeting.HostFunctions
type myHostFunctions struct{}

// HttpGet is embedded into the plugin and can be called by the plugin.
func (myHostFunctions) HttpGet(ctx context.Context, request greeting.HttpGetRequest) (greeting.HttpGetResponse, error) {
	resp, err := http.Get(request.Url)
	if err != nil {
		return greeting.HttpGetResponse{}, err
	}
	defer resp.Body.Close()

	buf, err := io.ReadAll(resp.Body)
	if err != nil {
		return greeting.HttpGetResponse{}, err
	}

	return greeting.HttpGetResponse{Response: buf}, nil
}

// Log is embedded into the plugin and can be called by the plugin.
func (myHostFunctions) Log(ctx context.Context, request greeting.LogRequest) (emptypb.Empty, error) {
	// Use the host logger
	log.Println(request.GetMessage())
	return emptypb.Empty{}, nil
}

Pass it to a plugin in Load().

// Pass my host functions that are embedded into the plugin.
greetingPlugin, err := p.Load(ctx, "plugin/plugin.wasm", myHostFunctions{})

Call host functions in a plugin

The embedded host functions can be called in plugins.

hostFunctions := greeting.NewHostFunctions()

// Logging via the host function
hostFunctions.Log(ctx, greeting.LogRequest{
	Message: "Sending a HTTP request...",
})

// HTTP GET via the host function
resp, err := hostFunctions.HttpGet(ctx, greeting.HttpGetRequest{Url: "http://ifconfig.me"})

Compile a plugin

Use TinyGo to compile the plugin to Wasm.

$ tinygo build -o plugin/plugin.wasm -scheduler=none -target=wasi --no-debug plugin/plugin.go

Run

main.go loads the above plugin.

$ go run main.go
2022/08/28 10:13:57 Sending a HTTP request...
Hello, go-plugin from x.x.x.x