[Discuss] Use gRPC gateway to generate REST endpoints

Introduction

Right now, feast-java uses RestController (example code) to implement REST endpoints for both feast-core and feast-serving. However, this approach will increase maintenance cost. Therefore, generating REST endpoint from proto definitions is a cleaner way.

Proposed Change

Use grpc-gateway to generate the REST endpoint.

Steps

1. Add gateway server to translate REST call into gRPC call to another process inside the container

The example gateway server will look like:

package main

import (
	"context"
	"flag"
	"fmt"
	"net/http"

	"github.com/golang/glog"
	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
	"google.golang.org/grpc"

	coreGateWay "pkg/gateway/generated/feast/core"
	servingGateWay "pkg/gateway/generated/feast/serving"
)

// gRPC serving endpoint
var servingServerEndpoint = flag.String("serving-server-endpoint", "localhost:6566", "gRPC server endpoint")

func run() error {
	ctx := context.Background()
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()

	mux := runtime.NewServeMux()
	opts := []grpc.DialOption{grpc.WithInsecure()}

	// Register ServingServing endpoint
	err = servingGateWay.RegisterServingServiceHandlerFromEndpoint(ctx, mux, *servingServerEndpoint, opts)
	if err != nil {
		return err
	}

	// Start HTTP server (and proxy calls to gRPC server endpoint)
	return http.ListenAndServe(":8000", mux)
}

func main() {
	flag.Parse()
	defer glog.Flush()

	if err := run(); err != nil {
		fmt.Println(err)
		glog.Fatal(err)
	}
}

2. Use supervisord to start two processes inside a container

We can provide instructions to use different entry points. Here's an example config:

[supervisord]
nodaemon=true

[program:serving]
directory=/
user=root
command=java -jar /opt/feast/feast-serving.jar --spring.config.location=classpath:/application.yml,file:/etc/feast/%(ENV_FEAST_SERVING_YML_NAME)s 

[program:gateway]
directory=/
user=root
command=/opt/feast/gateway_server 
            --serving-server-endpoint=%(ENV_FEAST_SERVING_URL)s
numprocs=1

3. Remove existing REST controllers' code

As titled.