gRPC Go Proxy server

Build a transparent reverse proxy for gRPC targets that will make it easy to expose gRPC services over the internet. This includes:

• no needed knowledge of the semantics of requests exchanged in the call (independent rollouts)
• easy, declarative definition of backends and their mappings to frontends
• simple round-robin load balancing of inbound requests from a single connection to multiple backends

The project now exists as a proof of concept, with the key piece being the proxy package that is a generic gRPC reverse proxy handler.

The package proxy contains a generic gRPC reverse proxy handler that allows a gRPC server to not know about registered handlers or their data types. Please consult the docs, here's an exaple usage.

You can call proxy.NewProxy to create a *grpc.Server that proxies requests.

proxy := proxy.NewProxy(clientConn)

More advanced users will want to define a StreamDirector that can make more complex decisions on what to do with the request.

director = func(ctx context.Context, fullMethodName string) (context.Context, *grpc.ClientConn, error) {
    md, _ := metadata.FromIncomingContext(ctx)
    outCtx = metadata.NewOutgoingContext(ctx, md.Copy())
    return outCtx, cc, nil
	
    // Make sure we never forward internal services.
    if strings.HasPrefix(fullMethodName, "/com.example.internal.") {
        return outCtx, nil, status.Errorf(codes.Unimplemented, "Unknown method")
    }
    
    if ok {
        // Decide on which backend to dial
        if val, exists := md[":authority"]; exists && val[0] == "staging.api.example.com" {
            // Make sure we use DialContext so the dialing can be cancelled/time out together with the context.
            return outCtx, grpc.DialContext(ctx, "api-service.staging.svc.local", grpc.WithCodec(proxy.Codec())), nil
        } else if val, exists := md[":authority"]; exists && val[0] == "api.example.com" {
            return outCtx, grpc.DialContext(ctx, "api-service.prod.svc.local", grpc.WithCodec(proxy.Codec())), nil
        }
    }
    return outCtx, nil, status.Errorf(codes.Unimplemented, "Unknown method")
}

Then you need to register it with a grpc.Server. The server may have other handlers that will be served locally.

server := grpc.NewServer(
    grpc.CustomCodec(proxy.Codec()),
    grpc.UnknownServiceHandler(proxy.TransparentHandler(director)))
pb_test.RegisterTestServiceServer(server, &testImpl{})

To make debugging a bit simpler, there are some helpers.

testservice contains a method TestTestServiceServerImpl which performs a complete test against the reference implementation of the TestServiceServer.

In proxy_test.go, the test framework spins up a TestServiceServer that it tests the proxy against. To make debugging a bit simpler (eg. if the developer needs to step into google.golang.org/grpc methods), this TestServiceServer can be provided by a server by passing -test-backend=addr to go test. A simple, local-only implementation of TestServiceServer exists in testservice/server.

grpc-proxy is released under the Apache 2.0 license. See LICENSE.txt.