gRPC Unary requests the hard way: using protorefelect, dynamicpb and wire-encoding to send messages


Simple tutorial in how to create a protobuf message and its wireformat by using reflection packages go provides.

Almost always, you just generate go packages for protobuf pb.go and its corresponding transport over gRPC echo_grpc.pb.go. You would then use both to ‘just invoke’ an API and all the gory details are taken care of for you:

import echo ""

c := echo.NewEchoServerClient(conn)
r, err := c.SayHello(ctx, &echo.EchoRequest{FirstName: "sal", LastName: "mander", MiddleName: &echo.Middle{
	Name: "a",

This article is just an exercise to show what you can do under the hood using ‘first principles’.

We manually construct a protobuf message for the following .proto:

syntax = "proto3";

package echo;

service EchoServer {
  rpc SayHello (EchoRequest) returns (EchoReply) {}

message Middle {
  string name = 1;

message EchoRequest {
  string first_name = 1;
  string last_name = 2;
  Middle middle_name = 3;

message EchoReply {
  string message = 1;


  1. Load and Register its binary protobuf definition echo.pb

  2. Create an EchoRequest message using protoreflect and dynamicpb

  3. Either

    • a. Create Message Descriptor add fields

    • b. Create Message using anypb and protojson

  4. Encode that message into gRPC’s Unary wireformat

  5. Send that to a gRPC server using _an ordinary net/http client over http2

  6. Recieve the wireformat response

  7. decode the wireformat to a protobuf message

  8. Convert the message to EchoReply

  9. print the contents of EchoReply

What does that prove? not much, its just academic thing i did…learning something new has its perpetual rewards..

What you can do is use this to construct an arbitrary Terraform DataSource

You can find the source here

for some background, also see

gRPC Request

So, let just see a normal gRPC client server:

Standard Client/Server

# run server
cd grpc_services/
go run src/grpc_server.go --grpcport :50051

# run client
go run src/grpc_client.go --host localhost:50051

What this does is just send back a unary response..nothing to see here, move along

The hard way

Next is what this article is about. grpc_client_dynamic.go does a couple of things:

  1. Load echo.pb

First step is for your go app to even know about the protobuf…so we need to load it so protoreflect knows about it

	protoFile, err := ioutil.ReadFile("grpc_services/src/echo/echo.pb")

	fileDescriptors := &descriptorpb.FileDescriptorSet{}
	err = proto.Unmarshal(protoFile, fileDescriptors)

	pb := fileDescriptors.GetFile()[0]
	fd, err := protodesc.NewFile(pb, protoregistry.GlobalFiles)

	err = protoregistry.GlobalFiles.RegisterFile(fd)
  1. Create Message

Next we construct our echo.EchoRequest using the protodescriptor from step 1.

I found two ways to do this: in 3a below, we will “strongly type” create a message and in 3b, we will create a message using a JSON string. (the latter is even more subject to simple typos)

  1. (a) Create Message Descriptor for both messages add fields

In the following, you know which type you want to create so we do this by hand:

    // create the inner message
	echoRequestInnerMessageType, err := protoregistry.GlobalTypes.FindMessageByName("echo.Middle")
	echoRequestInnerMessageDescriptor := echoRequestInnerMessageType.Descriptor()
	// add a field
	inner_name := echoRequestInnerMessageDescriptor.Fields().ByName("name")
	reflectEchoInnerRequest := echoRequestInnerMessageType.New()
	reflectEchoInnerRequest.Set(inner_name, protoreflect.ValueOfString("a"))

	// now create the outer EchoRequest message
	echoRequestMessageType, err := protoregistry.GlobalTypes.FindMessageByName("echo.EchoRequest")
	echoRequestMessageDescriptor := echoRequestMessageType.Descriptor()

	// setup the outer objects fields
	fname := echoRequestMessageDescriptor.Fields().ByName("first_name")
	lname := echoRequestMessageDescriptor.Fields().ByName("last_name")
	mname := echoRequestMessageDescriptor.Fields().ByName("middle_name")

	// now add the fields and the Middle message
	// note the types, the message is of type Message
	reflectEchoRequest := echoRequestMessageType.New()
	reflectEchoRequest.Set(fname, protoreflect.ValueOfString("sal"))
	reflectEchoRequest.Set(lname, protoreflect.ValueOfString("mander"))
	reflectEchoRequest.Set(mname, protoreflect.ValueOfMessage(reflectEchoInnerRequest))
	fmt.Printf("EchoRequest: %v\n", reflectEchoRequest)

	in, err := proto.Marshal(reflectEchoRequest.Interface())

	fmt.Printf("Encoded EchoRequest using protoreflect %s\n", hex.EncodeToString(in))

Note that we’re manually defining everything…its excruciating

  1. (b) Create Message using anypb and protojson

In the following, we will “just create” a message using its JSON format. Remember to set the @type: field in json

	j := `{	"@type": "echo.EchoRequest", "firstName": "sal", "lastName": "mander", "middleName": { "name": "a"}}`
	a, err := anypb.New(echoRequestMessageType.New().Interface())

	err = protojson.Unmarshal([]byte(j), a)
	fmt.Printf("Encoded EchoRequest using protojson and anypb %v\n", hex.EncodeToString(a.Value))
  1. Encode it to the wireformat

Either way, we need to convert the proto message into a wireformat. For this we use lencode

	var out bytes.Buffer
	enc := lencode.NewEncoder(&out, lencode.SeparatorOpt([]byte{0}))

	// (a) to send the manually generated message:
	err = enc.Encode(in)

	// (b) to send the json->protobuf message
	err = enc.Encode(a.Value)

You might be asking …WTF is lencode.SeparatorOpt([]byte{0})?!

…weeellll, thats just the wireformat position that signals works here. See parsing gRPC messages from Envoy TAP

  1. Send message

We’re now ready to transmit the wireformat message to our grpc server

	client := http.Client{
		Transport: &http2.Transport{
			TLSClientConfig: &tlsConfig,

	reader := bytes.NewReader(out.Bytes())
	resp, err := client.Post("https://localhost:50051/echo.EchoServer/SayHello", "application/grpc", reader)
  1. Recieve the wireformat response

read in the bytes inside resp.Body

  1. decode the wireformat to a protobuf message

Use lencode to unmarshall the payload

	respMessage := lencode.NewDecoder(bytesReader, lencode.SeparatorOpt([]byte{0}))
	respMessageBytes, err := respMessage.Decode()
  1. Convert the message to EchoReply

We now do the inverse of the outbound steps still using the descriptors we originally setup

	echoReplyMessageType, err := protoregistry.GlobalTypes.FindMessageByName("echo.EchoReply")

	echoReplyMessageDescriptor := echoReplyMessageType.Descriptor()
	pmr := echoReplyMessageType.New()

	err = proto.Unmarshal(respMessageBytes, pmr.Interface())

	msg := echoReplyMessageDescriptor.Fields().ByName("message")

	fmt.Printf("EchoReply.Message using protoreflect: %s\n", pmr.Get(msg).String())
  1. print the contents of EchoReply

We now have the message back…we can print it.


TO run it end-to end, keep the server running and invoke the client

$ go run grpc_client_dynamic.go 

Loading package echo
  Registering MessageType: Middle
  Registering MessageType: EchoRequest
  Registering MessageType: EchoReply
EchoRequest: first_name:"sal"  last_name:"mander"  middle_name:{name:"a"}

Encoded EchoRequest using protoreflect 0a0373616c12066d616e6465721a030a0161
Encoded EchoRequest using protojson and anypb 0a0373616c12066d616e6465721a030a0161

wire encoded EchoRequest: 00000000120a0373616c12066d616e6465721a030a0161
wire encoded EchoReply 00000000140a1248656c6c6f2073616c2061206d616e646572

Encoded EchoReply 0a1248656c6c6f2073616c2061206d616e646572
EchoReply.Message using protoreflect: Hello sal a mander
EchoReply as string JSON: {"message":"Hello sal a mander"}

What the output shows is how we loaded the echo.pb, then constructed the Message from either explicitly creating it or by converting a JSON Message over.

Once that was done, we sent the wire-encoded message to the server and reversed the process.

Did i mention you can also use curl to call the endpoint…

the trick is to use the wire encoded format (since, you know, curl send stuff on the wire

echo -n '00000000120a0373616c12066d616e6465721a030a0161' | xxd -r -p - frame.bin
curl -v  --raw -X POST --http2-prior-knowledge  \
    -H "Content-Type: application/grpc" \
    -H "TE: trailers" \
    --data-binary @frame.bin \
       http://localhost:50051/echo.EchoServer/SayHello -o resp.bin

to decode

$ xxd -p resp.bin 

## remove the prefix headers 0000000014
$ echo -n "0a1248656c6c6f2073616c2061206d616e646572" | \
   xxd -r -p | \
   protoc  --decode echo.EchoReply grpc_services/src/echo/echo.proto 

message: "Hello sal a mander"

Now look at the message decoded with wireshark

gRPC Response


This repo also contains an end-to-end sample of

Using that library makes certain things a lot easer as it wraps some of the legwork for you. You can also “just load” a .proto file that includes specifications of the Message and gRPC server.

To use that,

cd jhump_client/
$ go run grpc_client_jhump.go 
> service echo.EchoServer
  * method echo.EchoServer.SayHello (echo.EchoRequest) echo.EchoReply
- message echo.Middle
- message echo.EchoRequest
- message echo.EchoReply
Looking for serviceName echo.EchoServer methodName SayHello
Response: {
	"message": "Hello sal a mander"

gRPC Reflection

The default gRPC server here also has gRPC Reflection enabled for inspection.

To use this, you have to install grpc_cli (which, TBH, is way too cumbersome!)


$ grpc_cli ls localhost:50051

$ grpc_cli ls localhost:50051 echo.EchoServer -l
	filename: src/echo/echo.proto
	package: echo;
	service EchoServer {
		rpc SayHello(echo.EchoRequest) returns (echo.EchoReply) {}

$ grpc_cli ls localhost:50051 echo.EchoServer.SayHello -l
	rpc SayHello(echo.EchoRequest) returns (echo.EchoReply) {}

$ grpc_cli type localhost:50051 echo.EchoRequest
	message EchoRequest {
	string first_name = 1 [json_name = "firstName"];
	string last_name = 2 [json_name = "lastName"];
	.echo.Middle middle_name = 3 [json_name = "middleName"];

grpc_cli call localhost:50051 echo.EchoServer.SayHello "first_name: 'sal' last_name: 'mander' middle_name: {name: 'a'}"
	connecting to localhost:50051
	message: "Hello sal a mander"
	Rpc succeeded with OK status


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