Is “google/protobuf/struct.proto” the best way to send dynamic JSON over GRPC?

Question

I have a written a simple GRPC server and a client to call the server (both in Go). Please tell me if using golang/protobuf/struct is the best way to send a dynamic JSON with GR

Answer 1

I ended up using a two-step conversion with protojson, from map to json to struct:

m := map[string]interface{}{
  "foo":"bar",
  "baz":123,
}
b, err := json.Marshal(m)
s := &structpb.Struct{}
err = protojson.Unmarshal(b, s)

I don't find it elegant but could not really find any official documentation of how to do this differently. I also prefer to produce structures using "official" functions rather than trying to build a structure myself.

Answer 2

If you have JSON data already, you could also choose to encode it as a string field. Otherwise, using a google.protobuf.Struct seems pretty reasonable, and you should be able to use jsonpb to convert between the Struct and JSON easily on the client and server.

Answer 3

Based on this proto file.

syntax = "proto3";
package messages;
import "google/protobuf/struct.proto";

service UserService {
    rpc SendJson (SendJsonRequest) returns (SendJsonResponse) {}
}

message SendJsonRequest {
    string UserID = 1;
    google.protobuf.Struct Details = 2;
}

message SendJsonResponse {
    string Response = 1;
}

I think it is a good solution to use the google.protobuf.Struct type.

The folks with their answers, helped me a lot at the beginning, so I would like to say thanks for your work! :) I really appreciate both solutions! :) On the other hand, I think I found a better one to produce these kinds of Structs.

Anuj's Solution

This is a little bit overcomplicated but it can work.

var item = &structpb.Struct{
    Fields: map[string]*structpb.Value{
        "name": &structpb.Value{
            Kind: &structpb.Value_StringValue{
                StringValue: "Anuj",
            },
        },
        "age": &structpb.Value{
            Kind: &structpb.Value_StringValue{
                StringValue: "Anuj",
            },
        },
    },
}

Luke's Solution

This is a shorter one but still require more conversion than necessary. map[string]interface{} -> bytes -> Struct

m := map[string]interface{}{
  "foo":"bar",
  "baz":123,
}
b, err := json.Marshal(m)
s := &structpb.Struct{}
err = protojson.Unmarshal(b, s)

The solution from my perspective

My solution will use the official functions from the structpb package which is pretty well documented and user friendly.

Documentation: https://pkg.go.dev/google.golang.org/protobuf/types/known/structpb

For example, this code creates a *structpb.Struct via the function that was designed to do this.

m := map[string]interface{}{
    "name": "Anuj",
    "age":  23,
}

details, err := structpb.NewStruct(m) // Check to rules below to avoid errors
if err != nil {
    panic(err)
}

userGetRequest := &pb.SendJsonRequest{
    UserID: "A123",
    Details: details,
}

One of the most important thing, that we should keep in mind when we are building Struct from a map[string]interface{} is this:

https://pkg.go.dev/google.golang.org/protobuf/types/known/structpb#NewValue

// NewValue constructs a Value from a general-purpose Go interface.
//
//  ╔════════════════════════╤════════════════════════════════════════════╗
//  ║ Go type                │ Conversion                                 ║
//  ╠════════════════════════╪════════════════════════════════════════════╣
//  ║ nil                    │ stored as NullValue                        ║
//  ║ bool                   │ stored as BoolValue                        ║
//  ║ int, int32, int64      │ stored as NumberValue                      ║
//  ║ uint, uint32, uint64   │ stored as NumberValue                      ║
//  ║ float32, float64       │ stored as NumberValue                      ║
//  ║ string                 │ stored as StringValue; must be valid UTF-8 ║
//  ║ []byte                 │ stored as StringValue; base64-encoded      ║
//  ║ map[string]interface{} │ stored as StructValue                      ║
//  ║ []interface{}          │ stored as ListValue                        ║
//  ╚════════════════════════╧════════════════════════════════════════════╝
//
// When converting an int64 or uint64 to a NumberValue, numeric precision loss
// is possible since they are stored as a float64.

For example, if you would like to produce a Struct that has a string list in its JSON form, you should create the following map[string]interface{}

m := map[string]interface{}{
    "name": "Anuj",
    "age":  23,
    "cars": []interface{}{
        "Toyota",
        "Honda",
        "Dodge",
    }
}

Sorry for the long post, I hope it makes your work easier with proto3 and Go! :)