gRPC context on the client side

Question

I am building a client/server system in go, using gRPC and protobuf (and with a gRPC gateway to REST).

I use metadata in the context on the server side to carry authentication data from the client, and that works perfectly well.

Now, I'd like the server to set some metadata keys/values so that the client can get them, alongside with the response. How can I do that? Using SetHeader and SendHeader? Ideally, I'd like every single response from the server to integrate that metadata (can be seen as some kind of UnaryInterceptor, but on the response rather than the request?)

Here is the code for the server and for the client.

Answer 1

I finally found my way: https://github.com/grpc/grpc-go/blob/master/Documentation/grpc-metadata.md

So basically, grpc.SetHeader() + grpc.SendHeader() and grpc.SetTrailer() are totally what I was looking for. On the client side, grpc.Header() and grpc.Trailer() functions need to be passed to the RPC call, and their argument is a metadata.MD object to be filled.

On the client side, define your receiving metadata:

var header, trailer metadata.MD

Then, pass it to the SomeRPCCall() unary RPC:

response, err := client.SomeRPCCall(
    context.Background(),
    proto.MyMessage{},
    grpc.Header(&header),
    grpc.Trailer(&trailer),
)

And now, you can check what's in your metadata:

for key, value := range header {
    fmt.Printf("%s => %s", key, value)
}

for key, value := range trailer {
    fmt.Printf("%s => %s", key, value)
}

On the server side, you can:

• force the data to be sent right after the RPC is received (but before it's processed):

  grpc.SendHeader(ctx, metadata.New(map[string]string{"my-key": "my-value"}))

• or set & send the metadata at the end of the RPC process (along with the Status):

  grpc.SetTrailer(ctx, metadata.New(map[string]string{"my-key": "my-value"}))