how to read metadata in grpc on the server side? (golang example)

Question

How to read metadata (passed as a header) on the server side in grpc? Any example in golang?

I am writing something like this:

// this should be passed from the client side as a context and needs to accessed on server side to read the metadata
var headers = metadata.New(map[string]string{"authorization": "", "space":  "", "org": "", "limit": "", "offset": ""})

I want to pass the Authorization token to my validation function to validate the received token.

func validate_token(ctx context.Context, md *metadata.MD) (context.Context, error){
    token := headers["authorization"]
}

Solution 1

You have to insert your metadata into the client's context before calling the server.

For an unary RPC the client side looks like:

conn, _ := grpc.Dial(address, opts...)
client := NewMyClient(conn) // generated from your proto with the grpc protoc option

header := metadata.New(map[string]string{"authorization": "", "space":  "", "org": "", "limit": "", "offset": ""})
// this is the critical step that includes your headers
ctx := metadata.NewContext(context.Background(), header)

request := // construct a request for your service
response, err := client.MyMethod(ctx, request)

For a stream, it looks almost the same:

conn, _ := grpc.Dial(address, opts...)
client := NewMyClient(conn) // generated from your proto with the grpc protoc option

header := metadata.New(map[string]string{"authorization": "", "space":  "", "org": "", "limit": "", "offset": ""})
// this is the critical step that includes your headers
ctx := metadata.NewContext(context.Background(), header)
stream, err := client.MyMethodStream(ctx)

for {
    request :=  // construct a request for your service
    err := stream.Send(request)
    response := new(Response)
    err = stream.RecvMsg(response)
}

---

On the server side for an unary RPC:

func (s myServer) MyMethod(context.Context, *Request) (*Response, error) {
    md, ok := metadata.FromIncomingContext(ctx)
    token := md.Get("authorization")[0] // metadata.Get returns an array of values for the key
}

and for a streaming RPC:

func (s myServer) MyMethodStream(stream MyMethod_MyServiceStreamServer) error {
    md, ok := metadata.FromIncomingContext(stream.Context())
    token := md.Get("authorization")[0] // metadata.Get returns an array of values for the key
    for {
        request := new(Request)
        err := stream.RecvMsg(request)
        response := // do work
        err := stream.SendMsg(response)
    }
}

---

Note that for a stream there are only three times that headers can be sent: in the context used to open the initial stream, via grpc.SendHeader, and grpc.SetTrailer. It is not possible to set headers on arbitrary messages in a stream. For an unary RPC header are sent with every message and can be set in the initial context, with grpc.SendHeader and grpc.SetHeader, and grpc.SetTrailer.

Solution 2

original answer is correct but reading the headers is slightly outdated

import "google.golang.org/grpc/metadata"

func (s myServer) MyMethod(ctx context.Context, *Request) (*Response, error) {
  var values []string
  var token string

  md, ok := metadata.FromIncomingContext(ctx)
  if ok {
    values = md.Get("authorization")
  }

  if len(values) > 0 {
    token = values[0]
  }

  // do something with token
}

Comments

[Maxim Vladimirsky]

In later gRPC versions it is metadata.FromIncomingContext

[freitas]

what is the difference between incoming and outgoing context?