How to Handle gRPC Metadata (Headers) in Go

You handle gRPC metadata in Go by accessing the metadata.MD map stored in the request context. Use metadata.FromIncomingContext to retrieve headers on the server side or metadata.AppendToOutgoingContext to attach them on the client side.

import (
	"context"
	"google.golang.org/grpc/metadata"
)

// Server-side: Read metadata
func (s *Server) Handle(ctx context.Context, req *Request) (*Response, error) {
	md, ok := metadata.FromIncomingContext(ctx)
	if !ok {
		return nil, fmt.Errorf("no metadata found")
	}
	// Access header: md.Get("key") returns []string
	val := md.Get("x-custom-header")
	return &Response{}, nil
}

// Client-side: Send metadata
ctx := metadata.AppendToOutgoingContext(context.Background(), "x-custom-header", "value")
resp, err := client.Call(ctx, &Request{})

gRPC metadata acts like HTTP headers, allowing you to pass extra information like authentication tokens or tracing IDs alongside your main data. In Go, you don't access these directly on the request object; instead, you pull them from the request's context. Think of it as a side-channel attached to every request that carries small, non-body details needed for processing.