Metadata interceptor example

This example shows how to update metadata from unary and streaming interceptors on the server. Please see grpc-metadata.md for more information.

Try it

go run server/main.go

go run client/main.go

Explanation

Unary interceptor

The interceptor can read existing metadata from the RPC context passed to it. Since Go contexts are immutable, the interceptor will have to create a new context with updated metadata and pass it to the provided handler.

func SomeInterceptor(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
    // Get the incoming metadata from the RPC context, and add a new
    // key-value pair to it.
    md, ok := metadata.FromIncomingContext(ctx)
    md.Append("key1", "value1")

    // Create a context with the new metadata and pass it to handler.
    ctx = metadata.NewIncomingContext(ctx, md)
    return handler(ctx, req)
}

Streaming interceptor

grpc.ServerStream does not provide a way to modify its RPC context. The streaming interceptor therefore needs to implement the grpc.ServerStream interface and return a context with updated metadata.

The easiest way to do this would be to create a type which embeds the grpc.ServerStream interface and overrides only the Context() method to return a context with updated metadata. The streaming interceptor would then pass this wrapped stream to the provided handler.

type wrappedStream struct {
    grpc.ServerStream
    ctx context.Context
}

func (s *wrappedStream) Context() context.Context {
    return s.ctx
}

func SomeStreamInterceptor(srv interface{}, ss grpc.ServerStream, info *grpc.StreamServerInfo, handler grpc.StreamHandler) error {
    // Get the incoming metadata from the RPC context, and add a new 
    // key-value pair to it.
    md, ok := metadata.FromIncomingContext(ctx)
    md.Append("key1", "value1")

    // Create a context with the new metadata and pass it to handler.
    ctx = metadata.NewIncomingContext(ctx, md)

    return handler(srv, &wrappedStream{ss, ctx})
}