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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})
}