gRPC has been solid for us on the JVM, and streaming has been great when consuming from Apache Flink jobs, integrating with message queues, receiving push notifications and so on. For async work it's useful to have more than just request/response.

I've been playing the FoundationDB Record Layer for a personal project of mine, and with this setup I can generate not only the API implementation, but also the models used by the persistence layer:

Protobuf (Messages) -> gRPC -> Scala/Monix -> Protobuf (Models) -> FoundationDB