With the ISO/oasis standard AMQP you can actually do synchronous calls without an intermediary. You also have 'direct' message routing capability with component

With the ISO/oasis standard AMQP you can actually do synchronous calls without an intermediary. You also have 'direct' message routing capability with components like apache qpid dispatch router. I can't see a reason why one would use gRPC/thrift and then having to build flow control, delivery guarantees and other messaging features yourself.