Protobuf RPC callbacks

Question

Is there any way to do the invoke call from server to the client with Protobuf RPC(while custom call from client to the server, not directly)?

I'm meaning, for example, describe service method in *.proto file with callback param?

Im using gRPC library.

Answer 1

The general way to do server->client messages in gRPC is through "streaming". That is, the client makes a call to the server, and then the server can "stream" back a series of messages to the client before eventually completing the call.

See: http://www.grpc.io/docs/guides/concepts.html#server-streaming-rpc

Of course, "streaming" is a pretty different pattern from a traditional "callback", but you can often solve the same problems with it.

Note that Cap'n Proto RPC -- an alternative to Protobuf and gRPC -- fully supports calls in both directions by virtue of supporting object references as a first-class type: a client can, for example, make a call to the server in which it provides an object reference to use for callbacks, and then the server can later on make calls back to that object on the client.

(Disclosure: I am the author of Google's Protobuf v2 and also the author of Cap'n Proto, but I am not affiliated with gRPC.)