gRPC and schema evolution guarantees?

I am evaluating using gRPC. On the subject of compatibility with 'schema evolution', i did find the information that protocol buffers, which are exchanged by gRPC for data serialization, have a format such that different evolutions of a data in protobuf format can stay compatible, as long as the schemas evolution allows it.

But that does not tell me wether two iterations of a gRPC client/server will be able to exchange a command that did not change in the schema, regardless of the changes in the rest of the schema?

Does gRPC quarantee that an older generated version of a client or server code will always be able to launch/answer a command that has not changed in the schema file, with any more recent schema generated code on the interlocutor side, reglardless of the rest of the schema? (Assuming no other breaking changes like a non backward compatible gRPC version change)

gRPC has compatibility for a method as long as:

• the proto package, service name, and method name is unchanged
• the proto request and response messages are still compatible
• the cardinality (unary vs streaming) of the request and response message remains unchanged

New services and methods can be added at will and not impact compatibility of preexisting methods. This doesn't get discussed much because those restrictions are mostly what people would expect.

There is actually some wiggle room in changing cardinality as unary is encoded the same as streaming on-the-wire, but it's generally just better to assume you can't change cardinality and add a new method instead.

This topic is discussed in the Modifying gRPC Services over Time talk (PDF slides and Youtube links available). I'll note that the slides are not intended to stand alone.