Comment by ragall

The problem is different. Protobuf's unknown field support is useful if you want to forward a message in its entirety, and it allows you to copy an input message even though it has fields unknown to the middleware. The problem arises because at Google, in order to minimize payloads and storage sizes, they almost always create "intermediate" protobufs that are only used by middleware to talk to other middleware.

Example:

The service that manages the web frontend knows that the new checkbox is auth-related and therefore it has to go into the WebServiceAuthRequest PB message, but it doesn't have the new schema of the WebServiceAuthRequest message with the checkbox field, so it can't create a WebServiceAuthRequest message because it doesn't know which numeric ID to use for the value.

The "common wisdom" at Google was that you have to add a new field starting at the leaves (the storage backends) and work your way up to the middleware, then the web frontends and finally the JS code. And yes, in the worst case it can take two quarters and modifying 20 intermediate services (each with its own ServiceFooRequest protobuf) just to add a new checkbox in the UI.

And in writing this I came up with a way to avoid the problem, but it would require an incompatible change to the PB wire format. Hmmm...