> The goal is not to have a single source of truth schema. That is a means to some other goal, and it's not even a good means.

It’s about data transmission. Being able to encode and decode in a type safe manner between different languages (and so, different platforms) is a goal that makes a lot of sense.

> If you do change your schema from time to time, you need testing between the two systems

Or you could just use a defined format that doesn’t require testing. I rarely use protobuf but I can see why people do. The guaranteed backwards compatibility is huge for people who can’t just publish a new web frontend at the drop of a hat.

If you understand how to evolve protobuf schema definitions, then you don’t really need testing. You instinctively know how the parser works when it is parsing data with a different schema from what it expects. And that’s a powerful thing. If your things break even when using protobuf then you don’t grok protobuf.

It’s probably not an exaggeration to say that being able to avoid tests between different systems who have different versions of the schema is a core goal of protobuf. Why? These two different systems are probably owned by different teams, and introducing explicit tests between different versions of them increases coupling between them.

Both sibling comments say one type of assurance makes the other irrelevant, but I would wager they cover different territory.