> Then it's not really required if it can be ignored.

It can be required in v2 but not in v1 which was my point. If the client is running v2 while the server is still on v1 temporarily, then there is no problem. The server just ignores the new field until it is upgraded.

> This is the problem, software (and protos) can live for a long time). They might be used by other clients elsewhere that you don't control. What you thought might not required 10 years down the line is not anymore. What you "think" is not a huge deal then becomes a huge deal and can cause downtime.

Part of this is just that trying to create a format that is suitable both as an rpc wire serialization format and ALSO a format suitable for long term storage leads to something that is not great for either use case. But even taking that into account, RDBMS have been dealing with this problem for decades and every RDBMS lets you define fields as non-nullable.

> If a message is required for an interface to function, you should be checking it anyway and returning the correct error. How is that change with proto supporting require?

That's my point, you have to do that check in code which clutters the implementation with validation noise. That and you often can't use the wire message in your internal domain model since you now have to do that defensive null-check everywhere the object is used.

Aside from that, protocol buffers are an interface definition language so should be able to encode some of the validation logic at least (make invalid states unrepresentable and all that). If you are just looking at the proto IDL you have no way of knowing whether a field is really required or not because there is no way to specify that.